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transaction: register summary callbacks only at start of transaction (BC)...
transaction: register summary callbacks only at start of transaction (BC) We currently register summary callbacks every time localrepo.transaction() is called, so both when the transaction is started and when a nested transaction is created. That seems a little weirdly asymmetric, because the summary callbacks are thus not necessarily registred at the beginning of the outermost transaction, but they are only called when the outermost transaction closes (not when a nested transaction closes). I want to add another summary callback that records the repo state at the beginning of the transaction and compares to that when the transaction closes. However, because of the registration that happens when a nested transaction is created, that would need to go through extra trouble to not overwrite the callback and report the difference from the start time of the innermost transaction to the close of the outermost transaction. Also, the callbacks are registered with a name based on the order they are defined in the registersummarycallback(). For example, if both the "new changesets %s" and the "obsoleted %i changesets" hooks are registered, the first would be called 00-txnreport and the second would be called 01-txnreport. That gets really weird if registersummarycallback() gets called multiple times, because the last one wins, and a depending on which of the two callbacks get registered, we might hypothetically even overwrite on type of callback with another. For example, if when the outer transaction was started, we registered the "new changesets %s" callback first, and when the inner transaction was started, we registered only the "obsoleted %i changesets" callback, then only the latter message would get printed. What makes it hypothetical is that what gets registered depends on the transaction name, and the set of transaction names that we match on for the former latter message is a subset of the set of names we match on for the former. Still, that seems like a bug waiting to happen. That second issue could be solved independently, but the first issue seems enough for me to consider it a bug (affecting developers, not users), so this patch simply drops that extra registration. Note that this affects "hg transplant" in a user-visible way. When "hg transplant" is asked to transplant from a remote repo so it involves a pull, then the outermost transaction name is "transplant" and an inner transaction is created for "pull". That inner transaction is what led us to sometimes report "new changesets %s" from "hg transplant". After this patch, that no longer happens. That seems fine to me. We can make it instead print the message for all "hg transplant" invocations if we want (not just those involving a remote), but I'll leave that for someone else to do if they think it's important. Differential Revision: https://phab.mercurial-scm.org/D1866
Gregory Szorc - - Load All Authors

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bundles.txt
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A bundle is a container for repository data.
Bundles are used as standalone files as well as the interchange format
over the wire protocol used when two Mercurial peers communicate with
each other.
Headers
=======
Bundles produced since Mercurial 0.7 (September 2005) have a 4 byte
header identifying the major bundle type. The header always begins with
``HG`` and the follow 2 bytes indicate the bundle type/version. Some
bundle types have additional data after this 4 byte header.
The following sections describe each bundle header/type.
HG10
----
``HG10`` headers indicate a *changegroup bundle*. This is the original
bundle format, so it is sometimes referred to as *bundle1*. It has been
present since version 0.7 (released September 2005).
This header is followed by 2 bytes indicating the compression algorithm
used for data that follows. All subsequent data following this
compression identifier is compressed according to the algorithm/method
specified.
Supported algorithms include the following.
``BZ``
*bzip2* compression.
Bzip2 compressors emit a leading ``BZ`` header. Mercurial uses this
leading ``BZ`` as part of the bundle header. Therefore consumers
of bzip2 bundles need to *seed* the bzip2 decompressor with ``BZ`` or
seek the input stream back to the beginning of the algorithm component
of the bundle header so that decompressor input is valid. This behavior
is unique among supported compression algorithms.
Supported since version 0.7 (released December 2006).
``GZ``
*zlib* compression.
Supported since version 0.9.2 (released December 2006).
``UN``
*Uncompressed* or no compression. Unmodified changegroup data follows.
Supported since version 0.9.2 (released December 2006).
3rd party extensions may implement their own compression. However, no
authority reserves values for their compression algorithm identifiers.
HG2X
----
``HG2X`` headers (where ``X`` is any value) denote a *bundle2* bundle.
Bundle2 bundles are a container format for various kinds of repository
data and capabilities, beyond changegroup data (which was the only data
supported by ``HG10`` bundles.
``HG20`` is currently the only defined bundle2 version.
The ``HG20`` format is not yet documented here. See the inline comments
in ``mercurial/exchange.py`` for now.
Initial ``HG20`` support was added in Mercurial 3.0 (released May
2014). However, bundle2 bundles were hidden behind an experimental flag
until version 3.5 (released August 2015), when they were enabled in the
wire protocol. Various commands (including ``hg bundle``) did not
support generating bundle2 files until Mercurial 3.6 (released November
2015).
HGS1
----
*Experimental*
A ``HGS1`` header indicates a *streaming clone bundle*. This is a bundle
that contains raw revlog data from a repository store. (Typically revlog
data is exchanged in the form of changegroups.)
The purpose of *streaming clone bundles* are to *clone* repository data
very efficiently.
The ``HGS1`` header is always followed by 2 bytes indicating a
compression algorithm of the data that follows. Only ``UN``
(uncompressed data) is currently allowed.
``HGS1UN`` support was added as an experimental feature in version 3.6
(released November 2015) as part of the initial offering of the *clone
bundles* feature.