[PATCH 7 of 7] 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 1474057498 -32400
#      Sat Sep 17 05:24:58 2016 +0900
# Node ID a7e512ac0968ed851bff0d95c002dac140582899
# Parent  582f1b85d9fbc727ddc4fe2c24cbcf3ccb82c249
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: