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repo: avoid copying/updating a dict on every `repo.__getitem__`...
repo: avoid copying/updating a dict on every `repo.__getitem__` This has some mild performance benefits. I'm looking into a pathological case where one of our `hg log` invocations takes several seconds, and according to hyperfine this reduces the wall time of the entire operation (running in chg) from: ``` Time (mean ± σ): 7.390 s ± 0.106 s [User: 7.058 s, System: 0.271 s] Range (min … max): 7.300 s … 7.625 s ``` to: ``` Time (mean ± σ): 7.046 s ± 0.091 s [User: 6.714 s, System: 0.279 s] Range (min … max): 6.916 s … 7.169 s ``` Note: the log command is slow due to an issue in our custom stuff executing `repo[<arg>]` 298,800 times. This performance improvement is likely not noticeable during normal operation, but I don't feel like it's making the code more difficult to understand, and every small bit helps. Differential Revision: https://phab.mercurial-scm.org/D9022

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bundlespec.txt
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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 hyphen (``-``).
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 ``v1`` 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.