revlog: add an aggressivemergedelta option

This adds an option for delta'ing against both p1 and p2 when applying merge
revisions and picking whichever is smallest.

Some before and after stats on manifest.d size:

internal large repo:
before: 1.2 GB
after: 930 MB

mozilla-central:
before: 261 MB
after: 92 MB

mercurial/localrepo.py

@@ -354,6 +354,10 b' class localrepository(object):'
         manifestcachesize = self.ui.configint('format', 'manifestcachesize')
         if manifestcachesize is not None:
             self.svfs.options['manifestcachesize'] = manifestcachesize
+        # experimental config: format.aggressivemergedeltas
+        aggressivemergedeltas = self.ui.configbool('format',
+            'aggressivemergedeltas', False)
+        self.svfs.options['aggressivemergedeltas'] = aggressivemergedeltas
 
     def _writerequirements(self):
         scmutil.writerequires(self.vfs, self.requirements)

mercurial/revlog.py

@@ -210,6 +210,7 b' class revlog(object):'
         self._chunkcache = (0, '')
         self._chunkcachesize = 65536
         self._maxchainlen = None
+        self._aggressivemergedeltas = False
         self.index = []
         self._pcache = {}
         self._nodecache = {nullid: nullrev}
@@ -227,6 +228,8 b' class revlog(object):'
                 self._chunkcachesize = opts['chunkcachesize']
             if 'maxchainlen' in opts:
                 self._maxchainlen = opts['maxchainlen']
+            if 'aggressivemergedeltas' in opts:
+                self._aggressivemergedeltas = opts['aggressivemergedeltas']
 
         if self._chunkcachesize <= 0:
             raise RevlogError(_('revlog chunk cache size %r is not greater '
@@ -1343,15 +1346,34 b' class revlog(object):'
         # should we try to build a delta?
         if prev != nullrev:
             if self._generaldelta:
-                # Pick whichever parent is closer to us (to minimize the
-                # chance of having to build a fulltext). Since
-                # nullrev == -1, any non-merge commit will always pick p1r.
-                drev = p2r if p2r > p1r else p1r
-                d = builddelta(drev)
-                # If the chosen delta will result in us making a full text,
-                # give it one last try against prev.
-                if drev != prev and not self._isgooddelta(d, textlen):
-                    d = builddelta(prev)
+                if p2r != nullrev and self._aggressivemergedeltas:
+                    d = builddelta(p1r)
+                    d2 = builddelta(p2r)
+                    p1good = self._isgooddelta(d, textlen)
+                    p2good = self._isgooddelta(d2, textlen)
+                    if p1good and p2good:
+                        # If both are good deltas, choose the smallest
+                        if d2[1] < d[1]:
+                            d = d2
+                    elif p2good:
+                        # If only p2 is good, use it
+                        d = d2
+                    elif p1good:
+                        pass
+                    else:
+                        # Neither is good, try against prev to hopefully save us
+                        # a fulltext.
+                        d = builddelta(prev)
+                else:
+                    # Pick whichever parent is closer to us (to minimize the
+                    # chance of having to build a fulltext). Since
+                    # nullrev == -1, any non-merge commit will always pick p1r.
+                    drev = p2r if p2r > p1r else p1r
+                    d = builddelta(drev)
+                    # If the chosen delta will result in us making a full text,
+                    # give it one last try against prev.
+                    if drev != prev and not self._isgooddelta(d, textlen):
+                        d = builddelta(prev)
             else:
                 d = builddelta(prev)
             dist, l, data, base, chainbase, chainlen, compresseddeltalen = d

tests/test-generaldelta.t

@@ -69,3 +69,37 b' commit.'
      rev    offset  length   base linkrev nodeid       p1           p2
        0         0       3      0       1 1406e7411862 000000000000 000000000000
 
+  $ cd ..
+
+Test format.aggressivemergedeltas
+
+  $ hg init --config format.generaldelta=1 aggressive
+  $ cd aggressive
+  $ touch a b c d e
+  $ hg commit -Aqm side1
+  $ hg up -q null
+  $ touch x y
+  $ hg commit -Aqm side2
+
+- Verify non-aggressive merge uses p1 (commit 1) as delta parent
+  $ hg merge -q 0
+  $ hg commit -q -m merge
+  $ hg debugindex -m
+     rev    offset  length  delta linkrev nodeid       p1           p2
+       0         0      59     -1       0 8dde941edb6e 000000000000 000000000000
+       1        59      59     -1       1 315c023f341d 000000000000 000000000000
+       2       118      65      1       2 2ab389a983eb 315c023f341d 8dde941edb6e
+
+  $ hg strip -q -r . --config extensions.strip=
+
+- Verify aggressive merge uses p2 (commit 0) as delta parent
+  $ hg up -q -C 1
+  $ hg merge -q 0
+  $ hg commit -q -m merge --config format.aggressivemergedeltas=True
+  $ hg debugindex -m
+     rev    offset  length  delta linkrev nodeid       p1           p2
+       0         0      59     -1       0 8dde941edb6e 000000000000 000000000000
+       1        59      59     -1       1 315c023f341d 000000000000 000000000000
+       2       118      62      0       2 2ab389a983eb 315c023f341d 8dde941edb6e
+
+  $ cd ..