revlog: add config variable for limiting delta-chain length

The current heuristic for deciding between storing delta and full texts
is based on ratio of (sizeofdeltas)/(sizeoffulltext).

In some cases (for example a manifest for ahuge repo) this approach
can result in extremely long delta chains (~30,000) which are very slow to
read. (In the case of a manifest ~500ms are added to every hg command because of that).

This commit introduces "revlog.maxchainlength" configuration variable that will
limit delta chain length.

mercurial/localrepo.py

@@ -316,6 +316,9 b' class localrepository(object):'
         chunkcachesize = self.ui.configint('format', 'chunkcachesize')
         if chunkcachesize is not None:
             self.sopener.options['chunkcachesize'] = chunkcachesize
+        maxchainlen = self.ui.configint('revlog', 'maxchainlen')
+        if maxchainlen is not None:
+            self.sopener.options['maxchainlen'] = maxchainlen
 
     def _writerequirements(self):
         reqfile = self.opener("requires", "w")

mercurial/revlog.py

@@ -204,6 +204,7 b' class revlog(object):'
         self._basecache = None
         self._chunkcache = (0, '')
         self._chunkcachesize = 65536
+        self._maxchainlen = None
         self.index = []
         self._pcache = {}
         self._nodecache = {nullid: nullrev}
@@ -219,6 +220,8 b' class revlog(object):'
                 v = 0
             if 'chunkcachesize' in opts:
                 self._chunkcachesize = opts['chunkcachesize']
+            if 'maxchainlen' in opts:
+                self._maxchainlen = opts['maxchainlen']
 
         if self._chunkcachesize <= 0:
             raise RevlogError(_('revlog chunk cache size %r is not greater '
@@ -1216,11 +1219,13 b' class revlog(object):'
                 base = rev
             else:
                 base = chainbase
-            return dist, l, data, base, chainbase
+            chainlen = self.chainlen(rev) + 1
+            return dist, l, data, base, chainbase, chainlen
 
         curr = len(self)
         prev = curr - 1
         base = chainbase = curr
+        chainlen = None
         offset = self.end(prev)
         flags = 0
         d = None
@@ -1240,7 +1245,7 b' class revlog(object):'
                     d = builddelta(prev)
             else:
                 d = builddelta(prev)
-            dist, l, data, base, chainbase = d
+            dist, l, data, base, chainbase, chainlen = d
 
         # full versions are inserted when the needed deltas
         # become comparable to the uncompressed text
@@ -1249,7 +1254,8 b' class revlog(object):'
                                         cachedelta[1])
         else:
             textlen = len(text)
-        if d is None or dist > textlen * 2:
+        if (d is None or dist > textlen * 2 or
+            self._maxchainlen and chainlen > self._maxchainlen):
             text = buildtext()
             data = self.compress(text)
             l = len(data[1]) + len(data[0])

tests/test-debugcommands.t

@@ -24,6 +24,40 b''
   full revision size (min/max/avg)     : 44 / 44 / 44
   delta size (min/max/avg)             : 0 / 0 / 0
 
+Test max chain len
+  $ cat >> $HGRCPATH << EOF
+  > [revlog]
+  > maxchainlen=4
+  > EOF
+
+  $ echo "This test checks if maxchainlen config value is respected also it can serve as basic test for debugrevlog -d <file>.\n" >> a
+  $ hg ci -m a
+  $ echo "b\n" >> a
+  $ hg ci -m a
+  $ echo "c\n" >> a
+  $ hg ci -m a
+  $ echo "d\n" >> a
+  $ hg ci -m a
+  $ echo "e\n" >> a
+  $ hg ci -m a
+  $ echo "f\n" >> a
+  $ hg ci -m a
+  $ echo 'g\n' >> a
+  $ hg ci -m a
+  $ echo 'h\n' >> a
+  $ hg ci -m a
+  $ hg debugrevlog -d a
+  # rev p1rev p2rev start   end deltastart base   p1   p2 rawsize totalsize compression heads chainlen
+      0    -1    -1     0   ???          0    0    0    0     ???      ????           ?     1        0 (glob)
+      1     0    -1   ???   ???          0    0    0    0     ???      ????           ?     1        1 (glob)
+      2     1    -1   ???   ???        ???  ???  ???    0     ???      ????           ?     1        2 (glob)
+      3     2    -1   ???   ???        ???  ???  ???    0     ???      ????           ?     1        3 (glob)
+      4     3    -1   ???   ???        ???  ???  ???    0     ???      ????           ?     1        4 (glob)
+      5     4    -1   ???   ???        ???  ???  ???    0     ???      ????           ?     1        0 (glob)
+      6     5    -1   ???   ???        ???  ???  ???    0     ???      ????           ?     1        1 (glob)
+      7     6    -1   ???   ???        ???  ???  ???    0     ???      ????           ?     1        2 (glob)
+      8     7    -1   ???   ???        ???  ???  ???    0     ???      ????           ?     1        3 (glob)
+  $ cd ..
 
 Test internal debugstacktrace command