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streamclone: define first iteration of version 2 of stream format...
streamclone: define first iteration of version 2 of stream format (This patch is based on a first draft from Gregory Szorc, with deeper rework) Version 1 of the stream clone format was invented many years ago and suffers from a few deficiencies: 1) Filenames are stored in store-encoded (on filesystem) form rather than in their internal form. This makes future compatibility with new store filename encodings more difficult. 2) File entry "headers" consist of a newline of the file name followed by the string file size. Converting strings to integers is avoidable overhead. We can't store filenames with newlines (manifests have this limitation as well, so it isn't a major concern). But the big concern here is the necessity for readline(). Scanning for newlines means reading ahead and that means extra buffer allocations and slicing (in Python) and this makes performance suffer. 3) Filenames aren't compressed optimally. Filenames should be compressed well since there is a lot of repeated data. However, since they are scattered all over the stream (with revlog data in between), they typically fall outside the window size of the compressor and don't compress. 4) It can only exchange stored based content, being able to exchange caches too would be nice. 5) It is limited to a stream-based protocol and isn't suitable for an on-disk format for general repository reading because the offset of individual file entries requires scanning the entire file to find file records. As part of enabling streaming clones to work in bundle2, #2 proved to have a significant negative impact on performance. Since bundle2 provides the opportunity to start fresh, Gregory Szorc figured he would take the opportunity to invent a new streaming clone data format. The new format devised in this series addresses #1, #2, and #4. It punts on #3 because it was complex without yielding a significant gain and on #5 because devising a new store format that "packs" multiple revlogs into a single "packed revlog" is massive scope bloat. However, this v2 format might be suitable for streaming into a "packed revlog" with minimal processing. If it works, great. If not, we can always invent stream format when it is needed. This patch only introduces the bases of the format. We'll get it usable through bundle2 first, then we'll extend the format in future patches to bring it to its full potential (especially #4).
Gregory Szorc - - Load All Authors

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requirements.txt
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Repositories contain a file (``.hg/requires``) containing a list of
features/capabilities that are *required* for clients to interface
with the repository. This file has been present in Mercurial since
version 0.9.2 (released December 2006).
One of the first things clients do when opening a repository is read
``.hg/requires`` and verify that all listed requirements are supported,
aborting if not. Requirements are therefore a strong mechanism to
prevent incompatible clients from reading from unknown repository
formats or even corrupting them by writing to them.
Extensions may add requirements. When they do this, clients not running
an extension will be unable to read from repositories.
The following sections describe the requirements defined by the
Mercurial core distribution.
revlogv1
========
When present, revlogs are version 1 (RevlogNG). RevlogNG was introduced
in 2006. The ``revlogv1`` requirement has been enabled by default
since the ``requires`` file was introduced in Mercurial 0.9.2.
If this requirement is not present, version 0 revlogs are assumed.
store
=====
The *store* repository layout should be used.
This requirement has been enabled by default since the ``requires`` file
was introduced in Mercurial 0.9.2.
fncache
=======
The *fncache* repository layout should be used.
The *fncache* layout hash encodes filenames with long paths and
encodes reserved filenames.
This requirement is enabled by default when the *store* requirement is
enabled (which is the default behavior). It was introduced in Mercurial
1.1 (released December 2008).
shared
======
Denotes that the store for a repository is shared from another location
(defined by the ``.hg/sharedpath`` file).
This requirement is set when a repository is created via :hg:`share`.
The requirement was added in Mercurial 1.3 (released July 2009).
relshared
=========
Derivative of ``shared``; the location of the store is relative to the
store of this repository.
This requirement is set when a repository is created via :hg:`share`
using the ``--relative`` option.
The requirement was added in Mercurial 4.2 (released May 2017).
dotencode
=========
The *dotencode* repository layout should be used.
The *dotencode* layout encodes the first period or space in filenames
to prevent issues on OS X and Windows.
This requirement is enabled by default when the *store* requirement
is enabled (which is the default behavior). It was introduced in
Mercurial 1.7 (released November 2010).
parentdelta
===========
Denotes a revlog delta encoding format that was experimental and
replaced by *generaldelta*. It should not be seen in the wild because
it was never enabled by default.
This requirement was added in Mercurial 1.7 and removed in Mercurial
1.9.
generaldelta
============
Revlogs should be created with the *generaldelta* flag enabled. The
generaldelta flag will cause deltas to be encoded against a parent
revision instead of the previous revision in the revlog.
Support for this requirement was added in Mercurial 1.9 (released
July 2011). The requirement was disabled on new repositories by
default until Mercurial 3.7 (released February 2016).
manifestv2
==========
Denotes that version 2 of manifests are being used.
Support for this requirement was added in Mercurial 3.4 (released
May 2015). The requirement is currently experimental and is disabled
by default.
treemanifest
============
Denotes that tree manifests are being used. Tree manifests are
one manifest per directory (as opposed to a single flat manifest).
Support for this requirement was added in Mercurial 3.4 (released
August 2015). The requirement is currently experimental and is
disabled by default.
exp-sparse
==========
The working directory is sparse (only contains a subset of files).
Support for this requirement was added in Mercurial 4.3 (released
August 2017). This requirement and feature are experimental and may
disappear in a future Mercurial release. The requirement will only
be present on repositories that have opted in to a sparse working
directory.