[PATCH 7 of 9 V2] changelog: specify checkambig=True to revlog.__init__, to avoid ambiguity

[FUJIWARA Katsunori]

# HG changeset patch
# User FUJIWARA Katsunori <foozy at lares.dti.ne.jp>
# Date 1474548719 -32400
#      Thu Sep 22 21:51:59 2016 +0900
# Node ID 5ec24143ad2e85bf1567d9af83df9fc5187f0a4e
# Parent  550a2388d189a079822ee13d901f3cb9854e5a3e
changelog: specify checkambig=True to revlog.__init__, to avoid ambiguity

If steps below occurs at "the same time in sec", all of mtime, ctime
and size are same between (1) and (3).

  1. append data to 00changelog.i (and close transaction)
  2. discard appended data by truncation (strip or rollback)
  3. append same size but different data to 00changelog.i again

Therefore, cache validation doesn't work after (3) as expected.

To avoid such file stat ambiguity around truncation, this patch
specifies checkambig=True to revlog.__init__(). This makes revlog
write changes out with checkambig=True.

Even though changes of 00changelog.i themselves are written out at
changelog._finalize(), this checkambig=True is needed, because
revlog.checkinlinesize(), which is invoked at the end of
changelog._finalize(), might replace already changed 00changelog.i by
converted one.

Even after this patch, avoiding file stat ambiguity of 00changelog.i
around truncation isn't yet completed, because truncation side isn't
aware of this issue.

This is a part of ExactCacheValidationPlan.

    https://www.mercurial-scm.org/wiki/ExactCacheValidationPlan

diff --git a/mercurial/changelog.py b/mercurial/changelog.py
--- a/mercurial/changelog.py
+++ b/mercurial/changelog.py
@@ -257,7 +257,8 @@ class changelogrevision(object):
 
 class changelog(revlog.revlog):
     def __init__(self, opener):
-        revlog.revlog.__init__(self, opener, "00changelog.i")
+        revlog.revlog.__init__(self, opener, "00changelog.i",
+                               checkambig=True)
         if self._initempty:
             # changelogs don't benefit from generaldelta
             self.version &= ~revlog.REVLOGGENERALDELTA