wireproto: add frame flag to denote payloads as CBOR

We may eventually want a separate frame type for this. But for
now this is the easiest to implement.

Differential Revision: https://phab.mercurial-scm.org/D2986

mercurial/help/internals/wireprotocol.txt

@@ -1,1638 +1,1640
 The Mercurial wire protocol is a request-response based protocol
 with multiple wire representations.
 
 Each request is modeled as a command name, a dictionary of arguments, and
 optional raw input. Command arguments and their types are intrinsic
 properties of commands. So is the response type of the command. This means
 clients can't always send arbitrary arguments to servers and servers can't
 return multiple response types.
 
 The protocol is synchronous and does not support multiplexing (concurrent
 commands).
 
 Handshake
 =========
 
 It is required or common for clients to perform a *handshake* when connecting
 to a server. The handshake serves the following purposes:
 
 * Negotiating protocol/transport level options
 * Allows the client to learn about server capabilities to influence
   future requests
 * Ensures the underlying transport channel is in a *clean* state
 
 An important goal of the handshake is to allow clients to use more modern
 wire protocol features. By default, clients must assume they are talking
 to an old version of Mercurial server (possibly even the very first
 implementation). So, clients should not attempt to call or utilize modern
 wire protocol features until they have confirmation that the server
 supports them. The handshake implementation is designed to allow both
 ends to utilize the latest set of features and capabilities with as
 few round trips as possible.
 
 The handshake mechanism varies by transport and protocol and is documented
 in the sections below.
 
 HTTP Protocol
 =============
 
 Handshake
 ---------
 
 The client sends a ``capabilities`` command request (``?cmd=capabilities``)
 as soon as HTTP requests may be issued.
 
 The server responds with a capabilities string, which the client parses to
 learn about the server's abilities.
 
 HTTP Version 1 Transport
 ------------------------
 
 Commands are issued as HTTP/1.0 or HTTP/1.1 requests. Commands are
 sent to the base URL of the repository with the command name sent in
 the ``cmd`` query string parameter. e.g.
 ``https://example.com/repo?cmd=capabilities``. The HTTP method is ``GET``
 or ``POST`` depending on the command and whether there is a request
 body.
 
 Command arguments can be sent multiple ways.
 
 The simplest is part of the URL query string using ``x-www-form-urlencoded``
 encoding (see Python's ``urllib.urlencode()``. However, many servers impose
 length limitations on the URL. So this mechanism is typically only used if
 the server doesn't support other mechanisms.
 
 If the server supports the ``httpheader`` capability, command arguments can
 be sent in HTTP request headers named ``X-HgArg-<N>`` where ``<N>`` is an
 integer starting at 1. A ``x-www-form-urlencoded`` representation of the
 arguments is obtained. This full string is then split into chunks and sent
 in numbered ``X-HgArg-<N>`` headers. The maximum length of each HTTP header
 is defined by the server in the ``httpheader`` capability value, which defaults
 to ``1024``. The server reassembles the encoded arguments string by
 concatenating the ``X-HgArg-<N>`` headers then URL decodes them into a
 dictionary.
 
 The list of ``X-HgArg-<N>`` headers should be added to the ``Vary`` request
 header to instruct caches to take these headers into consideration when caching
 requests.
 
 If the server supports the ``httppostargs`` capability, the client
 may send command arguments in the HTTP request body as part of an
 HTTP POST request. The command arguments will be URL encoded just like
 they would for sending them via HTTP headers. However, no splitting is
 performed: the raw arguments are included in the HTTP request body.
 
 The client sends a ``X-HgArgs-Post`` header with the string length of the
 encoded arguments data. Additional data may be included in the HTTP
 request body immediately following the argument data. The offset of the
 non-argument data is defined by the ``X-HgArgs-Post`` header. The
 ``X-HgArgs-Post`` header is not required if there is no argument data.
 
 Additional command data can be sent as part of the HTTP request body. The
 default ``Content-Type`` when sending data is ``application/mercurial-0.1``.
 A ``Content-Length`` header is currently always sent.
 
 Example HTTP requests::
 
     GET /repo?cmd=capabilities
     X-HgArg-1: foo=bar&baz=hello%20world
 
 The request media type should be chosen based on server support. If the
 ``httpmediatype`` server capability is present, the client should send
 the newest mutually supported media type. If this capability is absent,
 the client must assume the server only supports the
 ``application/mercurial-0.1`` media type.
 
 The ``Content-Type`` HTTP response header identifies the response as coming
 from Mercurial and can also be used to signal an error has occurred.
 
 The ``application/mercurial-*`` media types indicate a generic Mercurial
 data type.
 
 The ``application/mercurial-0.1`` media type is raw Mercurial data. It is the
 predecessor of the format below.
 
 The ``application/mercurial-0.2`` media type is compression framed Mercurial
 data. The first byte of the payload indicates the length of the compression
 format identifier that follows. Next are N bytes indicating the compression
 format. e.g. ``zlib``. The remaining bytes are compressed according to that
 compression format. The decompressed data behaves the same as with
 ``application/mercurial-0.1``.
 
 The ``application/hg-error`` media type indicates a generic error occurred.
 The content of the HTTP response body typically holds text describing the
 error.
 
 The ``application/hg-changegroup`` media type indicates a changegroup response
 type.
 
 Clients also accept the ``text/plain`` media type. All other media
 types should cause the client to error.
 
 Behavior of media types is further described in the ``Content Negotiation``
 section below.
 
 Clients should issue a ``User-Agent`` request header that identifies the client.
 The server should not use the ``User-Agent`` for feature detection.
 
 A command returning a ``string`` response issues a
 ``application/mercurial-0.*`` media type and the HTTP response body contains
 the raw string value (after compression decoding, if used). A
 ``Content-Length`` header is typically issued, but not required.
 
 A command returning a ``stream`` response issues a
 ``application/mercurial-0.*`` media type and the HTTP response is typically
 using *chunked transfer* (``Transfer-Encoding: chunked``).
 
 HTTP Version 2 Transport
 ------------------------
 
 **Experimental - feature under active development**
 
 Version 2 of the HTTP protocol is exposed under the ``/api/*`` URL space.
 It's final API name is not yet formalized.
 
 Commands are triggered by sending HTTP POST requests against URLs of the
 form ``<permission>/<command>``, where ``<permission>`` is ``ro`` or
 ``rw``, meaning read-only and read-write, respectively and ``<command>``
 is a named wire protocol command.
 
 Non-POST request methods MUST be rejected by the server with an HTTP
 405 response.
 
 Commands that modify repository state in meaningful ways MUST NOT be
 exposed under the ``ro`` URL prefix. All available commands MUST be
 available under the ``rw`` URL prefix.
 
 Server adminstrators MAY implement blanket HTTP authentication keyed
 off the URL prefix. For example, a server may require authentication
 for all ``rw/*`` URLs and let unauthenticated requests to ``ro/*``
 URL proceed. A server MAY issue an HTTP 401, 403, or 407 response
 in accordance with RFC 7235. Clients SHOULD recognize the HTTP Basic
 (RFC 7617) and Digest (RFC 7616) authentication schemes. Clients SHOULD
 make an attempt to recognize unknown schemes using the
 ``WWW-Authenticate`` response header on a 401 response, as defined by
 RFC 7235.
 
 Read-only commands are accessible under ``rw/*`` URLs so clients can
 signal the intent of the operation very early in the connection
 lifecycle. For example, a ``push`` operation - which consists of
 various read-only commands mixed with at least one read-write command -
 can perform all commands against ``rw/*`` URLs so that any server-side
 authentication requirements are discovered upon attempting the first
 command - not potentially several commands into the exchange. This
 allows clients to fail faster or prompt for credentials as soon as the
 exchange takes place. This provides a better end-user experience.
 
 Requests to unknown commands or URLS result in an HTTP 404.
 TODO formally define response type, how error is communicated, etc.
 
 HTTP request and response bodies use the *Unified Frame-Based Protocol*
 (defined below) for media exchange. The entirety of the HTTP message
 body is 0 or more frames as defined by this protocol.
 
 Clients and servers MUST advertise the ``TBD`` media type via the
 ``Content-Type`` request and response headers. In addition, clients MUST
 advertise this media type value in their ``Accept`` request header in all
 requests.
 TODO finalize the media type. For now, it is defined in wireprotoserver.py.
 
 Servers receiving requests without an ``Accept`` header SHOULD respond with
 an HTTP 406.
 
 Servers receiving requests with an invalid ``Content-Type`` header SHOULD
 respond with an HTTP 415.
 
 The command to run is specified in the POST payload as defined by the
 *Unified Frame-Based Protocol*. This is redundant with data already
 encoded in the URL. This is by design, so server operators can have
 better understanding about server activity from looking merely at
 HTTP access logs.
 
 In most circumstances, the command specified in the URL MUST match
 the command specified in the frame-based payload or the server will
 respond with an error. The exception to this is the special
 ``multirequest`` URL. (See below.) In addition, HTTP requests
 are limited to one command invocation. The exception is the special
 ``multirequest`` URL.
 
 The ``multirequest`` command endpoints (``ro/multirequest`` and
 ``rw/multirequest``) are special in that they allow the execution of
 *any* command and allow the execution of multiple commands. If the
 HTTP request issues multiple commands across multiple frames, all
 issued commands will be processed by the server. Per the defined
 behavior of the *Unified Frame-Based Protocol*, commands may be
 issued interleaved and responses may come back in a different order
 than they were issued. Clients MUST be able to deal with this.
 
 SSH Protocol
 ============
 
 Handshake
 ---------
 
 For all clients, the handshake consists of the client sending 1 or more
 commands to the server using version 1 of the transport. Servers respond
 to commands they know how to respond to and send an empty response (``0\n``)
 for unknown commands (per standard behavior of version 1 of the transport).
 Clients then typically look for a response to the newest sent command to
 determine which transport version to use and what the available features for
 the connection and server are.
 
 Preceding any response from client-issued commands, the server may print
 non-protocol output. It is common for SSH servers to print banners, message
 of the day announcements, etc when clients connect. It is assumed that any
 such *banner* output will precede any Mercurial server output. So clients
 must be prepared to handle server output on initial connect that isn't
 in response to any client-issued command and doesn't conform to Mercurial's
 wire protocol. This *banner* output should only be on stdout. However,
 some servers may send output on stderr.
 
 Pre 0.9.1 clients issue a ``between`` command with the ``pairs`` argument
 having the value
 ``0000000000000000000000000000000000000000-0000000000000000000000000000000000000000``.
 
 The ``between`` command has been supported since the original Mercurial
 SSH server. Requesting the empty range will return a ``\n`` string response,
 which will be encoded as ``1\n\n`` (value length of ``1`` followed by a newline
 followed by the value, which happens to be a newline).
 
 For pre 0.9.1 clients and all servers, the exchange looks like::
 
    c: between\n
    c: pairs 81\n
    c: 0000000000000000000000000000000000000000-0000000000000000000000000000000000000000
    s: 1\n
    s: \n
 
 0.9.1+ clients send a ``hello`` command (with no arguments) before the
 ``between`` command. The response to this command allows clients to
 discover server capabilities and settings.
 
 An example exchange between 0.9.1+ clients and a ``hello`` aware server looks
 like::
 
    c: hello\n
    c: between\n
    c: pairs 81\n
    c: 0000000000000000000000000000000000000000-0000000000000000000000000000000000000000
    s: 324\n
    s: capabilities: lookup changegroupsubset branchmap pushkey known getbundle ...\n
    s: 1\n
    s: \n
 
 And a similar scenario but with servers sending a banner on connect::
 
    c: hello\n
    c: between\n
    c: pairs 81\n
    c: 0000000000000000000000000000000000000000-0000000000000000000000000000000000000000
    s: welcome to the server\n
    s: if you find any issues, email [e-mail]\n
    s: 324\n
    s: capabilities: lookup changegroupsubset branchmap pushkey known getbundle ...\n
    s: 1\n
    s: \n
 
 Note that output from the ``hello`` command is terminated by a ``\n``. This is
 part of the response payload and not part of the wire protocol adding a newline
 after responses. In other words, the length of the response contains the
 trailing ``\n``.
 
 Clients supporting version 2 of the SSH transport send a line beginning
 with ``upgrade`` before the ``hello`` and ``between`` commands. The line
 (which isn't a well-formed command line because it doesn't consist of a
 single command name) serves to both communicate the client's intent to
 switch to transport version 2 (transports are version 1 by default) as
 well as to advertise the client's transport-level capabilities so the
 server may satisfy that request immediately.
 
 The upgrade line has the form:
 
     upgrade <token> <transport capabilities>
 
 That is the literal string ``upgrade`` followed by a space, followed by
 a randomly generated string, followed by a space, followed by a string
 denoting the client's transport capabilities.
 
 The token can be anything. However, a random UUID is recommended. (Use
 of version 4 UUIDs is recommended because version 1 UUIDs can leak the
 client's MAC address.)
 
 The transport capabilities string is a URL/percent encoded string
 containing key-value pairs defining the client's transport-level
 capabilities. The following capabilities are defined:
 
 proto
    A comma-delimited list of transport protocol versions the client
    supports. e.g. ``ssh-v2``.
 
 If the server does not recognize the ``upgrade`` line, it should issue
 an empty response and continue processing the ``hello`` and ``between``
 commands. Here is an example handshake between a version 2 aware client
 and a non version 2 aware server:
 
    c: upgrade 2e82ab3f-9ce3-4b4e-8f8c-6fd1c0e9e23a proto=ssh-v2
    c: hello\n
    c: between\n
    c: pairs 81\n
    c: 0000000000000000000000000000000000000000-0000000000000000000000000000000000000000
    s: 0\n
    s: 324\n
    s: capabilities: lookup changegroupsubset branchmap pushkey known getbundle ...\n
    s: 1\n
    s: \n
 
 (The initial ``0\n`` line from the server indicates an empty response to
 the unknown ``upgrade ..`` command/line.)
 
 If the server recognizes the ``upgrade`` line and is willing to satisfy that
 upgrade request, it replies to with a payload of the following form:
 
    upgraded <token> <transport name>\n
 
 This line is the literal string ``upgraded``, a space, the token that was
 specified by the client in its ``upgrade ...`` request line, a space, and the
 name of the transport protocol that was chosen by the server. The transport
 name MUST match one of the names the client specified in the ``proto`` field
 of its ``upgrade ...`` request line.
 
 If a server issues an ``upgraded`` response, it MUST also read and ignore
 the lines associated with the ``hello`` and ``between`` command requests
 that were issued by the server. It is assumed that the negotiated transport
 will respond with equivalent requested information following the transport
 handshake.
 
 All data following the ``\n`` terminating the ``upgraded`` line is the
 domain of the negotiated transport. It is common for the data immediately
 following to contain additional metadata about the state of the transport and
 the server. However, this isn't strictly speaking part of the transport
 handshake and isn't covered by this section.
 
 Here is an example handshake between a version 2 aware client and a version
 2 aware server:
 
    c:  upgrade 2e82ab3f-9ce3-4b4e-8f8c-6fd1c0e9e23a proto=ssh-v2
    c:  hello\n
    c:  between\n
    c:  pairs 81\n
    c:  0000000000000000000000000000000000000000-0000000000000000000000000000000000000000
    s: upgraded 2e82ab3f-9ce3-4b4e-8f8c-6fd1c0e9e23a ssh-v2\n
    s: <additional transport specific data>
 
 The client-issued token that is echoed in the response provides a more
 resilient mechanism for differentiating *banner* output from Mercurial
 output. In version 1, properly formatted banner output could get confused
 for Mercurial server output. By submitting a randomly generated token
 that is then present in the response, the client can look for that token
 in response lines and have reasonable certainty that the line did not
 originate from a *banner* message.
 
 SSH Version 1 Transport
 -----------------------
 
 The SSH transport (version 1) is a custom text-based protocol suitable for
 use over any bi-directional stream transport. It is most commonly used with
 SSH.
 
 A SSH transport server can be started with ``hg serve --stdio``. The stdin,
 stderr, and stdout file descriptors of the started process are used to exchange
 data. When Mercurial connects to a remote server over SSH, it actually starts
 a ``hg serve --stdio`` process on the remote server.
 
 Commands are issued by sending the command name followed by a trailing newline
 ``\n`` to the server. e.g. ``capabilities\n``.
 
 Command arguments are sent in the following format::
 
     <argument> <length>\n<value>
 
 That is, the argument string name followed by a space followed by the
 integer length of the value (expressed as a string) followed by a newline
 (``\n``) followed by the raw argument value.
 
 Dictionary arguments are encoded differently::
 
     <argument> <# elements>\n
     <key1> <length1>\n<value1>
     <key2> <length2>\n<value2>
     ...
 
 Non-argument data is sent immediately after the final argument value. It is
 encoded in chunks::
 
     <length>\n<data>
 
 Each command declares a list of supported arguments and their types. If a
 client sends an unknown argument to the server, the server should abort
 immediately. The special argument ``*`` in a command's definition indicates
 that all argument names are allowed.
 
 The definition of supported arguments and types is initially made when a
 new command is implemented. The client and server must initially independently
 agree on the arguments and their types. This initial set of arguments can be
 supplemented through the presence of *capabilities* advertised by the server.
 
 Each command has a defined expected response type.
 
 A ``string`` response type is a length framed value. The response consists of
 the string encoded integer length of a value followed by a newline (``\n``)
 followed by the value. Empty values are allowed (and are represented as
 ``0\n``).
 
 A ``stream`` response type consists of raw bytes of data. There is no framing.
 
 A generic error response type is also supported. It consists of a an error
 message written to ``stderr`` followed by ``\n-\n``. In addition, ``\n`` is
 written to ``stdout``.
 
 If the server receives an unknown command, it will send an empty ``string``
 response.
 
 The server terminates if it receives an empty command (a ``\n`` character).
 
 SSH Version 2 Transport
 -----------------------
 
 **Experimental and under development**
 
 Version 2 of the SSH transport behaves identically to version 1 of the SSH
 transport with the exception of handshake semantics. See above for how
 version 2 of the SSH transport is negotiated.
 
 Immediately following the ``upgraded`` line signaling a switch to version
 2 of the SSH protocol, the server automatically sends additional details
 about the capabilities of the remote server. This has the form:
 
    <integer length of value>\n
    capabilities: ...\n
 
 e.g.
 
    s: upgraded 2e82ab3f-9ce3-4b4e-8f8c-6fd1c0e9e23a ssh-v2\n
    s: 240\n
    s: capabilities: known getbundle batch ...\n
 
 Following capabilities advertisement, the peers communicate using version
 1 of the SSH transport.
 
 Unified Frame-Based Protocol
 ============================
 
 **Experimental and under development**
 
 The *Unified Frame-Based Protocol* is a communications protocol between
 Mercurial peers. The protocol aims to be mostly transport agnostic
 (works similarly on HTTP, SSH, etc).
 
 To operate the protocol, a bi-directional, half-duplex pipe supporting
 ordered sends and receives is required. That is, each peer has one pipe
 for sending data and another for receiving.
 
 All data is read and written in atomic units called *frames*. These
 are conceptually similar to TCP packets. Higher-level functionality
 is built on the exchange and processing of frames.
 
 All frames are associated with a *stream*. A *stream* provides a
 unidirectional grouping of frames. Streams facilitate two goals:
 content encoding and parallelism. There is a dedicated section on
 streams below.
 
 The protocol is request-response based: the client issues requests to
 the server, which issues replies to those requests. Server-initiated
 messaging is not currently supported, but this specification carves
 out room to implement it.
 
 All frames are associated with a numbered request. Frames can thus
 be logically grouped by their request ID.
 
 Frames begin with an 8 octet header followed by a variable length
 payload::
 
     +------------------------------------------------+
     |                 Length (24)                    |
     +--------------------------------+---------------+
     |         Request ID (16)        | Stream ID (8) |
     +------------------+-------------+---------------+
     | Stream Flags (8) |
     +-----------+------+
     | Type (4)  |
     +-----------+
     | Flags (4) |
     +===========+===================================================|
     |                     Frame Payload (0...)                    ...
     +---------------------------------------------------------------+
 
 The length of the frame payload is expressed as an unsigned 24 bit
 little endian integer. Values larger than 65535 MUST NOT be used unless
 given permission by the server as part of the negotiated capabilities
 during the handshake. The frame header is not part of the advertised
 frame length. The payload length is the over-the-wire length. If there
 is content encoding applied to the payload as part of the frame's stream,
 the length is the output of that content encoding, not the input.
 
 The 16-bit ``Request ID`` field denotes the integer request identifier,
 stored as an unsigned little endian integer. Odd numbered requests are
 client-initiated. Even numbered requests are server-initiated. This
 refers to where the *request* was initiated - not where the *frame* was
 initiated, so servers will send frames with odd ``Request ID`` in
 response to client-initiated requests. Implementations are advised to
 start ordering request identifiers at ``1`` and ``0``, increment by
 ``2``, and wrap around if all available numbers have been exhausted.
 
 The 8-bit ``Stream ID`` field denotes the stream that the frame is
 associated with. Frames belonging to a stream may have content
 encoding applied and the receiver may need to decode the raw frame
 payload to obtain the original data. Odd numbered IDs are
 client-initiated. Even numbered IDs are server-initiated.
 
 The 8-bit ``Stream Flags`` field defines stream processing semantics.
 See the section on streams below.
 
 The 4-bit ``Type`` field denotes the type of frame being sent.
 
 The 4-bit ``Flags`` field defines special, per-type attributes for
 the frame.
 
 The sections below define the frame types and their behavior.
 
 Command Request (``0x01``)
 --------------------------
 
 This frame contains a request to run a command.
 
 The payload consists of a CBOR map defining the command request. The
 bytestring keys of that map are:
 
 name
    Name of the command that should be executed (bytestring).
 args
    Map of bytestring keys to various value types containing the named
    arguments to this command.
 
    Each command defines its own set of argument names and their expected
    types.
 
 This frame type MUST ONLY be sent from clients to servers: it is illegal
 for a server to send this frame to a client.
 
 The following flag values are defined for this type:
 
 0x01
    New command request. When set, this frame represents the beginning
    of a new request to run a command. The ``Request ID`` attached to this
    frame MUST NOT be active.
 0x02
    Command request continuation. When set, this frame is a continuation
    from a previous command request frame for its ``Request ID``. This
    flag is set when the CBOR data for a command request does not fit
    in a single frame.
 0x04
    Additional frames expected. When set, the command request didn't fit
    into a single frame and additional CBOR data follows in a subsequent
    frame.
 0x08
    Command data frames expected. When set, command data frames are
    expected to follow the final command request frame for this request.
 
 ``0x01`` MUST be set on the initial command request frame for a
 ``Request ID``.
 
 ``0x01`` or ``0x02`` MUST be set to indicate this frame's role in
 a series of command request frames.
 
 If command data frames are to be sent, ``0x10`` MUST be set on ALL
 command request frames.
 
 Command Data (``0x03``)
 -----------------------
 
 This frame contains raw data for a command.
 
 Most commands can be executed by specifying arguments. However,
 arguments have an upper bound to their length. For commands that
 accept data that is beyond this length or whose length isn't known
 when the command is initially sent, they will need to stream
 arbitrary data to the server. This frame type facilitates the sending
 of this data.
 
 The payload of this frame type consists of a stream of raw data to be
 consumed by the command handler on the server. The format of the data
 is command specific.
 
 The following flag values are defined for this type:
 
 0x01
    Command data continuation. When set, the data for this command
    continues into a subsequent frame.
 
 0x02
    End of data. When set, command data has been fully sent to the
    server. The command has been fully issued and no new data for this
    command will be sent. The next frame will belong to a new command.
 
-Bytes Response Data (``0x04``)
-------------------------------
+Response Data (``0x04``)
+------------------------
 
-This frame contains raw bytes response data to an issued command.
+This frame contains raw response data to an issued command.
 
 The following flag values are defined for this type:
 
 0x01
-   Data continuation. When set, an additional frame containing raw
-   response data will follow.
+   Data continuation. When set, an additional frame containing response data
+   will follow.
 0x02
-   End of data. When sent, the response data has been fully sent and
+   End of data. When set, the response data has been fully sent and
    no additional frames for this response will be sent.
+0x04
+   CBOR data. When set, the frame payload consists of CBOR data.
 
 The ``0x01`` flag is mutually exclusive with the ``0x02`` flag.
 
 Error Response (``0x05``)
 -------------------------
 
 An error occurred when processing a request. This could indicate
 a protocol-level failure or an application level failure depending
 on the flags for this message type.
 
 The payload for this type is an error message that should be
 displayed to the user.
 
 The following flag values are defined for this type:
 
 0x01
    The error occurred at the transport/protocol level. If set, the
    connection should be closed.
 0x02
    The error occurred at the application level. e.g. invalid command.
 
 Human Output Side-Channel (``0x06``)
 ------------------------------------
 
 This frame contains a message that is intended to be displayed to
 people. Whereas most frames communicate machine readable data, this
 frame communicates textual data that is intended to be shown to
 humans.
 
 The frame consists of a series of *formatting requests*. Each formatting
 request consists of a formatting string, arguments for that formatting
 string, and labels to apply to that formatting string.
 
 A formatting string is a printf()-like string that allows variable
 substitution within the string. Labels allow the rendered text to be
 *decorated*. Assuming use of the canonical Mercurial code base, a
 formatting string can be the input to the ``i18n._`` function. This
 allows messages emitted from the server to be localized. So even if
 the server has different i18n settings, people could see messages in
 their *native* settings. Similarly, the use of labels allows
 decorations like coloring and underlining to be applied using the
 client's configured rendering settings.
 
 Formatting strings are similar to ``printf()`` strings or how
 Python's ``%`` operator works. The only supported formatting sequences
 are ``%s`` and ``%%``. ``%s`` will be replaced by whatever the string
 at that position resolves to. ``%%`` will be replaced by ``%``. All
 other 2-byte sequences beginning with ``%`` represent a literal
 ``%`` followed by that character. However, future versions of the
 wire protocol reserve the right to allow clients to opt in to receiving
 formatting strings with additional formatters, hence why ``%%`` is
 required to represent the literal ``%``.
 
 The raw frame consists of a series of data structures representing
 textual atoms to print. Each atom begins with a struct defining the
 size of the data that follows:
 
 * A 16-bit little endian unsigned integer denoting the length of the
   formatting string.
 * An 8-bit unsigned integer denoting the number of label strings
   that follow.
 * An 8-bit unsigned integer denoting the number of formatting string
   arguments strings that follow.
 * An array of 8-bit unsigned integers denoting the lengths of
   *labels* data.
 * An array of 16-bit unsigned integers denoting the lengths of
   formatting strings.
 * The formatting string, encoded as UTF-8.
 * 0 or more ASCII strings defining labels to apply to this atom.
 * 0 or more UTF-8 strings that will be used as arguments to the
   formatting string.
 
 TODO use ASCII for formatting string.
 
 All data to be printed MUST be encoded into a single frame: this frame
 does not support spanning data across multiple frames.
 
 All textual data encoded in these frames is assumed to be line delimited.
 The last atom in the frame SHOULD end with a newline (``\n``). If it
 doesn't, clients MAY add a newline to facilitate immediate printing.
 
 Progress Update (``0x07``)
 --------------------------
 
 This frame holds the progress of an operation on the peer. Consumption
 of these frames allows clients to display progress bars, estimated
 completion times, etc.
 
 Each frame defines the progress of a single operation on the peer. The
 payload consists of a CBOR map with the following bytestring keys:
 
 topic
    Topic name (string)
 pos
    Current numeric position within the topic (integer)
 total
    Total/end numeric position of this topic (unsigned integer)
 label (optional)
    Unit label (string)
 item (optional)
    Item name (string)
 
 Progress state is created when a frame is received referencing a
 *topic* that isn't currently tracked. Progress tracking for that
 *topic* is finished when a frame is received reporting the current
 position of that topic as ``-1``.
 
 Multiple *topics* may be active at any given time.
 
 Rendering of progress information is not mandated or governed by this
 specification: implementations MAY render progress information however
 they see fit, including not at all.
 
 The string data describing the topic SHOULD be static strings to
 facilitate receivers localizing that string data. The emitter
 MUST normalize all string data to valid UTF-8 and receivers SHOULD
 validate that received data conforms to UTF-8. The topic name
 SHOULD be ASCII.
 
 Stream Encoding Settings (``0x08``)
 -----------------------------------
 
 This frame type holds information defining the content encoding
 settings for a *stream*.
 
 This frame type is likely consumed by the protocol layer and is not
 passed on to applications.
 
 This frame type MUST ONLY occur on frames having the *Beginning of Stream*
 ``Stream Flag`` set.
 
 The payload of this frame defines what content encoding has (possibly)
 been applied to the payloads of subsequent frames in this stream.
 
 The payload begins with an 8-bit integer defining the length of the
 encoding *profile*, followed by the string name of that profile, which
 must be an ASCII string. All bytes that follow can be used by that
 profile for supplemental settings definitions. See the section below
 on defined encoding profiles.
 
 Stream States and Flags
 -----------------------
 
 Streams can be in two states: *open* and *closed*. An *open* stream
 is active and frames attached to that stream could arrive at any time.
 A *closed* stream is not active. If a frame attached to a *closed*
 stream arrives, that frame MUST have an appropriate stream flag
 set indicating beginning of stream. All streams are in the *closed*
 state by default.
 
 The ``Stream Flags`` field denotes a set of bit flags for defining
 the relationship of this frame within a stream. The following flags
 are defined:
 
 0x01
    Beginning of stream. The first frame in the stream MUST set this
    flag. When received, the ``Stream ID`` this frame is attached to
    becomes ``open``.
 
 0x02
    End of stream. The last frame in a stream MUST set this flag. When
    received, the ``Stream ID`` this frame is attached to becomes
    ``closed``. Any content encoding context associated with this stream
    can be destroyed after processing the payload of this frame.
 
 0x04
    Apply content encoding. When set, any content encoding settings
    defined by the stream should be applied when attempting to read
    the frame. When not set, the frame payload isn't encoded.
 
 Streams
 -------
 
 Streams - along with ``Request IDs`` - facilitate grouping of frames.
 But the purpose of each is quite different and the groupings they
 constitute are independent.
 
 A ``Request ID`` is essentially a tag. It tells you which logical
 request a frame is associated with.
 
 A *stream* is a sequence of frames grouped for the express purpose
 of applying a stateful encoding or for denoting sub-groups of frames.
 
 Unlike ``Request ID``s which span the request and response, a stream
 is unidirectional and stream IDs are independent from client to
 server.
 
 There is no strict hierarchical relationship between ``Request IDs``
 and *streams*. A stream can contain frames having multiple
 ``Request IDs``. Frames belonging to the same ``Request ID`` can
 span multiple streams.
 
 One goal of streams is to facilitate content encoding. A stream can
 define an encoding to be applied to frame payloads. For example, the
 payload transmitted over the wire may contain output from a
 zstandard compression operation and the receiving end may decompress
 that payload to obtain the original data.
 
 The other goal of streams is to facilitate concurrent execution. For
 example, a server could spawn 4 threads to service a request that can
 be easily parallelized. Each of those 4 threads could write into its
 own stream. Those streams could then in turn be delivered to 4 threads
 on the receiving end, with each thread consuming its stream in near
 isolation. The *main* thread on both ends merely does I/O and
 encodes/decodes frame headers: the bulk of the work is done by worker
 threads.
 
 In addition, since content encoding is defined per stream, each
 *worker thread* could perform potentially CPU bound work concurrently
 with other threads. This approach of applying encoding at the
 sub-protocol / stream level eliminates a potential resource constraint
 on the protocol stream as a whole (it is common for the throughput of
 a compression engine to be smaller than the throughput of a network).
 
 Having multiple streams - each with their own encoding settings - also
 facilitates the use of advanced data compression techniques. For
 example, a transmitter could see that it is generating data faster
 and slower than the receiving end is consuming it and adjust its
 compression settings to trade CPU for compression ratio accordingly.
 
 While streams can define a content encoding, not all frames within
 that stream must use that content encoding. This can be useful when
 data is being served from caches and being derived dynamically. A
 cache could pre-compressed data so the server doesn't have to
 recompress it. The ability to pick and choose which frames are
 compressed allows servers to easily send data to the wire without
 involving potentially expensive encoding overhead.
 
 Content Encoding Profiles
 -------------------------
 
 Streams can have named content encoding *profiles* associated with
 them. A profile defines a shared understanding of content encoding
 settings and behavior.
 
 The following profiles are defined:
 
 TBD
 
 Issuing Commands
 ----------------
 
 A client can request that a remote run a command by sending it
 frames defining that command. This logical stream is composed of
 1 or more ``Command Request`` frames and and 0 or more ``Command Data``
 frames.
 
 All frames composing a single command request MUST be associated with
 the same ``Request ID``.
 
 Clients MAY send additional command requests without waiting on the
 response to a previous command request. If they do so, they MUST ensure
 that the ``Request ID`` field of outbound frames does not conflict
 with that of an active ``Request ID`` whose response has not yet been
 fully received.
 
 Servers MAY respond to commands in a different order than they were
 sent over the wire. Clients MUST be prepared to deal with this. Servers
 also MAY start executing commands in a different order than they were
 received, or MAY execute multiple commands concurrently.
 
 If there is a dependency between commands or a race condition between
 commands executing (e.g. a read-only command that depends on the results
 of a command that mutates the repository), then clients MUST NOT send
 frames issuing a command until a response to all dependent commands has
 been received.
 TODO think about whether we should express dependencies between commands
 to avoid roundtrip latency.
 
 A command is defined by a command name, 0 or more command arguments,
 and optional command data.
 
 Arguments are the recommended mechanism for transferring fixed sets of
 parameters to a command. Data is appropriate for transferring variable
 data. Thinking in terms of HTTP, arguments would be headers and data
 would be the message body.
 
 It is recommended for servers to delay the dispatch of a command
 until all argument have been received. Servers MAY impose limits on the
 maximum argument size.
 TODO define failure mechanism.
 
 Servers MAY dispatch to commands immediately once argument data
 is available or delay until command data is received in full.
 
 Capabilities
 ============
 
 Servers advertise supported wire protocol features. This allows clients to
 probe for server features before blindly calling a command or passing a
 specific argument.
 
 The server's features are exposed via a *capabilities* string. This is a
 space-delimited string of tokens/features. Some features are single words
 like ``lookup`` or ``batch``. Others are complicated key-value pairs
 advertising sub-features. e.g. ``httpheader=2048``. When complex, non-word
 values are used, each feature name can define its own encoding of sub-values.
 Comma-delimited and ``x-www-form-urlencoded`` values are common.
 
 The following document capabilities defined by the canonical Mercurial server
 implementation.
 
 batch
 -----
 
 Whether the server supports the ``batch`` command.
 
 This capability/command was introduced in Mercurial 1.9 (released July 2011).
 
 branchmap
 ---------
 
 Whether the server supports the ``branchmap`` command.
 
 This capability/command was introduced in Mercurial 1.3 (released July 2009).
 
 bundle2-exp
 -----------
 
 Precursor to ``bundle2`` capability that was used before bundle2 was a
 stable feature.
 
 This capability was introduced in Mercurial 3.0 behind an experimental
 flag. This capability should not be observed in the wild.
 
 bundle2
 -------
 
 Indicates whether the server supports the ``bundle2`` data exchange format.
 
 The value of the capability is a URL quoted, newline (``\n``) delimited
 list of keys or key-value pairs.
 
 A key is simply a URL encoded string.
 
 A key-value pair is a URL encoded key separated from a URL encoded value by
 an ``=``. If the value is a list, elements are delimited by a ``,`` after
 URL encoding.
 
 For example, say we have the values::
 
   {'HG20': [], 'changegroup': ['01', '02'], 'digests': ['sha1', 'sha512']}
 
 We would first construct a string::
 
   HG20\nchangegroup=01,02\ndigests=sha1,sha512
 
 We would then URL quote this string::
 
   HG20%0Achangegroup%3D01%2C02%0Adigests%3Dsha1%2Csha512
 
 This capability was introduced in Mercurial 3.4 (released May 2015).
 
 changegroupsubset
 -----------------
 
 Whether the server supports the ``changegroupsubset`` command.
 
 This capability was introduced in Mercurial 0.9.2 (released December
 2006).
 
 This capability was introduced at the same time as the ``lookup``
 capability/command.
 
 compression
 -----------
 
 Declares support for negotiating compression formats.
 
 Presence of this capability indicates the server supports dynamic selection
 of compression formats based on the client request.
 
 Servers advertising this capability are required to support the
 ``application/mercurial-0.2`` media type in response to commands returning
 streams. Servers may support this media type on any command.
 
 The value of the capability is a comma-delimited list of strings declaring
 supported compression formats. The order of the compression formats is in
 server-preferred order, most preferred first.
 
 The identifiers used by the official Mercurial distribution are:
 
 bzip2
    bzip2
 none
    uncompressed / raw data
 zlib
    zlib (no gzip header)
 zstd
    zstd
 
 This capability was introduced in Mercurial 4.1 (released February 2017).
 
 getbundle
 ---------
 
 Whether the server supports the ``getbundle`` command.
 
 This capability was introduced in Mercurial 1.9 (released July 2011).
 
 httpheader
 ----------
 
 Whether the server supports receiving command arguments via HTTP request
 headers.
 
 The value of the capability is an integer describing the max header
 length that clients should send. Clients should ignore any content after a
 comma in the value, as this is reserved for future use.
 
 This capability was introduced in Mercurial 1.9 (released July 2011).
 
 httpmediatype
 -------------
 
 Indicates which HTTP media types (``Content-Type`` header) the server is
 capable of receiving and sending.
 
 The value of the capability is a comma-delimited list of strings identifying
 support for media type and transmission direction. The following strings may
 be present:
 
 0.1rx
    Indicates server support for receiving ``application/mercurial-0.1`` media
    types.
 
 0.1tx
    Indicates server support for sending ``application/mercurial-0.1`` media
    types.
 
 0.2rx
    Indicates server support for receiving ``application/mercurial-0.2`` media
    types.
 
 0.2tx
    Indicates server support for sending ``application/mercurial-0.2`` media
    types.
 
 minrx=X
    Minimum media type version the server is capable of receiving. Value is a
    string like ``0.2``.
 
    This capability can be used by servers to limit connections from legacy
    clients not using the latest supported media type. However, only clients
    with knowledge of this capability will know to consult this value. This
    capability is present so the client may issue a more user-friendly error
    when the server has locked out a legacy client.
 
 mintx=X
    Minimum media type version the server is capable of sending. Value is a
    string like ``0.1``.
 
 Servers advertising support for the ``application/mercurial-0.2`` media type
 should also advertise the ``compression`` capability.
 
 This capability was introduced in Mercurial 4.1 (released February 2017).
 
 httppostargs
 ------------
 
 **Experimental**
 
 Indicates that the server supports and prefers clients send command arguments
 via a HTTP POST request as part of the request body.
 
 This capability was introduced in Mercurial 3.8 (released May 2016).
 
 known
 -----
 
 Whether the server supports the ``known`` command.
 
 This capability/command was introduced in Mercurial 1.9 (released July 2011).
 
 lookup
 ------
 
 Whether the server supports the ``lookup`` command.
 
 This capability was introduced in Mercurial 0.9.2 (released December
 2006).
 
 This capability was introduced at the same time as the ``changegroupsubset``
 capability/command.
 
 pushkey
 -------
 
 Whether the server supports the ``pushkey`` and ``listkeys`` commands.
 
 This capability was introduced in Mercurial 1.6 (released July 2010).
 
 standardbundle
 --------------
 
 **Unsupported**
 
 This capability was introduced during the Mercurial 0.9.2 development cycle in
 2006. It was never present in a release, as it was replaced by the ``unbundle``
 capability. This capability should not be encountered in the wild.
 
 stream-preferred
 ----------------
 
 If present the server prefers that clients clone using the streaming clone
 protocol (``hg clone --stream``) rather than the standard
 changegroup/bundle based protocol.
 
 This capability was introduced in Mercurial 2.2 (released May 2012).
 
 streamreqs
 ----------
 
 Indicates whether the server supports *streaming clones* and the *requirements*
 that clients must support to receive it.
 
 If present, the server supports the ``stream_out`` command, which transmits
 raw revlogs from the repository instead of changegroups. This provides a faster
 cloning mechanism at the expense of more bandwidth used.
 
 The value of this capability is a comma-delimited list of repo format
 *requirements*. These are requirements that impact the reading of data in
 the ``.hg/store`` directory. An example value is
 ``streamreqs=generaldelta,revlogv1`` indicating the server repo requires
 the ``revlogv1`` and ``generaldelta`` requirements.
 
 If the only format requirement is ``revlogv1``, the server may expose the
 ``stream`` capability instead of the ``streamreqs`` capability.
 
 This capability was introduced in Mercurial 1.7 (released November 2010).
 
 stream
 ------
 
 Whether the server supports *streaming clones* from ``revlogv1`` repos.
 
 If present, the server supports the ``stream_out`` command, which transmits
 raw revlogs from the repository instead of changegroups. This provides a faster
 cloning mechanism at the expense of more bandwidth used.
 
 This capability was introduced in Mercurial 0.9.1 (released July 2006).
 
 When initially introduced, the value of the capability was the numeric
 revlog revision. e.g. ``stream=1``. This indicates the changegroup is using
 ``revlogv1``. This simple integer value wasn't powerful enough, so the
 ``streamreqs`` capability was invented to handle cases where the repo
 requirements have more than just ``revlogv1``. Newer servers omit the
 ``=1`` since it was the only value supported and the value of ``1`` can
 be implied by clients.
 
 unbundlehash
 ------------
 
 Whether the ``unbundle`` commands supports receiving a hash of all the
 heads instead of a list.
 
 For more, see the documentation for the ``unbundle`` command.
 
 This capability was introduced in Mercurial 1.9 (released July 2011).
 
 unbundle
 --------
 
 Whether the server supports pushing via the ``unbundle`` command.
 
 This capability/command has been present since Mercurial 0.9.1 (released
 July 2006).
 
 Mercurial 0.9.2 (released December 2006) added values to the capability
 indicating which bundle types the server supports receiving. This value is a
 comma-delimited list. e.g. ``HG10GZ,HG10BZ,HG10UN``. The order of values
 reflects the priority/preference of that type, where the first value is the
 most preferred type.
 
 Content Negotiation
 ===================
 
 The wire protocol has some mechanisms to help peers determine what content
 types and encoding the other side will accept. Historically, these mechanisms
 have been built into commands themselves because most commands only send a
 well-defined response type and only certain commands needed to support
 functionality like compression.
 
 Currently, only the HTTP version 1 transport supports content negotiation
 at the protocol layer.
 
 HTTP requests advertise supported response formats via the ``X-HgProto-<N>``
 request header, where ``<N>`` is an integer starting at 1 allowing the logical
 value to span multiple headers. This value consists of a list of
 space-delimited parameters. Each parameter denotes a feature or capability.
 
 The following parameters are defined:
 
 0.1
    Indicates the client supports receiving ``application/mercurial-0.1``
    responses.
 
 0.2
    Indicates the client supports receiving ``application/mercurial-0.2``
    responses.
 
 comp
    Indicates compression formats the client can decode. Value is a list of
    comma delimited strings identifying compression formats ordered from
    most preferential to least preferential. e.g. ``comp=zstd,zlib,none``.
 
    This parameter does not have an effect if only the ``0.1`` parameter
    is defined, as support for ``application/mercurial-0.2`` or greater is
    required to use arbitrary compression formats.
 
    If this parameter is not advertised, the server interprets this as
    equivalent to ``zlib,none``.
 
 Clients may choose to only send this header if the ``httpmediatype``
 server capability is present, as currently all server-side features
 consulting this header require the client to opt in to new protocol features
 advertised via the ``httpmediatype`` capability.
 
 A server that doesn't receive an ``X-HgProto-<N>`` header should infer a
 value of ``0.1``. This is compatible with legacy clients.
 
 A server receiving a request indicating support for multiple media type
 versions may respond with any of the supported media types. Not all servers
 may support all media types on all commands.
 
 Commands
 ========
 
 This section contains a list of all wire protocol commands implemented by
 the canonical Mercurial server.
 
 batch
 -----
 
 Issue multiple commands while sending a single command request. The purpose
 of this command is to allow a client to issue multiple commands while avoiding
 multiple round trips to the server therefore enabling commands to complete
 quicker.
 
 The command accepts a ``cmds`` argument that contains a list of commands to
 execute.
 
 The value of ``cmds`` is a ``;`` delimited list of strings. Each string has the
 form ``<command> <arguments>``. That is, the command name followed by a space
 followed by an argument string.
 
 The argument string is a ``,`` delimited list of ``<key>=<value>`` values
 corresponding to command arguments. Both the argument name and value are
 escaped using a special substitution map::
 
    : -> :c
    , -> :o
    ; -> :s
    = -> :e
 
 The response type for this command is ``string``. The value contains a
 ``;`` delimited list of responses for each requested command. Each value
 in this list is escaped using the same substitution map used for arguments.
 
 If an error occurs, the generic error response may be sent.
 
 between
 -------
 
 (Legacy command used for discovery in old clients)
 
 Obtain nodes between pairs of nodes.
 
 The ``pairs`` arguments contains a space-delimited list of ``-`` delimited
 hex node pairs. e.g.::
 
    a072279d3f7fd3a4aa7ffa1a5af8efc573e1c896-6dc58916e7c070f678682bfe404d2e2d68291a18
 
 Return type is a ``string``. Value consists of lines corresponding to each
 requested range. Each line contains a space-delimited list of hex nodes.
 A newline ``\n`` terminates each line, including the last one.
 
 branchmap
 ---------
 
 Obtain heads in named branches.
 
 Accepts no arguments. Return type is a ``string``.
 
 Return value contains lines with URL encoded branch names followed by a space
 followed by a space-delimited list of hex nodes of heads on that branch.
 e.g.::
 
     default a072279d3f7fd3a4aa7ffa1a5af8efc573e1c896 6dc58916e7c070f678682bfe404d2e2d68291a18
     stable baae3bf31522f41dd5e6d7377d0edd8d1cf3fccc
 
 There is no trailing newline.
 
 branches
 --------
 
 (Legacy command used for discovery in old clients. Clients with ``getbundle``
 use the ``known`` and ``heads`` commands instead.)
 
 Obtain ancestor changesets of specific nodes back to a branch point.
 
 Despite the name, this command has nothing to do with Mercurial named branches.
 Instead, it is related to DAG branches.
 
 The command accepts a ``nodes`` argument, which is a string of space-delimited
 hex nodes.
 
 For each node requested, the server will find the first ancestor node that is
 a DAG root or is a merge.
 
 Return type is a ``string``. Return value contains lines with result data for
 each requested node. Each line contains space-delimited nodes followed by a
 newline (``\n``). The 4 nodes reported on each line correspond to the requested
 node, the ancestor node found, and its 2 parent nodes (which may be the null
 node).
 
 capabilities
 ------------
 
 Obtain the capabilities string for the repo.
 
 Unlike the ``hello`` command, the capabilities string is not prefixed.
 There is no trailing newline.
 
 This command does not accept any arguments. Return type is a ``string``.
 
 This command was introduced in Mercurial 0.9.1 (released July 2006).
 
 changegroup
 -----------
 
 (Legacy command: use ``getbundle`` instead)
 
 Obtain a changegroup version 1 with data for changesets that are
 descendants of client-specified changesets.
 
 The ``roots`` arguments contains a list of space-delimited hex nodes.
 
 The server responds with a changegroup version 1 containing all
 changesets between the requested root/base nodes and the repo's head nodes
 at the time of the request.
 
 The return type is a ``stream``.
 
 changegroupsubset
 -----------------
 
 (Legacy command: use ``getbundle`` instead)
 
 Obtain a changegroup version 1 with data for changesetsets between
 client specified base and head nodes.
 
 The ``bases`` argument contains a list of space-delimited hex nodes.
 The ``heads`` argument contains a list of space-delimited hex nodes.
 
 The server responds with a changegroup version 1 containing all
 changesets between the requested base and head nodes at the time of the
 request.
 
 The return type is a ``stream``.
 
 clonebundles
 ------------
 
 Obtains a manifest of bundle URLs available to seed clones.
 
 Each returned line contains a URL followed by metadata. See the
 documentation in the ``clonebundles`` extension for more.
 
 The return type is a ``string``.
 
 getbundle
 ---------
 
 Obtain a bundle containing repository data.
 
 This command accepts the following arguments:
 
 heads
    List of space-delimited hex nodes of heads to retrieve.
 common
    List of space-delimited hex nodes that the client has in common with the
    server.
 obsmarkers
    Boolean indicating whether to include obsolescence markers as part
    of the response. Only works with bundle2.
 bundlecaps
    Comma-delimited set of strings defining client bundle capabilities.
 listkeys
    Comma-delimited list of strings of ``pushkey`` namespaces. For each
    namespace listed, a bundle2 part will be included with the content of
    that namespace.
 cg
    Boolean indicating whether changegroup data is requested.
 cbattempted
    Boolean indicating whether the client attempted to use the *clone bundles*
    feature before performing this request.
 bookmarks
    Boolean indicating whether bookmark data is requested.
 phases
    Boolean indicating whether phases data is requested.
 
 The return type on success is a ``stream`` where the value is bundle.
 On the HTTP version 1 transport, the response is zlib compressed.
 
 If an error occurs, a generic error response can be sent.
 
 Unless the client sends a false value for the ``cg`` argument, the returned
 bundle contains a changegroup with the nodes between the specified ``common``
 and ``heads`` nodes. Depending on the command arguments, the type and content
 of the returned bundle can vary significantly.
 
 The default behavior is for the server to send a raw changegroup version
 ``01`` response.
 
 If the ``bundlecaps`` provided by the client contain a value beginning
 with ``HG2``, a bundle2 will be returned. The bundle2 data may contain
 additional repository data, such as ``pushkey`` namespace values.
 
 heads
 -----
 
 Returns a list of space-delimited hex nodes of repository heads followed
 by a newline. e.g.
 ``a9eeb3adc7ddb5006c088e9eda61791c777cbf7c 31f91a3da534dc849f0d6bfc00a395a97cf218a1\n``
 
 This command does not accept any arguments. The return type is a ``string``.
 
 hello
 -----
 
 Returns lines describing interesting things about the server in an RFC-822
 like format.
 
 Currently, the only line defines the server capabilities. It has the form::
 
     capabilities: <value>
 
 See above for more about the capabilities string.
 
 SSH clients typically issue this command as soon as a connection is
 established.
 
 This command does not accept any arguments. The return type is a ``string``.
 
 This command was introduced in Mercurial 0.9.1 (released July 2006).
 
 listkeys
 --------
 
 List values in a specified ``pushkey`` namespace.
 
 The ``namespace`` argument defines the pushkey namespace to operate on.
 
 The return type is a ``string``. The value is an encoded dictionary of keys.
 
 Key-value pairs are delimited by newlines (``\n``). Within each line, keys and
 values are separated by a tab (``\t``). Keys and values are both strings.
 
 lookup
 ------
 
 Try to resolve a value to a known repository revision.
 
 The ``key`` argument is converted from bytes to an
 ``encoding.localstr`` instance then passed into
 ``localrepository.__getitem__`` in an attempt to resolve it.
 
 The return type is a ``string``.
 
 Upon successful resolution, returns ``1 <hex node>\n``. On failure,
 returns ``0 <error string>\n``. e.g.::
 
    1 273ce12ad8f155317b2c078ec75a4eba507f1fba\n
 
    0 unknown revision 'foo'\n
 
 known
 -----
 
 Determine whether multiple nodes are known.
 
 The ``nodes`` argument is a list of space-delimited hex nodes to check
 for existence.
 
 The return type is ``string``.
 
 Returns a string consisting of ``0``s and ``1``s indicating whether nodes
 are known. If the Nth node specified in the ``nodes`` argument is known,
 a ``1`` will be returned at byte offset N. If the node isn't known, ``0``
 will be present at byte offset N.
 
 There is no trailing newline.
 
 pushkey
 -------
 
 Set a value using the ``pushkey`` protocol.
 
 Accepts arguments ``namespace``, ``key``, ``old``, and ``new``, which
 correspond to the pushkey namespace to operate on, the key within that
 namespace to change, the old value (which may be empty), and the new value.
 All arguments are string types.
 
 The return type is a ``string``. The value depends on the transport protocol.
 
 The SSH version 1 transport sends a string encoded integer followed by a
 newline (``\n``) which indicates operation result. The server may send
 additional output on the ``stderr`` stream that should be displayed to the
 user.
 
 The HTTP version 1 transport sends a string encoded integer followed by a
 newline followed by additional server output that should be displayed to
 the user. This may include output from hooks, etc.
 
 The integer result varies by namespace. ``0`` means an error has occurred
 and there should be additional output to display to the user.
 
 stream_out
 ----------
 
 Obtain *streaming clone* data.
 
 The return type is either a ``string`` or a ``stream``, depending on
 whether the request was fulfilled properly.
 
 A return value of ``1\n`` indicates the server is not configured to serve
 this data. If this is seen by the client, they may not have verified the
 ``stream`` capability is set before making the request.
 
 A return value of ``2\n`` indicates the server was unable to lock the
 repository to generate data.
 
 All other responses are a ``stream`` of bytes. The first line of this data
 contains 2 space-delimited integers corresponding to the path count and
 payload size, respectively::
 
     <path count> <payload size>\n
 
 The ``<payload size>`` is the total size of path data: it does not include
 the size of the per-path header lines.
 
 Following that header are ``<path count>`` entries. Each entry consists of a
 line with metadata followed by raw revlog data. The line consists of::
 
     <store path>\0<size>\n
 
 The ``<store path>`` is the encoded store path of the data that follows.
 ``<size>`` is the amount of data for this store path/revlog that follows the
 newline.
 
 There is no trailer to indicate end of data. Instead, the client should stop
 reading after ``<path count>`` entries are consumed.
 
 unbundle
 --------
 
 Send a bundle containing data (usually changegroup data) to the server.
 
 Accepts the argument ``heads``, which is a space-delimited list of hex nodes
 corresponding to server repository heads observed by the client. This is used
 to detect race conditions and abort push operations before a server performs
 too much work or a client transfers too much data.
 
 The request payload consists of a bundle to be applied to the repository,
 similarly to as if :hg:`unbundle` were called.
 
 In most scenarios, a special ``push response`` type is returned. This type
 contains an integer describing the change in heads as a result of the
 operation. A value of ``0`` indicates nothing changed. ``1`` means the number
 of heads remained the same. Values ``2`` and larger indicate the number of
 added heads minus 1. e.g. ``3`` means 2 heads were added. Negative values
 indicate the number of fewer heads, also off by 1. e.g. ``-2`` means there
 is 1 fewer head.
 
 The encoding of the ``push response`` type varies by transport.
 
 For the SSH version 1 transport, this type is composed of 2 ``string``
 responses: an empty response (``0\n``) followed by the integer result value.
 e.g. ``1\n2``. So the full response might be ``0\n1\n2``.
 
 For the HTTP version 1 transport, the response is a ``string`` type composed
 of an integer result value followed by a newline (``\n``) followed by string
 content holding server output that should be displayed on the client (output
 hooks, etc).
 
 In some cases, the server may respond with a ``bundle2`` bundle. In this
 case, the response type is ``stream``. For the HTTP version 1 transport, the
 response is zlib compressed.
 
 The server may also respond with a generic error type, which contains a string
 indicating the failure.

mercurial/wireprotoframing.py

@@ -1,883 +1,885
 # wireprotoframing.py - unified framing protocol for wire protocol
 #
 # Copyright 2018 Gregory Szorc <[e-mail]>
 #
 # This software may be used and distributed according to the terms of the
 # GNU General Public License version 2 or any later version.
 
 # This file contains functionality to support the unified frame-based wire
 # protocol. For details about the protocol, see
 # `hg help internals.wireprotocol`.
 
 from __future__ import absolute_import
 
 import struct
 
 from .i18n import _
 from .thirdparty import (
     attr,
     cbor,
 )
 from . import (
     error,
     util,
 )
 from .utils import (
     stringutil,
 )
 
 FRAME_HEADER_SIZE = 8
 DEFAULT_MAX_FRAME_SIZE = 32768
 
 STREAM_FLAG_BEGIN_STREAM = 0x01
 STREAM_FLAG_END_STREAM = 0x02
 STREAM_FLAG_ENCODING_APPLIED = 0x04
 
 STREAM_FLAGS = {
     b'stream-begin': STREAM_FLAG_BEGIN_STREAM,
     b'stream-end': STREAM_FLAG_END_STREAM,
     b'encoded': STREAM_FLAG_ENCODING_APPLIED,
 }
 
 FRAME_TYPE_COMMAND_REQUEST = 0x01
 FRAME_TYPE_COMMAND_DATA = 0x03
 FRAME_TYPE_BYTES_RESPONSE = 0x04
 FRAME_TYPE_ERROR_RESPONSE = 0x05
 FRAME_TYPE_TEXT_OUTPUT = 0x06
 FRAME_TYPE_PROGRESS = 0x07
 FRAME_TYPE_STREAM_SETTINGS = 0x08
 
 FRAME_TYPES = {
     b'command-request': FRAME_TYPE_COMMAND_REQUEST,
     b'command-data': FRAME_TYPE_COMMAND_DATA,
     b'bytes-response': FRAME_TYPE_BYTES_RESPONSE,
     b'error-response': FRAME_TYPE_ERROR_RESPONSE,
     b'text-output': FRAME_TYPE_TEXT_OUTPUT,
     b'progress': FRAME_TYPE_PROGRESS,
     b'stream-settings': FRAME_TYPE_STREAM_SETTINGS,
 }
 
 FLAG_COMMAND_REQUEST_NEW = 0x01
 FLAG_COMMAND_REQUEST_CONTINUATION = 0x02
 FLAG_COMMAND_REQUEST_MORE_FRAMES = 0x04
 FLAG_COMMAND_REQUEST_EXPECT_DATA = 0x08
 
 FLAGS_COMMAND_REQUEST = {
     b'new': FLAG_COMMAND_REQUEST_NEW,
     b'continuation': FLAG_COMMAND_REQUEST_CONTINUATION,
     b'more': FLAG_COMMAND_REQUEST_MORE_FRAMES,
     b'have-data': FLAG_COMMAND_REQUEST_EXPECT_DATA,
 }
 
 FLAG_COMMAND_DATA_CONTINUATION = 0x01
 FLAG_COMMAND_DATA_EOS = 0x02
 
 FLAGS_COMMAND_DATA = {
     b'continuation': FLAG_COMMAND_DATA_CONTINUATION,
     b'eos': FLAG_COMMAND_DATA_EOS,
 }
 
 FLAG_BYTES_RESPONSE_CONTINUATION = 0x01
 FLAG_BYTES_RESPONSE_EOS = 0x02
+FLAG_BYTES_RESPONSE_CBOR = 0x04
 
 FLAGS_BYTES_RESPONSE = {
     b'continuation': FLAG_BYTES_RESPONSE_CONTINUATION,
     b'eos': FLAG_BYTES_RESPONSE_EOS,
+    b'cbor': FLAG_BYTES_RESPONSE_CBOR,
 }
 
 FLAG_ERROR_RESPONSE_PROTOCOL = 0x01
 FLAG_ERROR_RESPONSE_APPLICATION = 0x02
 
 FLAGS_ERROR_RESPONSE = {
     b'protocol': FLAG_ERROR_RESPONSE_PROTOCOL,
     b'application': FLAG_ERROR_RESPONSE_APPLICATION,
 }
 
 # Maps frame types to their available flags.
 FRAME_TYPE_FLAGS = {
     FRAME_TYPE_COMMAND_REQUEST: FLAGS_COMMAND_REQUEST,
     FRAME_TYPE_COMMAND_DATA: FLAGS_COMMAND_DATA,
     FRAME_TYPE_BYTES_RESPONSE: FLAGS_BYTES_RESPONSE,
     FRAME_TYPE_ERROR_RESPONSE: FLAGS_ERROR_RESPONSE,
     FRAME_TYPE_TEXT_OUTPUT: {},
     FRAME_TYPE_PROGRESS: {},
     FRAME_TYPE_STREAM_SETTINGS: {},
 }
 
 ARGUMENT_RECORD_HEADER = struct.Struct(r'<HH')
 
 def humanflags(mapping, value):
     """Convert a numeric flags value to a human value, using a mapping table."""
     flags = []
     for val, name in sorted({v: k for k, v in mapping.iteritems()}.iteritems()):
         if value & val:
             flags.append(name)
 
     return b'|'.join(flags)
 
 @attr.s(slots=True)
 class frameheader(object):
     """Represents the data in a frame header."""
 
     length = attr.ib()
     requestid = attr.ib()
     streamid = attr.ib()
     streamflags = attr.ib()
     typeid = attr.ib()
     flags = attr.ib()
 
 @attr.s(slots=True, repr=False)
 class frame(object):
     """Represents a parsed frame."""
 
     requestid = attr.ib()
     streamid = attr.ib()
     streamflags = attr.ib()
     typeid = attr.ib()
     flags = attr.ib()
     payload = attr.ib()
 
     def __repr__(self):
         typename = '<unknown>'
         for name, value in FRAME_TYPES.iteritems():
             if value == self.typeid:
                 typename = name
                 break
 
         return ('frame(size=%d; request=%d; stream=%d; streamflags=%s; '
                 'type=%s; flags=%s)' % (
             len(self.payload), self.requestid, self.streamid,
             humanflags(STREAM_FLAGS, self.streamflags), typename,
             humanflags(FRAME_TYPE_FLAGS[self.typeid], self.flags)))
 
 def makeframe(requestid, streamid, streamflags, typeid, flags, payload):
     """Assemble a frame into a byte array."""
     # TODO assert size of payload.
     frame = bytearray(FRAME_HEADER_SIZE + len(payload))
 
     # 24 bits length
     # 16 bits request id
     # 8 bits stream id
     # 8 bits stream flags
     # 4 bits type
     # 4 bits flags
 
     l = struct.pack(r'<I', len(payload))
     frame[0:3] = l[0:3]
     struct.pack_into(r'<HBB', frame, 3, requestid, streamid, streamflags)
     frame[7] = (typeid << 4) | flags
     frame[8:] = payload
 
     return frame
 
 def makeframefromhumanstring(s):
     """Create a frame from a human readable string
 
     DANGER: NOT SAFE TO USE WITH UNTRUSTED INPUT BECAUSE OF POTENTIAL
     eval() USAGE. DO NOT USE IN CORE.
 
     Strings have the form:
 
         <request-id> <stream-id> <stream-flags> <type> <flags> <payload>
 
     This can be used by user-facing applications and tests for creating
     frames easily without having to type out a bunch of constants.
 
     Request ID and stream IDs are integers.
 
     Stream flags, frame type, and flags can be specified by integer or
     named constant.
 
     Flags can be delimited by `|` to bitwise OR them together.
 
     If the payload begins with ``cbor:``, the following string will be
     evaluated as Python code and the resulting object will be fed into
     a CBOR encoder. Otherwise, the payload is interpreted as a Python
     byte string literal.
     """
     fields = s.split(b' ', 5)
     requestid, streamid, streamflags, frametype, frameflags, payload = fields
 
     requestid = int(requestid)
     streamid = int(streamid)
 
     finalstreamflags = 0
     for flag in streamflags.split(b'|'):
         if flag in STREAM_FLAGS:
             finalstreamflags |= STREAM_FLAGS[flag]
         else:
             finalstreamflags |= int(flag)
 
     if frametype in FRAME_TYPES:
         frametype = FRAME_TYPES[frametype]
     else:
         frametype = int(frametype)
 
     finalflags = 0
     validflags = FRAME_TYPE_FLAGS[frametype]
     for flag in frameflags.split(b'|'):
         if flag in validflags:
             finalflags |= validflags[flag]
         else:
             finalflags |= int(flag)
 
     if payload.startswith(b'cbor:'):
         payload = cbor.dumps(stringutil.evalpython(payload[5:]), canonical=True)
 
     else:
         payload = stringutil.unescapestr(payload)
 
     return makeframe(requestid=requestid, streamid=streamid,
                      streamflags=finalstreamflags, typeid=frametype,
                      flags=finalflags, payload=payload)
 
 def parseheader(data):
     """Parse a unified framing protocol frame header from a buffer.
 
     The header is expected to be in the buffer at offset 0 and the
     buffer is expected to be large enough to hold a full header.
     """
     # 24 bits payload length (little endian)
     # 16 bits request ID
     # 8 bits stream ID
     # 8 bits stream flags
     # 4 bits frame type
     # 4 bits frame flags
     # ... payload
     framelength = data[0] + 256 * data[1] + 16384 * data[2]
     requestid, streamid, streamflags = struct.unpack_from(r'<HBB', data, 3)
     typeflags = data[7]
 
     frametype = (typeflags & 0xf0) >> 4
     frameflags = typeflags & 0x0f
 
     return frameheader(framelength, requestid, streamid, streamflags,
                        frametype, frameflags)
 
 def readframe(fh):
     """Read a unified framing protocol frame from a file object.
 
     Returns a 3-tuple of (type, flags, payload) for the decoded frame or
     None if no frame is available. May raise if a malformed frame is
     seen.
     """
     header = bytearray(FRAME_HEADER_SIZE)
 
     readcount = fh.readinto(header)
 
     if readcount == 0:
         return None
 
     if readcount != FRAME_HEADER_SIZE:
         raise error.Abort(_('received incomplete frame: got %d bytes: %s') %
                           (readcount, header))
 
     h = parseheader(header)
 
     payload = fh.read(h.length)
     if len(payload) != h.length:
         raise error.Abort(_('frame length error: expected %d; got %d') %
                           (h.length, len(payload)))
 
     return frame(h.requestid, h.streamid, h.streamflags, h.typeid, h.flags,
                  payload)
 
 def createcommandframes(stream, requestid, cmd, args, datafh=None,
                         maxframesize=DEFAULT_MAX_FRAME_SIZE):
     """Create frames necessary to transmit a request to run a command.
 
     This is a generator of bytearrays. Each item represents a frame
     ready to be sent over the wire to a peer.
     """
     data = {b'name': cmd}
     if args:
         data[b'args'] = args
 
     data = cbor.dumps(data, canonical=True)
 
     offset = 0
 
     while True:
         flags = 0
 
         # Must set new or continuation flag.
         if not offset:
             flags |= FLAG_COMMAND_REQUEST_NEW
         else:
             flags |= FLAG_COMMAND_REQUEST_CONTINUATION
 
         # Data frames is set on all frames.
         if datafh:
             flags |= FLAG_COMMAND_REQUEST_EXPECT_DATA
 
         payload = data[offset:offset + maxframesize]
         offset += len(payload)
 
         if len(payload) == maxframesize and offset < len(data):
             flags |= FLAG_COMMAND_REQUEST_MORE_FRAMES
 
         yield stream.makeframe(requestid=requestid,
                                typeid=FRAME_TYPE_COMMAND_REQUEST,
                                flags=flags,
                                payload=payload)
 
         if not (flags & FLAG_COMMAND_REQUEST_MORE_FRAMES):
             break
 
     if datafh:
         while True:
             data = datafh.read(DEFAULT_MAX_FRAME_SIZE)
 
             done = False
             if len(data) == DEFAULT_MAX_FRAME_SIZE:
                 flags = FLAG_COMMAND_DATA_CONTINUATION
             else:
                 flags = FLAG_COMMAND_DATA_EOS
                 assert datafh.read(1) == b''
                 done = True
 
             yield stream.makeframe(requestid=requestid,
                                    typeid=FRAME_TYPE_COMMAND_DATA,
                                    flags=flags,
                                    payload=data)
 
             if done:
                 break
 
 def createbytesresponseframesfrombytes(stream, requestid, data,
                                        maxframesize=DEFAULT_MAX_FRAME_SIZE):
     """Create a raw frame to send a bytes response from static bytes input.
 
     Returns a generator of bytearrays.
     """
 
     # Simple case of a single frame.
     if len(data) <= maxframesize:
         yield stream.makeframe(requestid=requestid,
                                typeid=FRAME_TYPE_BYTES_RESPONSE,
                                flags=FLAG_BYTES_RESPONSE_EOS,
                                payload=data)
         return
 
     offset = 0
     while True:
         chunk = data[offset:offset + maxframesize]
         offset += len(chunk)
         done = offset == len(data)
 
         if done:
             flags = FLAG_BYTES_RESPONSE_EOS
         else:
             flags = FLAG_BYTES_RESPONSE_CONTINUATION
 
         yield stream.makeframe(requestid=requestid,
                                typeid=FRAME_TYPE_BYTES_RESPONSE,
                                flags=flags,
                                payload=chunk)
 
         if done:
             break
 
 def createerrorframe(stream, requestid, msg, protocol=False, application=False):
     # TODO properly handle frame size limits.
     assert len(msg) <= DEFAULT_MAX_FRAME_SIZE
 
     flags = 0
     if protocol:
         flags |= FLAG_ERROR_RESPONSE_PROTOCOL
     if application:
         flags |= FLAG_ERROR_RESPONSE_APPLICATION
 
     yield stream.makeframe(requestid=requestid,
                            typeid=FRAME_TYPE_ERROR_RESPONSE,
                            flags=flags,
                            payload=msg)
 
 def createtextoutputframe(stream, requestid, atoms):
     """Create a text output frame to render text to people.
 
     ``atoms`` is a 3-tuple of (formatting string, args, labels).
 
     The formatting string contains ``%s`` tokens to be replaced by the
     corresponding indexed entry in ``args``. ``labels`` is an iterable of
     formatters to be applied at rendering time. In terms of the ``ui``
     class, each atom corresponds to a ``ui.write()``.
     """
     bytesleft = DEFAULT_MAX_FRAME_SIZE
     atomchunks = []
 
     for (formatting, args, labels) in atoms:
         if len(args) > 255:
             raise ValueError('cannot use more than 255 formatting arguments')
         if len(labels) > 255:
             raise ValueError('cannot use more than 255 labels')
 
         # TODO look for localstr, other types here?
 
         if not isinstance(formatting, bytes):
             raise ValueError('must use bytes formatting strings')
         for arg in args:
             if not isinstance(arg, bytes):
                 raise ValueError('must use bytes for arguments')
         for label in labels:
             if not isinstance(label, bytes):
                 raise ValueError('must use bytes for labels')
 
         # Formatting string must be UTF-8.
         formatting = formatting.decode(r'utf-8', r'replace').encode(r'utf-8')
 
         # Arguments must be UTF-8.
         args = [a.decode(r'utf-8', r'replace').encode(r'utf-8') for a in args]
 
         # Labels must be ASCII.
         labels = [l.decode(r'ascii', r'strict').encode(r'ascii')
                   for l in labels]
 
         if len(formatting) > 65535:
             raise ValueError('formatting string cannot be longer than 64k')
 
         if any(len(a) > 65535 for a in args):
             raise ValueError('argument string cannot be longer than 64k')
 
         if any(len(l) > 255 for l in labels):
             raise ValueError('label string cannot be longer than 255 bytes')
 
         chunks = [
             struct.pack(r'<H', len(formatting)),
             struct.pack(r'<BB', len(labels), len(args)),
             struct.pack(r'<' + r'B' * len(labels), *map(len, labels)),
             struct.pack(r'<' + r'H' * len(args), *map(len, args)),
         ]
         chunks.append(formatting)
         chunks.extend(labels)
         chunks.extend(args)
 
         atom = b''.join(chunks)
         atomchunks.append(atom)
         bytesleft -= len(atom)
 
     if bytesleft < 0:
         raise ValueError('cannot encode data in a single frame')
 
     yield stream.makeframe(requestid=requestid,
                            typeid=FRAME_TYPE_TEXT_OUTPUT,
                            flags=0,
                            payload=b''.join(atomchunks))
 
 class stream(object):
     """Represents a logical unidirectional series of frames."""
 
     def __init__(self, streamid, active=False):
         self.streamid = streamid
         self._active = False
 
     def makeframe(self, requestid, typeid, flags, payload):
         """Create a frame to be sent out over this stream.
 
         Only returns the frame instance. Does not actually send it.
         """
         streamflags = 0
         if not self._active:
             streamflags |= STREAM_FLAG_BEGIN_STREAM
             self._active = True
 
         return makeframe(requestid, self.streamid, streamflags, typeid, flags,
                          payload)
 
 def ensureserverstream(stream):
     if stream.streamid % 2:
         raise error.ProgrammingError('server should only write to even '
                                      'numbered streams; %d is not even' %
                                      stream.streamid)
 
 class serverreactor(object):
     """Holds state of a server handling frame-based protocol requests.
 
     This class is the "brain" of the unified frame-based protocol server
     component. While the protocol is stateless from the perspective of
     requests/commands, something needs to track which frames have been
     received, what frames to expect, etc. This class is that thing.
 
     Instances are modeled as a state machine of sorts. Instances are also
     reactionary to external events. The point of this class is to encapsulate
     the state of the connection and the exchange of frames, not to perform
     work. Instead, callers tell this class when something occurs, like a
     frame arriving. If that activity is worthy of a follow-up action (say
     *run a command*), the return value of that handler will say so.
 
     I/O and CPU intensive operations are purposefully delegated outside of
     this class.
 
     Consumers are expected to tell instances when events occur. They do so by
     calling the various ``on*`` methods. These methods return a 2-tuple
     describing any follow-up action(s) to take. The first element is the
     name of an action to perform. The second is a data structure (usually
     a dict) specific to that action that contains more information. e.g.
     if the server wants to send frames back to the client, the data structure
     will contain a reference to those frames.
 
     Valid actions that consumers can be instructed to take are:
 
     sendframes
        Indicates that frames should be sent to the client. The ``framegen``
        key contains a generator of frames that should be sent. The server
        assumes that all frames are sent to the client.
 
     error
        Indicates that an error occurred. Consumer should probably abort.
 
     runcommand
        Indicates that the consumer should run a wire protocol command. Details
        of the command to run are given in the data structure.
 
     wantframe
        Indicates that nothing of interest happened and the server is waiting on
        more frames from the client before anything interesting can be done.
 
     noop
        Indicates no additional action is required.
 
     Known Issues
     ------------
 
     There are no limits to the number of partially received commands or their
     size. A malicious client could stream command request data and exhaust the
     server's memory.
 
     Partially received commands are not acted upon when end of input is
     reached. Should the server error if it receives a partial request?
     Should the client send a message to abort a partially transmitted request
     to facilitate graceful shutdown?
 
     Active requests that haven't been responded to aren't tracked. This means
     that if we receive a command and instruct its dispatch, another command
     with its request ID can come in over the wire and there will be a race
     between who responds to what.
     """
 
     def __init__(self, deferoutput=False):
         """Construct a new server reactor.
 
         ``deferoutput`` can be used to indicate that no output frames should be
         instructed to be sent until input has been exhausted. In this mode,
         events that would normally generate output frames (such as a command
         response being ready) will instead defer instructing the consumer to
         send those frames. This is useful for half-duplex transports where the
         sender cannot receive until all data has been transmitted.
         """
         self._deferoutput = deferoutput
         self._state = 'idle'
         self._nextoutgoingstreamid = 2
         self._bufferedframegens = []
         # stream id -> stream instance for all active streams from the client.
         self._incomingstreams = {}
         self._outgoingstreams = {}
         # request id -> dict of commands that are actively being received.
         self._receivingcommands = {}
         # Request IDs that have been received and are actively being processed.
         # Once all output for a request has been sent, it is removed from this
         # set.
         self._activecommands = set()
 
     def onframerecv(self, frame):
         """Process a frame that has been received off the wire.
 
         Returns a dict with an ``action`` key that details what action,
         if any, the consumer should take next.
         """
         if not frame.streamid % 2:
             self._state = 'errored'
             return self._makeerrorresult(
                 _('received frame with even numbered stream ID: %d') %
                   frame.streamid)
 
         if frame.streamid not in self._incomingstreams:
             if not frame.streamflags & STREAM_FLAG_BEGIN_STREAM:
                 self._state = 'errored'
                 return self._makeerrorresult(
                     _('received frame on unknown inactive stream without '
                       'beginning of stream flag set'))
 
             self._incomingstreams[frame.streamid] = stream(frame.streamid)
 
         if frame.streamflags & STREAM_FLAG_ENCODING_APPLIED:
             # TODO handle decoding frames
             self._state = 'errored'
             raise error.ProgrammingError('support for decoding stream payloads '
                                          'not yet implemented')
 
         if frame.streamflags & STREAM_FLAG_END_STREAM:
             del self._incomingstreams[frame.streamid]
 
         handlers = {
             'idle': self._onframeidle,
             'command-receiving': self._onframecommandreceiving,
             'errored': self._onframeerrored,
         }
 
         meth = handlers.get(self._state)
         if not meth:
             raise error.ProgrammingError('unhandled state: %s' % self._state)
 
         return meth(frame)
 
     def onbytesresponseready(self, stream, requestid, data):
         """Signal that a bytes response is ready to be sent to the client.
 
         The raw bytes response is passed as an argument.
         """
         ensureserverstream(stream)
 
         def sendframes():
             for frame in createbytesresponseframesfrombytes(stream, requestid,
                                                             data):
                 yield frame
 
             self._activecommands.remove(requestid)
 
         result = sendframes()
 
         if self._deferoutput:
             self._bufferedframegens.append(result)
             return 'noop', {}
         else:
             return 'sendframes', {
                 'framegen': result,
             }
 
     def oninputeof(self):
         """Signals that end of input has been received.
 
         No more frames will be received. All pending activity should be
         completed.
         """
         # TODO should we do anything about in-flight commands?
 
         if not self._deferoutput or not self._bufferedframegens:
             return 'noop', {}
 
         # If we buffered all our responses, emit those.
         def makegen():
             for gen in self._bufferedframegens:
                 for frame in gen:
                     yield frame
 
         return 'sendframes', {
             'framegen': makegen(),
         }
 
     def onapplicationerror(self, stream, requestid, msg):
         ensureserverstream(stream)
 
         return 'sendframes', {
             'framegen': createerrorframe(stream, requestid, msg,
                                          application=True),
         }
 
     def makeoutputstream(self):
         """Create a stream to be used for sending data to the client."""
         streamid = self._nextoutgoingstreamid
         self._nextoutgoingstreamid += 2
 
         s = stream(streamid)
         self._outgoingstreams[streamid] = s
 
         return s
 
     def _makeerrorresult(self, msg):
         return 'error', {
             'message': msg,
         }
 
     def _makeruncommandresult(self, requestid):
         entry = self._receivingcommands[requestid]
 
         if not entry['requestdone']:
             self._state = 'errored'
             raise error.ProgrammingError('should not be called without '
                                          'requestdone set')
 
         del self._receivingcommands[requestid]
 
         if self._receivingcommands:
             self._state = 'command-receiving'
         else:
             self._state = 'idle'
 
         # Decode the payloads as CBOR.
         entry['payload'].seek(0)
         request = cbor.load(entry['payload'])
 
         if b'name' not in request:
             self._state = 'errored'
             return self._makeerrorresult(
                 _('command request missing "name" field'))
 
         if b'args' not in request:
             request[b'args'] = {}
 
         assert requestid not in self._activecommands
         self._activecommands.add(requestid)
 
         return 'runcommand', {
             'requestid': requestid,
             'command': request[b'name'],
             'args': request[b'args'],
             'data': entry['data'].getvalue() if entry['data'] else None,
         }
 
     def _makewantframeresult(self):
         return 'wantframe', {
             'state': self._state,
         }
 
     def _validatecommandrequestframe(self, frame):
         new = frame.flags & FLAG_COMMAND_REQUEST_NEW
         continuation = frame.flags & FLAG_COMMAND_REQUEST_CONTINUATION
 
         if new and continuation:
             self._state = 'errored'
             return self._makeerrorresult(
                 _('received command request frame with both new and '
                   'continuation flags set'))
 
         if not new and not continuation:
             self._state = 'errored'
             return self._makeerrorresult(
                 _('received command request frame with neither new nor '
                   'continuation flags set'))
 
     def _onframeidle(self, frame):
         # The only frame type that should be received in this state is a
         # command request.
         if frame.typeid != FRAME_TYPE_COMMAND_REQUEST:
             self._state = 'errored'
             return self._makeerrorresult(
                 _('expected command request frame; got %d') % frame.typeid)
 
         res = self._validatecommandrequestframe(frame)
         if res:
             return res
 
         if frame.requestid in self._receivingcommands:
             self._state = 'errored'
             return self._makeerrorresult(
                 _('request with ID %d already received') % frame.requestid)
 
         if frame.requestid in self._activecommands:
             self._state = 'errored'
             return self._makeerrorresult(
                 _('request with ID %d is already active') % frame.requestid)
 
         new = frame.flags & FLAG_COMMAND_REQUEST_NEW
         moreframes = frame.flags & FLAG_COMMAND_REQUEST_MORE_FRAMES
         expectingdata = frame.flags & FLAG_COMMAND_REQUEST_EXPECT_DATA
 
         if not new:
             self._state = 'errored'
             return self._makeerrorresult(
                 _('received command request frame without new flag set'))
 
         payload = util.bytesio()
         payload.write(frame.payload)
 
         self._receivingcommands[frame.requestid] = {
             'payload': payload,
             'data': None,
             'requestdone': not moreframes,
             'expectingdata': bool(expectingdata),
         }
 
         # This is the final frame for this request. Dispatch it.
         if not moreframes and not expectingdata:
             return self._makeruncommandresult(frame.requestid)
 
         assert moreframes or expectingdata
         self._state = 'command-receiving'
         return self._makewantframeresult()
 
     def _onframecommandreceiving(self, frame):
         if frame.typeid == FRAME_TYPE_COMMAND_REQUEST:
             # Process new command requests as such.
             if frame.flags & FLAG_COMMAND_REQUEST_NEW:
                 return self._onframeidle(frame)
 
             res = self._validatecommandrequestframe(frame)
             if res:
                 return res
 
         # All other frames should be related to a command that is currently
         # receiving but is not active.
         if frame.requestid in self._activecommands:
             self._state = 'errored'
             return self._makeerrorresult(
                 _('received frame for request that is still active: %d') %
                 frame.requestid)
 
         if frame.requestid not in self._receivingcommands:
             self._state = 'errored'
             return self._makeerrorresult(
                 _('received frame for request that is not receiving: %d') %
                   frame.requestid)
 
         entry = self._receivingcommands[frame.requestid]
 
         if frame.typeid == FRAME_TYPE_COMMAND_REQUEST:
             moreframes = frame.flags & FLAG_COMMAND_REQUEST_MORE_FRAMES
             expectingdata = bool(frame.flags & FLAG_COMMAND_REQUEST_EXPECT_DATA)
 
             if entry['requestdone']:
                 self._state = 'errored'
                 return self._makeerrorresult(
                     _('received command request frame when request frames '
                       'were supposedly done'))
 
             if expectingdata != entry['expectingdata']:
                 self._state = 'errored'
                 return self._makeerrorresult(
                     _('mismatch between expect data flag and previous frame'))
 
             entry['payload'].write(frame.payload)
 
             if not moreframes:
                 entry['requestdone'] = True
 
             if not moreframes and not expectingdata:
                 return self._makeruncommandresult(frame.requestid)
 
             return self._makewantframeresult()
 
         elif frame.typeid == FRAME_TYPE_COMMAND_DATA:
             if not entry['expectingdata']:
                 self._state = 'errored'
                 return self._makeerrorresult(_(
                     'received command data frame for request that is not '
                     'expecting data: %d') % frame.requestid)
 
             if entry['data'] is None:
                 entry['data'] = util.bytesio()
 
             return self._handlecommanddataframe(frame, entry)
         else:
             self._state = 'errored'
             return self._makeerrorresult(_(
                 'received unexpected frame type: %d') % frame.typeid)
 
     def _handlecommanddataframe(self, frame, entry):
         assert frame.typeid == FRAME_TYPE_COMMAND_DATA
 
         # TODO support streaming data instead of buffering it.
         entry['data'].write(frame.payload)
 
         if frame.flags & FLAG_COMMAND_DATA_CONTINUATION:
             return self._makewantframeresult()
         elif frame.flags & FLAG_COMMAND_DATA_EOS:
             entry['data'].seek(0)
             return self._makeruncommandresult(frame.requestid)
         else:
             self._state = 'errored'
             return self._makeerrorresult(_('command data frame without '
                                            'flags'))
 
     def _onframeerrored(self, frame):
         return self._makeerrorresult(_('server already errored'))