diff mercurial/help/internals/requirements.txt @ 28523:045fe7042510

help: document requirements

We didn't have unified documentation of the various repository
requirements. This patch changes that.

--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/mercurial/help/internals/requirements.txt	Sat Mar 12 18:51:07 2016 -0800
@@ -0,0 +1,110 @@
+Requirements
+============
+
+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).
+
+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.