[PATCH 1 of 3] vfs: add the possibility to have a "ward" to check vfs usage

[Pierre-Yves David]

# HG changeset patch
# User Pierre-Yves David <pierre-yves.david at ens-lyon.org>
# Date 1470323266 -7200
#      Thu Aug 04 17:07:46 2016 +0200
# Node ID 678c3cf029eceec20325928cff063ab71ea99761
# Parent  c0410814002f467c24ef07ce73850ba15b306f8e
# EXP-Topic vfs.ward
vfs: add the possibility to have a "ward" to check vfs usage

The function will be called anytime we open a file. The first usage of this
'ward' will be to check that lock are properly taken before accessing file.
Later we might use it to ensure we use the right vfs to access files, allowing
more vfs to be introduced.

We currently only apply the ward on 'open' operation. We will extend this to
other operations like copy, creation and removal later. The current readonlyvfs
seems to have the same shortcoming.

diff --git a/mercurial/scmutil.py b/mercurial/scmutil.py
--- a/mercurial/scmutil.py
+++ b/mercurial/scmutil.py
@@ -482,7 +482,8 @@ class vfs(abstractvfs):
     This class is used to hide the details of COW semantics and
     remote file access from higher level code.
     '''
-    def __init__(self, base, audit=True, expandpath=False, realpath=False):
+    def __init__(self, base, audit=True, expandpath=False, realpath=False,
+                 ward=None):
         if expandpath:
             base = util.expandpath(base)
         if realpath:
@@ -491,6 +492,11 @@ class vfs(abstractvfs):
         self.mustaudit = audit
         self.createmode = None
         self._trustnlink = None
+        # optional function to validate operation on file
+        # intended to be user for developer checks.
+        #
+        # XXX should be call for other things than 'open'
+        self._ward = ward
 
     @property
     def mustaudit(self):
@@ -552,6 +558,9 @@ class vfs(abstractvfs):
         if not text and "b" not in mode:
             mode += "b" # for that other OS
 
+        if self._ward is not None:
+            self._ward(f, mode, atomictemp)
+
         nlink = -1
         if mode not in ('r', 'rb'):
             dirname, basename = util.split(f)