sidedata: add a new module with basic documentation

For now the storage strategy is very simple. We can augment it in the future if
needed.

Code to actually support what is described will be introduced in later
changesets.

Differential Revision: https://phab.mercurial-scm.org/D6889

mercurial/revlogutils/sidedata.py

@@ -0,0 +1,34 b''
+# sidedata.py - Logic around store extra data alongside revlog revisions
+#
+# Copyright 2019 Pierre-Yves David <[e-mail])
+#
+# This software may be used and distributed according to the terms of the
+# GNU General Public License version 2 or any later version.
+"""core code for "sidedata" support
+
+The "sidedata" are stored alongside the revision without actually being part of
+its content and not affecting its hash. It's main use cases is to cache
+important information related to a changesets.
+
+The current implementation is experimental and subject to changes. Do not rely
+on it in production.
+
+Sidedata are stored in the revlog itself, withing the revision rawtext. They
+are inserted, removed from it using the flagprocessors mechanism. The following
+format is currently used::
+
+    initial header:
+        <number of sidedata; 2 bytes>
+    sidedata (repeated N times):
+        <sidedata-key; 2 bytes>
+        <sidedata-entry-length: 4 bytes>
+        <sidedata-content-sha1-digest: 20 bytes>
+        <sidedata-content; X bytes>
+    normal raw text:
+        <all bytes remaining in the rawtext>
+
+This is a simple and effective format. It should be enought to experiment with
+the concept.
+"""
+
+from __future__ import absolute_import