[PATCH 1 of 2] dirstate: add callback to notify extensions about wd parent change

[Mateusz Kwapich]

# HG changeset patch
# User Mateusz Kwapich <mitrandir at fb.com>
# Date 1470681797 25200
#      Mon Aug 08 11:43:17 2016 -0700
# Node ID e7d2b86e2ee35fcb7b4e6ef6a4d0c6f696331f42
# Parent  5e2365698d448c2a1d75f6a58e11ec65f66a0266
dirstate: add callback to notify extensions about wd parent change

The journal extension had to touch the dirstate internals to be notified about
wd parent change. To make that detection cleaner and reusable let's move it core.
Now the extension can register to be notified about parent changes.

diff --git a/mercurial/dirstate.py b/mercurial/dirstate.py
--- a/mercurial/dirstate.py
+++ b/mercurial/dirstate.py
@@ -101,6 +101,8 @@ class dirstate(object):
         self._parentwriters = 0
         self._filename = 'dirstate'
         self._pendingfilename = '%s.pending' % self._filename
+        self._parentchangecallbacks = []
+        self._origpl = None
 
         # for consistent view between _pl() and _read() invocations
         self._pendingmode = None
@@ -347,6 +349,8 @@ class dirstate(object):
 
         self._dirty = self._dirtypl = True
         oldp2 = self._pl[1]
+        if self._origpl is None:
+            self._origpl = self._pl
         self._pl = p1, p2
         copies = {}
         if oldp2 != nullid and p2 == nullid:
@@ -442,6 +446,7 @@ class dirstate(object):
         self._lastnormaltime = 0
         self._dirty = False
         self._parentwriters = 0
+        self._origpl = None
 
     def copy(self, source, dest):
         """Mark dest as a copy of source. Unmark dest if source is None."""
@@ -721,7 +726,16 @@ class dirstate(object):
         st = self._opener(filename, "w", atomictemp=True, checkambig=True)
         self._writedirstate(st)
 
+    def addparentchangecallback(self, callback):
+        self._parentchangecallbacks.append(callback)
+
     def _writedirstate(self, st):
+        # notify callbacks about parents change
+        if self._origpl is not None:
+            if self._origpl != self._pl:
+                for callback in self._parentchangecallbacks:
+                    callback(self, self._origpl, self._pl)
+                self._origpl = None
         # use the modification time of the newly created temporary file as the
         # filesystem's notion of 'now'
         now = util.fstat(st).st_mtime & _rangemask