help: document bundle specifications

I softly formalized the concept of a "bundle specification" a while
ago when I was working on clone bundles and stream clone bundles and
wanted a more robust way to define what exactly is in a bundle file.

The concept has existed for a while. Since it is part of the clone
bundles feature and exposed to the user via the "-t" argument to
`hg bundle`, it is something we need to support for the long haul.

After the 4.1 release, I heard a few people comment that they didn't
realize you could generate zstd bundles with `hg bundle`. I'm
partially to blame for not documenting it in bundle's docstring.

Additionally, I added a hacky, experimental feature for controlling
the compression level of bundles in 76104a4899ad. As the commit
message says, I went with a quick and dirty solution out of time
constraints. Furthermore, I wanted to eventually store this
configuration in the "bundlespec" so it could be made more flexible.

Given:

a) bundlespecs are here to stay
b) we don't have great documentation over what they are, despite being
   a user-facing feature
c) the list of available compression engines and their behavior isn't
   exposed
d) we need an extensible place to modify behavior of compression
   engines

I want to move forward with formalizing bundlespecs as a user-facing
feature. This commit does that by introducing a "bundlespec" help
page. Leaning on the just-added compression engine documentation
and API, the topic also conveniently lists available compression
engines and details about them. This makes features like zstd
bundle compression more discoverable. e.g. you can now
`hg help -k zstd` and it lists the "bundlespec" topic.

mercurial/help/bundlespec.txt

@@ -0,0 +1,84 b''
+Mercurial supports generating standalone "bundle" files that hold repository
+data. These "bundles" are typically saved locally and used later or exchanged
+between different repositories, possibly on different machines. Example
+commands using bundles are :hg:`bundle` and :hg:`unbundle`.
+
+Generation of bundle files is controlled by a "bundle specification"
+("bundlespec") string. This string tells the bundle generation process how
+to create the bundle.
+
+A "bundlespec" string is composed of the following elements:
+
+type
+    A string denoting the bundle format to use.
+
+compression
+    Denotes the compression engine to use compressing the raw bundle data.
+
+parameters
+    Arbitrary key-value parameters to further control bundle generation.
+
+A "bundlespec" string has the following formats:
+
+<type>
+    The literal bundle format string is used.
+
+<compression>-<type>
+    The compression engine and format are delimited by a hypthen (``-``).
+
+Optional parameters follow the ``<type>``. Parameters are URI escaped
+``key=value`` pairs. Each pair is delimited by a semicolon (``;``). The
+first parameter begins after a ``;`` immediately following the ``<type>``
+value.
+
+Available Types
+===============
+
+The following bundle <type> strings are available:
+
+v1
+    Produces a legacy "changegroup" version 1 bundle.
+
+    This format is compatible with nearly all Mercurial clients because it is
+    the oldest. However, it has some limitations, which is why it is no longer
+    the default for new repositories.
+
+    ``v1`` bundles can be used with modern repositories using the "generaldelta"
+    storage format. However, it may take longer to produce the bundle and the
+    resulting bundle may be significantly larger than a ``v2`` bundle.
+
+    ``v1`` bundles can only use the ``gzip``, ``bzip2``, and ``none`` compression
+    formats.
+
+v2
+    Produces a version 2 bundle.
+
+    Version 2 bundles are an extensible format that can store additional
+    repository data (such as bookmarks and phases information) and they can
+    store data more efficiently, resulting in smaller bundles.
+
+    Version 2 bundles can also use modern compression engines, such as
+    ``zstd``, making them faster to compress and often smaller.
+
+Available Compression Engines
+=============================
+
+The following bundle <compression> engines can be used:
+
+.. bundlecompressionmarker
+
+Examples
+========
+
+``v2``
+    Produce a ``v2`` bundle using default options, including compression.
+
+``none-v1``
+    Produce a ``v2`` bundle with no compression.
+
+``zstd-v2``
+    Produce a ``v2`` bundle with zstandard compression using default
+    settings.
+
+``zstd-v1``
+    This errors because ``zstd`` is not supported for ``v1`` types.

contrib/wix/help.wxs

@@ -15,6 +15,7 b''
     <DirectoryRef Id="INSTALLDIR">
       <Directory Id="helpdir" Name="help" FileSource="$(var.SourceDir)">
         <Component Id="help.root" Guid="$(var.help.root.guid)" Win64='$(var.IsX64)'>
+          <File Name="bundlespec.txt" />
           <File Name="color.txt" />
           <File Name="config.txt" KeyPath="yes" />
           <File Name="dates.txt" />

mercurial/help.py

@@ -210,6 +210,7 b' def internalshelp(ui):'
     return ''.join(lines)
 
 helptable = sorted([
+    (['bundlespec'], _("Bundle File Formats"), loaddoc('bundlespec')),
     (['color'], _("Colorizing Outputs"), loaddoc('color')),
     (["config", "hgrc"], _("Configuration Files"), loaddoc('config')),
     (["dates"], _("Date Formats"), loaddoc('dates')),
@@ -283,6 +284,8 b' def addtopicsymbols(topic, marker, symbo'
         return makeitemsdoc(ui, topic, doc, marker, symbols, dedent=dedent)
     addtopichook(topic, add)
 
+addtopicsymbols('bundlespec', '.. bundlecompressionmarker',
+                util.bundlecompressiontopics())
 addtopicsymbols('filesets', '.. predicatesmarker', fileset.symbols)
 addtopicsymbols('merge-tools', '.. internaltoolsmarker',
                 filemerge.internalsdoc)

tests/test-globalopts.t

@@ -340,6 +340,7 b' Testing -h/--help:'
   
   additional help topics:
   
+   bundlespec    Bundle File Formats
    color         Colorizing Outputs
    config        Configuration Files
    dates         Date Formats
@@ -423,6 +424,7 b' Testing -h/--help:'
   
   additional help topics:
   
+   bundlespec    Bundle File Formats
    color         Colorizing Outputs
    config        Configuration Files
    dates         Date Formats

tests/test-help.t

@@ -102,6 +102,7 b' Short help:'
   
   additional help topics:
   
+   bundlespec    Bundle File Formats
    color         Colorizing Outputs
    config        Configuration Files
    dates         Date Formats
@@ -179,6 +180,7 b' Short help:'
   
   additional help topics:
   
+   bundlespec    Bundle File Formats
    color         Colorizing Outputs
    config        Configuration Files
    dates         Date Formats
@@ -827,6 +829,7 b' Test that default list of commands omits'
   
   additional help topics:
   
+   bundlespec    Bundle File Formats
    color         Colorizing Outputs
    config        Configuration Files
    dates         Date Formats
@@ -1838,6 +1841,15 b' Test dynamic list of merge tools only sh'
       See the merge-tools and ui sections of hgrc(5) for details on the
       configuration of merge tools.
 
+Compression engines listed in `hg help bundlespec`
+
+  $ hg help bundlespec | grep gzip
+          "v1" bundles can only use the "gzip", "bzip2", and "none" compression
+        An algorithm that produces smaller bundles than "gzip".
+        This engine will likely produce smaller bundles than "gzip" but will be
+      "gzip"
+        better compression than "gzip". It also frequently yields better
+
 Test usage of section marks in help documents
 
   $ cd "$TESTDIR"/../doc
@@ -1899,6 +1911,13 b' Dish up an empty repo; serve it cold.'
   <tr><td colspan="2"><h2><a name="topics" href="#topics">Topics</a></h2></td></tr>
   
   <tr><td>
+  <a href="/help/bundlespec">
+  bundlespec
+  </a>
+  </td><td>
+  Bundle File Formats
+  </td></tr>
+  <tr><td>
   <a href="/help/color">
   color
   </a>

tests/test-hgweb-json.t

@@ -1549,6 +1549,10 b' help/ shows help topics'
     ],
     "topics": [
       {
+        "summary": "Bundle File Formats",
+        "topic": "bundlespec"
+      },
+      {
         "summary": "Colorizing Outputs",
         "topic": "color"
       },