[PATCH 3 of 3 fix-default] addrevision: only use the incoming base if it is a good delta (issue4975)

[Pierre-Yves David]

# HG changeset patch
# User Pierre-Yves David <pierre-yves.david at fb.com>
# Date 1449014780 28800
#      Tue Dec 01 16:06:20 2015 -0800
# Node ID b907edfa2c846952ff06a00abd888ce1cdca395c
# Parent  31aafeaa7ac49cbc98d3b526b6ae67f98cc803d2
# EXP-Topic generaldelta
# Available At http://hg.netv6.net/marmoute-wip/mercurial/
#              hg pull http://hg.netv6.net/marmoute-wip/mercurial/ -r b907edfa2c84
addrevision: only use the incoming base if it is a good delta (issue4975)

Before this change, the 'lazydeltabase' would blindly build a delta using the
base provided by the incoming bundle and try to use it. If that base was far
down the revlog, the delta would be seen as "no good" and we would fall back to
a full text revision.

We now check if the delta is good and fallback to a computing a delta again the
tipmost revision otherwise (as we would do without general delta).

Later changesets will improve the logic to compute the fallback delta using the
general delta logic.

diff --git a/mercurial/revlog.py b/mercurial/revlog.py
--- a/mercurial/revlog.py
+++ b/mercurial/revlog.py
@@ -1425,11 +1425,16 @@ class revlog(object):
         # should we try to build a delta?
         if prev != nullrev:
             if cachedelta and self._generaldelta and self._lazydeltabase:
                 # Assume what we received from the server is a good choice
                 # build delta will reuse the cache
-                delta = builddelta(cachedelta[0])
+                candidatedelta = builddelta(cachedelta[0])
+                if self._isgooddelta(candidatedelta, textlen):
+                    delta = candidatedelta
+                elif prev != candidatedelta[3]:
+                    # Try against prev to hopefully save us a fulltext.
+                    delta = builddelta(prev)
             elif self._generaldelta:
                 if p2r != nullrev and self._aggressivemergedeltas:
                     delta = builddelta(p1r)
                     delta2 = builddelta(p2r)
                     p1good = self._isgooddelta(delta, textlen)