transaction: add 'writepending' logic

transaction: add 'writepending' logic

The contents of the transaction must be flushed to disk before running
a hook. But it must be flushed to a special file so that the normal
reader does not use it. This logic is currently in the changelog only.
We add some facility to register such operations in the transaction
itself.

--- a/mercurial/transaction.py	Sat Oct 18 01:12:18 2014 -0700
+++ b/mercurial/transaction.py	Fri Oct 17 21:19:54 2014 -0700
@@ -101,6 +101,10 @@
 
         # hold file generations to be performed on commit
         self._filegenerators = {}
+        # hold callbalk to write pending data for hooks
+        self._pendingcallback = {}
+        # True is any pending data have been written ever
+        self._anypending = False
 
     def __del__(self):
         if self.journal:
@@ -263,6 +267,26 @@
     def running(self):
         return self.count > 0
 
+    def addpending(self, category, callback):
+        """add a callback to be called when the transaction is pending
+
+        Category is a unique identifier to allow overwriting an old callback
+        with a newer callback.
+        """
+        self._pendingcallback[category] = callback
+
+    @active
+    def writepending(self):
+        '''write pending file to temporary version
+
+        This is used to allow hooks to view a transaction before commit'''
+        categories = sorted(self._pendingcallback)
+        for cat in categories:
+            # remove callback since the data will have been flushed
+            any = self._pendingcallback.pop(cat)()
+            self._anypending = self._anypending or any
+        return self._anypending
+
     @active
     def close(self):
         '''commit the transaction'''