[PATCH 6 of 7] repair: 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 582f1b85d9fbc727ddc4fe2c24cbcf3ccb82c249
# Parent  2033102d2ea9b32e88b5289b4ad359b4e3e7b400
repair: 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 strip
  3. append same size but different data to revlog-style file again

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

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

This patch also introduces "with" statement style, to ensure immediate
invocation of close() after truncation, because closing file is the
only trigger to check (and get rid of) file stat ambiguity.

This is a part of ExactCacheValidationPlan.

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

diff --git a/mercurial/repair.py b/mercurial/repair.py
--- a/mercurial/repair.py
+++ b/mercurial/repair.py
@@ -180,7 +180,8 @@ def strip(ui, repo, nodelist, backup=Tru
 
             for i in xrange(offset, len(tr.entries)):
                 file, troffset, ignore = tr.entries[i]
-                repo.svfs(file, 'a').truncate(troffset)
+                with repo.svfs(file, 'a', checkambig=True) as fp:
+                    fp.truncate(troffset)
                 if troffset == 0:
                     repo.store.markremoved(file)