[PATCH 02 of 14] cache: introduce a changelogsourcebase class

[Boris Feld]

# HG changeset patch
# User Boris Feld <boris.feld at octobus.net>
# Date 1499458552 -7200
#      Fri Jul 07 22:15:52 2017 +0200
# Node ID 8b71290526ddb77f157e075191dd748793d85601
# Parent  6edb62505c697329de034c2fdc47befd5896f31f
# EXP-Topic obs-cache
cache: introduce a changelogsourcebase class

This abstract class will help code that need a cache tracking the changelog
content (eg: the branchmap cache). The cache key used is the same as what the
branchmap uses.

diff -r 6edb62505c69 -r 8b71290526dd mercurial/cache.py
--- a/mercurial/cache.py	Fri Jul 07 22:14:01 2017 +0200
+++ b/mercurial/cache.py	Fri Jul 07 22:15:52 2017 +0200
@@ -10,6 +10,7 @@
 import struct
 
 from . import (
+    node,
     util,
 )
 
@@ -125,3 +126,41 @@
     def _deserializecachekey(self, data):
         """read the cachekey from bytes"""
         return self._cachekeystruct.unpack(data)
+
+class changelogsourcebase(incrementalcachebase):
+    """an abstract class for cache sourcing data from the changelog
+
+    For this purpose it use a cache key covering changelog content.
+    The cache key parts are: (tiprev, tipnode)
+    """
+
+    __metaclass__ = abc.ABCMeta
+
+    # default key used for an empty cache
+    emptykey = (0, node.nullid)
+    _cachekeyspec = 'i20s'
+    _cachename = None # used for debug message
+
+    # Useful "public" function (no need to override them)
+
+    def _fetchchangelogdata(self, cachekey, cl):
+        """use a cachekey to fetch incremental data
+
+        Exists as its own method to help subclass to reuse it."""
+        tiprev = len(cl) - 1
+        tipnode = cl.node(tiprev)
+        newkey = (tiprev, tipnode)
+        tiprev = len(cl) - 1
+        if newkey == cachekey:
+            return False, [], newkey
+        keyrev, keynode = cachekey
+        if tiprev < keyrev or cl.node(keyrev) != keynode:
+            revs = ()
+            if len(cl):
+                revs = list(cl.revs(stop=tiprev))
+            return True, revs, newkey
+        else:
+            return False, list(cl.revs(start=keyrev + 1, stop=tiprev)), newkey
+
+    def _fetchupdatedata(self, repo):
+        return self._fetchchangelogdata(self._cachekey, repo.changelog)