diff mercurial/revlog.py @ 27251:d9bfe6289acf

revlog: don't consider nullrev when choosing delta base

In the most complex case, we try using the incoming delta base, then
we try both parents, and then we try the previous revlog entry. If
none of these result in a good delta, we natually use the null
revision as base. However, we sometimes consider the nullrev before we
have exhausted our other options. Specifically, when both parents are
null, we use the nullrev as delta base if it produces a good delta
(according to _isgooddelta()), and we fail to try the previous revlog
entry as delta base. After 20a9226bdc8a (addrevision: use general
delta when the incoming base delta is bad, 2015-12-01), it can also
happen for non-merge commits when the incoming delta is not good.

The Firefox repo (from many months back) shrinks a tiny bit with this
patch: from 1.855GB to 1.830GB (1.4%). The hg repo itself shrinks even
less: by less than 0.1%. There may be repos that get larger instead.

This undoes the unexplained test change in 20a9226bdc8a.

--- a/mercurial/revlog.py	Fri Dec 04 17:14:14 2015 -0800
+++ b/mercurial/revlog.py	Fri Dec 04 17:46:56 2015 -0800
@@ -1432,9 +1432,9 @@
                 if self._isgooddelta(candidatedelta, textlen):
                     delta = candidatedelta
             if delta is None and self._generaldelta:
-                parents = [p1r, p2r]
                 # exclude already lazy tested base if any
-                parents = [p for p in parents if p not in tested]
+                parents = [p for p in (p1r, p2r)
+                           if p != nullrev and p not in tested]
                 if parents and not self._aggressivemergedeltas:
                     # Pick whichever parent is closer to us (to minimize the
                     # chance of having to build a fulltext).