view mercurial/helptext/internals/requirements.txt @ 45095:8e04607023e5

procutil: ensure that procutil.std{out,err}.write() writes all bytes

Python 3 offers different kind of streams and it’s not guaranteed for all of
them that calling write() writes all bytes.

When Python is started in unbuffered mode, sys.std{out,err}.buffer are
instances of io.FileIO, whose write() can write less bytes for
platform-specific reasons (e.g. Linux has a 0x7ffff000 bytes maximum and could
write less if interrupted by a signal; when writing to Windows consoles, it’s
limited to 32767 bytes to avoid the "not enough space" error). This can lead to
silent loss of data, both when using sys.std{out,err}.buffer (which may in fact
not be a buffered stream) and when using the text streams sys.std{out,err}
(I’ve created a CPython bug report for that:
https://bugs.python.org/issue41221).

Python may fix the problem at some point. For now, we implement our own wrapper
for procutil.std{out,err} that calls the raw stream’s write() method until all
bytes have been written. We don’t use sys.std{out,err} for larger writes, so I
think it’s not worth the effort to patch them.

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 new format failed to meet expectations and support
for the format and requirement were removed in Mercurial 4.6
(released May 2018) since the feature never graduated frome experiment
status.

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.

bookmarksinstore
==================

Bookmarks are stored in ``.hg/store/`` instead of directly in ``.hg/``
where they used to be stored. The active bookmark is still stored
directly in ``.hg/``. This makes them always shared by ``hg share``,
whether or not ``-B`` was passed.

Support for this requirement was added in Mercurial 5.1 (released
August 2019). The requirement will only be present on repositories
that have opted in to this format (by having
``format.bookmarks-in-store=true`` set when they were created).

persistent-nodemap
==================

The `nodemap` index (mapping nodeid to local revision number) is persisted on
disk. This provides speed benefit (if the associated native code is used). The
persistent nodemap is only used for two revlogs: the changelog and the
manifestlog.

Support for this requirement was added in Mercurial 5.5 (released August 2020).
Note that as of 5.5, only installations compiled with the Rust extension will
benefit from a speedup. The other installations will do the necessary work to
keep the index up to date, but will suffer a slowdown.