[PATCH 9 of 9 V2] transaction: open a file with checkambig=True to avoid file stat ambiguity

[FUJIWARA Katsunori]

# HG changeset patch
# User FUJIWARA Katsunori <foozy at lares.dti.ne.jp>
# Date 1474548720 -32400
#      Thu Sep 22 21:52:00 2016 +0900
# Node ID c47d67e9412df24914ff4caaf1437a2c35a0af6b
# Parent  338febde64d5fe6a5aac79aff54741e201b5d88f
transaction: open a file with checkambig=True to avoid file stat ambiguity

Before this patch, 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 revlog-style file (and close transaction)
  2. discard appended data by truncation of rollback
  3. append same size but different data to revlog-style file again

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

To avoid file stat ambiguity around truncation, this patch opens a
file with checkambig=True.

This is a part of ExactCacheValidationPlan.

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

diff --git a/mercurial/transaction.py b/mercurial/transaction.py
--- a/mercurial/transaction.py
+++ b/mercurial/transaction.py
@@ -48,7 +48,7 @@ def _playback(journal, report, opener, v
     for f, o, _ignore in entries:
         if o or not unlink:
             try:
-                fp = opener(f, 'a')
+                fp = opener(f, 'a', checkambig=True)
                 fp.truncate(o)
                 fp.close()
             except IOError: