Patchwork [2,of,9,V2] scmutil: add file object wrapper class to check ambiguity at closing

diff --git a/mercurial/scmutil.py b/mercurial/scmutil.py
--- a/mercurial/scmutil.py
+++ b/mercurial/scmutil.py
@@ -1431,3 +1431,34 @@  class backgroundfilecloser(object):
             return
 
         self._queue.put(fh, block=True, timeout=None)
+
+class checkambigatclosing(closewrapbase):
+    """Proxy for a file object, to avoid ambiguity of file stat
+
+    See also util.filestat for detail about "ambiguity of file stat".
+
+    This proxy is useful only if the target file is guarded by any
+    lock (e.g. repo.lock or repo.wlock)
+
+    Do not instantiate outside of the vfs layer.
+    """
+    def __init__(self, fh):
+        super(checkambigatclosing, self).__init__(fh)
+        object.__setattr__(self, '_oldstat', util.filestat(fh.name))
+
+    def _checkambig(self):
+        oldstat = self._oldstat
+        if oldstat.stat:
+            newstat = util.filestat(self._origfh.name)
+            if newstat.isambig(oldstat):
+                # stat of changed file is ambiguous to original one
+                advanced = (oldstat.stat.st_mtime + 1) & 0x7fffffff
+                os.utime(self._origfh.name, (advanced, advanced))
+
+    def __exit__(self, exc_type, exc_value, exc_tb):
+        self._origfh.__exit__(exc_type, exc_value, exc_tb)
+        self._checkambig()
+
+    def close(self):
+        self._origfh.close()
+        self._checkambig()
diff --git a/tests/test-filecache.py b/tests/test-filecache.py
--- a/tests/test-filecache.py
+++ b/tests/test-filecache.py
@@ -179,6 +179,56 @@  def setbeforeget(repo):
     print("* file y created")
     print(repo.cached)
 
+def antiambiguity():
+    filename = 'ambigcheck'
+
+    # try some times, because reproduction of ambiguity depends on
+    # "filesystem time"
+    for i in xrange(5):
+        fp = open(filename, 'w')
+        fp.write('FOO')
+        fp.close()
+
+        oldstat = os.stat(filename)
+        if oldstat.st_ctime != oldstat.st_mtime:
+            # subsequent changing never causes ambiguity
+            continue
+
+        repetition = 3
+
+        # repeat changing via checkambigatclosing, to examine whether
+        # st_mtime is advanced multiple times as expecetd
+        for i in xrange(repetition):
+            # explicit closing
+            fp = scmutil.checkambigatclosing(open(filename, 'a'))
+            fp.write('FOO')
+            fp.close()
+
+            # implicit closing by "with" statement
+            with scmutil.checkambigatclosing(open(filename, 'a')) as fp:
+                fp.write('BAR')
+
+        newstat = os.stat(filename)
+        if oldstat.st_ctime != newstat.st_ctime:
+            # timestamp ambiguity was naturally avoided while repetition
+            continue
+
+        # st_mtime should be advanced "repetition * 2" times, because
+        # all changes occured at same time (in sec)
+        expected = (oldstat.st_mtime + repetition * 2) & 0x7fffffff
+        if newstat.st_mtime != expected:
+            print("'newstat.st_mtime %s is not %s (as %s + %s * 2)" %
+                  (newstat.st_mtime, expected, oldstat.st_mtime, repetition))
+
+        # no more examination is needed regardless of result
+        break
+    else:
+        # This platform seems too slow to examine anti-ambiguity
+        # of file timestamp (or test happened to be executed at
+        # bad timing). Exit silently in this case, because running
+        # on other faster platforms can detect problems
+        pass
+
 print('basic:')
 print()
 basic(fakerepo())
@@ -191,3 +241,7 @@  print()
 print('setbeforeget:')
 print()
 setbeforeget(fakerepo())
+print()
+print('antiambiguity:')
+print()
+antiambiguity()
diff --git a/tests/test-filecache.py.out b/tests/test-filecache.py.out
--- a/tests/test-filecache.py.out
+++ b/tests/test-filecache.py.out
@@ -58,3 +58,6 @@  string 2 set externally
 * file y created
 creating
 string from function
+
+antiambiguity:
+