[PATCH 3 of 7] vfs: use checkambigatclosing in checkambig=True but atomictemp=False case

[FUJIWARA Katsunori]

# HG changeset patch
# User FUJIWARA Katsunori <foozy at lares.dti.ne.jp>
# Date 1474057495 -32400
#      Sat Sep 17 05:24:55 2016 +0900
# Node ID 71b6b49f8a7ab6c894028b9153290f4bbf0f54f6
# Parent  ad999fb789fcb86b11c98334ab98b31a17ee2d25
vfs: use checkambigatclosing in checkambig=True but atomictemp=False case

In Mercurial source tree, opening a file in "a"/"a+" mode like below
doesn't specify atomictemp=True for vfs, and this avoids file stat
ambiguity check by atomictempfile.

  - writing changes out in revlog layer uses "a+" mode
  - truncation in repair.strip() uses "a" mode
  - truncation in transaction._playback() uses "a" mode

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 (strip or rollback)
  3. append same size but different data to revlog-style file again

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

This patch uses checkambigatclosing in checkambig=True but
atomictemp=False case, to check (and get rid of) file stat ambiguity
at closing.

This is a part of ExactCacheValidationPlan.

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

diff --git a/mercurial/scmutil.py b/mercurial/scmutil.py
--- a/mercurial/scmutil.py
+++ b/mercurial/scmutil.py
@@ -587,6 +587,10 @@ class vfs(abstractvfs):
         if nlink == 0:
             self._fixfilemode(f)
 
+        if checkambig:
+            assert mode not in ('r', 'rb'), "valid only at writing"
+            fp = checkambigatclosing(fp)
+
         if backgroundclose:
             if not self._backgroundfilecloser:
                 raise error.Abort(_('backgroundclose can only be used when a '