clonebundles: improve BUNDLESPEC documentation

Before the existence of `hg debugbundle --spec`, the process for
defining the BUNDLESPEC value in manifests was not very clear and not
trivial to automate, especially in the case of stream clone bundles.

This patch adds documentation to note the existence of
`hg debugbundle --spec`. We drop the reference to stream clone
requirements handling because it is now redundant with
`hg debugbundle --spec`. While we are here, we further reinforce the
importance of defining BUNDLESPEC.

hgext/clonebundles.py

@@ -1,184 +1,184 b''
 # This software may be used and distributed according to the terms of the
 # GNU General Public License version 2 or any later version.
 
 """advertise pre-generated bundles to seed clones
 
 "clonebundles" is a server-side extension used to advertise the existence
 of pre-generated, externally hosted bundle files to clients that are
 cloning so that cloning can be faster, more reliable, and require less
 resources on the server.
 
 Cloning can be a CPU and I/O intensive operation on servers. Traditionally,
 the server, in response to a client's request to clone, dynamically generates
 a bundle containing the entire repository content and sends it to the client.
 There is no caching on the server and the server will have to redundantly
 generate the same outgoing bundle in response to each clone request. For
 servers with large repositories or with high clone volume, the load from
 clones can make scaling the server challenging and costly.
 
 This extension provides server operators the ability to offload potentially
 expensive clone load to an external service. Here's how it works.
 
 1. A server operator establishes a mechanism for making bundle files available
    on a hosting service where Mercurial clients can fetch them.
 2. A manifest file listing available bundle URLs and some optional metadata
    is added to the Mercurial repository on the server.
 3. A client initiates a clone against a clone bundles aware server.
 4. The client sees the server is advertising clone bundles and fetches the
    manifest listing available bundles.
 5. The client filters and sorts the available bundles based on what it
    supports and prefers.
 6. The client downloads and applies an available bundle from the
    server-specified URL.
 7. The client reconnects to the original server and performs the equivalent
    of :hg:`pull` to retrieve all repository data not in the bundle. (The
    repository could have been updated between when the bundle was created
    and when the client started the clone.)
 
 Instead of the server generating full repository bundles for every clone
 request, it generates full bundles once and they are subsequently reused to
 bootstrap new clones. The server may still transfer data at clone time.
 However, this is only data that has been added/changed since the bundle was
 created. For large, established repositories, this can reduce server load for
 clones to less than 1% of original.
 
 To work, this extension requires the following of server operators:
 
 * Generating bundle files of repository content (typically periodically,
   such as once per day).
 * A file server that clients have network access to and that Python knows
   how to talk to through its normal URL handling facility (typically an
   HTTP server).
 * A process for keeping the bundles manifest in sync with available bundle
   files.
 
 Strictly speaking, using a static file hosting server isn't required: a server
 operator could use a dynamic service for retrieving bundle data. However,
 static file hosting services are simple and scalable and should be sufficient
 for most needs.
 
 Bundle files can be generated with the :hg:`bundle` command. Typically
 :hg:`bundle --all` is used to produce a bundle of the entire repository.
 
 :hg:`debugcreatestreamclonebundle` can be used to produce a special
 *streaming clone bundle*. These are bundle files that are extremely efficient
 to produce and consume (read: fast). However, they are larger than
 traditional bundle formats and require that clients support the exact set
 of repository data store formats in use by the repository that created them.
 Typically, a newer server can serve data that is compatible with older clients.
 However, *streaming clone bundles* don't have this guarantee. **Server
 operators need to be aware that newer versions of Mercurial may produce
 streaming clone bundles incompatible with older Mercurial versions.**
 
-The list of requirements printed by :hg:`debugcreatestreamclonebundle` should
-be specified in the ``requirements`` parameter of the *bundle specification
-string* for the ``BUNDLESPEC`` manifest property described below. e.g.
-``BUNDLESPEC=none-packed1;requirements%3Drevlogv1``.
-
 A server operator is responsible for creating a ``.hg/clonebundles.manifest``
 file containing the list of available bundle files suitable for seeding
 clones. If this file does not exist, the repository will not advertise the
 existence of clone bundles when clients connect.
 
 The manifest file contains a newline (\n) delimited list of entries.
 
 Each line in this file defines an available bundle. Lines have the format:
 
     <URL> [<key>=<value>[ <key>=<value>]]
 
 That is, a URL followed by an optional, space-delimited list of key=value
 pairs describing additional properties of this bundle. Both keys and values
 are URI encoded.
 
 Keys in UPPERCASE are reserved for use by Mercurial and are defined below.
 All non-uppercase keys can be used by site installations. An example use
 for custom properties is to use the *datacenter* attribute to define which
 data center a file is hosted in. Clients could then prefer a server in the
 data center closest to them.
 
 The following reserved keys are currently defined:
 
 BUNDLESPEC
    A "bundle specification" string that describes the type of the bundle.
 
    These are string values that are accepted by the "--type" argument of
    :hg:`bundle`.
 
    The values are parsed in strict mode, which means they must be of the
    "<compression>-<type>" form. See
    mercurial.exchange.parsebundlespec() for more details.
 
+   :hg:`debugbundle --spec` can be used to print the bundle specification
+   string for a bundle file. The output of this command can be used verbatim
+   for the value of ``BUNDLESPEC`` (it is already escaped).
+
    Clients will automatically filter out specifications that are unknown or
    unsupported so they won't attempt to download something that likely won't
    apply.
 
    The actual value doesn't impact client behavior beyond filtering:
    clients will still sniff the bundle type from the header of downloaded
    files.
 
    **Use of this key is highly recommended**, as it allows clients to
-   easily skip unsupported bundles.
+   easily skip unsupported bundles. If this key is not defined, an old
+   client may attempt to apply a bundle that it is incapable of reading.
 
 REQUIRESNI
    Whether Server Name Indication (SNI) is required to connect to the URL.
    SNI allows servers to use multiple certificates on the same IP. It is
    somewhat common in CDNs and other hosting providers. Older Python
    versions do not support SNI. Defining this attribute enables clients
    with older Python versions to filter this entry without experiencing
    an opaque SSL failure at connection time.
 
    If this is defined, it is important to advertise a non-SNI fallback
    URL or clients running old Python releases may not be able to clone
    with the clonebundles facility.
 
    Value should be "true".
 
 Manifests can contain multiple entries. Assuming metadata is defined, clients
 will filter entries from the manifest that they don't support. The remaining
 entries are optionally sorted by client preferences
 (``experimental.clonebundleprefers`` config option). The client then attempts
 to fetch the bundle at the first URL in the remaining list.
 
 **Errors when downloading a bundle will fail the entire clone operation:
 clients do not automatically fall back to a traditional clone.** The reason
 for this is that if a server is using clone bundles, it is probably doing so
 because the feature is necessary to help it scale. In other words, there
 is an assumption that clone load will be offloaded to another service and
 that the Mercurial server isn't responsible for serving this clone load.
 If that other service experiences issues and clients start mass falling back to
 the original Mercurial server, the added clone load could overwhelm the server
 due to unexpected load and effectively take it offline. Not having clients
 automatically fall back to cloning from the original server mitigates this
 scenario.
 
 Because there is no automatic Mercurial server fallback on failure of the
 bundle hosting service, it is important for server operators to view the bundle
 hosting service as an extension of the Mercurial server in terms of
 availability and service level agreements: if the bundle hosting service goes
 down, so does the ability for clients to clone. Note: clients will see a
 message informing them how to bypass the clone bundles facility when a failure
 occurs. So server operators should prepare for some people to follow these
 instructions when a failure occurs, thus driving more load to the original
 Mercurial server when the bundle hosting service fails.
 """
 
 from mercurial import (
     extensions,
     wireproto,
 )
 
 testedwith = 'internal'
 
 def capabilities(orig, repo, proto):
     caps = orig(repo, proto)
 
     # Only advertise if a manifest exists. This does add some I/O to requests.
     # But this should be cheaper than a wasted network round trip due to
     # missing file.
     if repo.opener.exists('clonebundles.manifest'):
         caps.append('clonebundles')
 
     return caps
 
 def extsetup(ui):
     extensions.wrapfunction(wireproto, '_capabilities', capabilities)