dirstate: add begin/endparentchange to dirstate

It's possible for the dirstate to become incoherent (issue4353) if there is an
exception in the middle of the dirstate parent and entries being written (like
if the user ctrl+c's). This change adds begin/endparentchange which a future
patch will require to be set before changing the dirstate parent.  This will
allow us to prevent writing the dirstate in the event of an exception while
changing the parent.

mercurial/dirstate.py

@@ -44,6 +44,30 b' class dirstate(object):'
         self._lastnormaltime = 0
         self._ui = ui
         self._filecache = {}
+        self._parentwriters = 0
+
+    def beginparentchange(self):
+        '''Marks the beginning of a set of changes that involve changing
+        the dirstate parents. If there is an exception during this time,
+        the dirstate will not be written when the wlock is released. This
+        prevents writing an incoherent dirstate where the parent doesn't
+        match the contents.
+        '''
+        self._parentwriters += 1
+
+    def endparentchange(self):
+        '''Marks the end of a set of changes that involve changing the
+        dirstate parents. Once all parent changes have been marked done,
+        the wlock will be free to write the dirstate on release.
+        '''
+        if self._parentwriters > 0:
+            self._parentwriters -= 1
+
+    def pendingparentchange(self):
+        '''Returns true if the dirstate is in the middle of a set of changes
+        that modify the dirstate parent.
+        '''
+        return self._parentwriters > 0
 
     @propertycache
     def _map(self):
@@ -300,6 +324,7 b' class dirstate(object):'
                 delattr(self, a)
         self._lastnormaltime = 0
         self._dirty = False
+        self._parentwriters = 0
 
     def copy(self, source, dest):
         """Mark dest as a copy of source. Unmark dest if source is None."""

mercurial/localrepo.py

@@ -1102,7 +1102,11 b' class localrepository(object):'
             return l
 
         def unlock():
-            self.dirstate.write()
+            if self.dirstate.pendingparentchange():
+                self.dirstate.invalidate()
+            else:
+                self.dirstate.write()
+
             self._filecache['dirstate'].refresh()
 
         l = self._lock(self.vfs, "wlock", wait, unlock,