vfs: copy if EPERM to avoid file stat ambiguity forcibly at closing

Now, files (to be truncated) are opened with checkambig=True, only if
localrepository caches it.

Therefore, straightforward "copy if EPERM" is always reasonable to
avoid file stat ambiguity at closing.

This patch makes checkambigatclosing close wrapper copy the target
file, and advance mtime on it after renaming, if EPERM. This can avoid
file stat ambiguity, even if the target file is owned by another (see
issue5418 and issue5584 for detail).

This patch factors main logic out instead of changing
checkambigatclosing._checkambig() directly, in order to reuse it.

mercurial/vfs.py

@@ -22,6 +22,23 b' from . import ('
     util,
 )
 
+def _avoidambig(path, oldstat):
+    """Avoid file stat ambiguity forcibly
+
+    This function causes copying ``path`` file, if it is owned by
+    another (see issue5418 and issue5584 for detail).
+    """
+    def checkandavoid():
+        newstat = util.filestat.frompath(path)
+        # return whether file stat ambiguity is (already) avoided
+        return (not newstat.isambig(oldstat) or
+                newstat.avoidambig(path, oldstat))
+    if not checkandavoid():
+        # simply copy to change owner of path to get privilege to
+        # advance mtime (see issue5418)
+        util.rename(util.mktempcopy(path), path)
+        checkandavoid()
+
 class abstractvfs(object):
     """Abstract base class; cannot be instantiated"""
 
@@ -613,10 +630,7 b' class checkambigatclosing(closewrapbase)'
     def _checkambig(self):
         oldstat = self._oldstat
         if oldstat.stat:
-            newstat = util.filestat.frompath(self._origfh.name)
-            if newstat.isambig(oldstat):
-                # stat of changed file is ambiguous to original one
-                newstat.avoidambig(self._origfh.name, oldstat)
+            _avoidambig(self._origfh.name, oldstat)
 
     def __exit__(self, exc_type, exc_value, exc_tb):
         self._origfh.__exit__(exc_type, exc_value, exc_tb)