revlog: fix bug in chainbase cache

The bug didn't cause corruption, and thus wasn't caught in hg verify or in
tests. It could lead to delta chains longer than normally allowed, by
affecting the code that decides when to add a full revision. This could,
in turn, lead to performance regression.

mercurial/revlog.py

@@ -218,7 +218,7 b' class revlog(object):'
         self.datafile = indexfile[:-2] + ".d"
         self.opener = opener
         self._cache = None
-        self._basecache = None
+        self._basecache = (0, 0)
         self._chunkcache = (0, '')
         self.index = []
         self._pcache = {}
@@ -1010,8 +1010,7 b' class revlog(object):'
                 delta = mdiff.textdiff(ptext, t)
             data = compress(delta)
             l = len(data[1]) + len(data[0])
-            basecache = self._basecache
-            if basecache and basecache[0] == rev:
+            if basecache[0] == rev:
                 chainbase = basecache[1]
             else:
                 chainbase = self.chainbase(rev)
@@ -1020,14 +1019,15 b' class revlog(object):'
                 base = rev
             else:
                 base = chainbase
-            return dist, l, data, base
+            return dist, l, data, base, chainbase
 
         curr = len(self)
         prev = curr - 1
-        base = curr
+        base = chainbase = curr
         offset = self.end(prev)
         flags = 0
         d = None
+        basecache = self._basecache
         p1r, p2r = self.rev(p1), self.rev(p2)
 
         # should we try to build a delta?
@@ -1036,7 +1036,7 b' class revlog(object):'
                 d = builddelta(p1r)
             else:
                 d = builddelta(prev)
-            dist, l, data, base = d
+            dist, l, data, base, chainbase = d
 
         # full versions are inserted when the needed deltas
         # become comparable to the uncompressed text
@@ -1049,7 +1049,7 b' class revlog(object):'
             text = buildtext()
             data = compress(text)
             l = len(data[1]) + len(data[0])
-            base = curr
+            base = chainbase = curr
 
         e = (offset_type(offset, flags), l, textlen,
              base, link, p1r, p2r, node)
@@ -1075,7 +1075,7 b' class revlog(object):'
 
         if type(text) == str: # only accept immutable objects
             self._cache = (node, curr, text)
-        self._basecache = (curr, base)
+        self._basecache = (curr, chainbase)
         return node
 
     def group(self, nodelist, bundler):