transaction: add onclose/onabort hook for pre-close logic

Adds an optional onclose parameter to transactions that gets called just before
the transaction is committed. This allows things that build up data over the
course of the transaction (like the fncache) to commit their data.

Also adds onabort. It's not used, but will allow extensions to hook into onclose
and onabort to provide transaction support.

mercurial/transaction.py

@@ -42,12 +42,27 b' def _playback(journal, report, opener, e'
     opener.unlink(journal)
 
 class transaction(object):
-    def __init__(self, report, opener, journal, after=None, createmode=None):
+    def __init__(self, report, opener, journal, after=None, createmode=None,
+            onclose=None, onabort=None):
+        """Begin a new transaction
+
+        Begins a new transaction that allows rolling back writes in the event of
+        an exception.
+
+        * `after`: called after the transaction has been committed
+        * `createmode`: the mode of the journal file that will be created
+        * `onclose`: called as the transaction is closing, but before it is
+        closed
+        * `onabort`: called as the transaction is aborting, but before any files
+        have been truncated
+        """
         self.count = 1
         self.usages = 1
         self.report = report
         self.opener = opener
         self.after = after
+        self.onclose = onclose
+        self.onabort = onabort
         self.entries = []
         self.map = {}
         self.journal = journal
@@ -126,6 +141,9 b' class transaction(object):'
     @active
     def close(self):
         '''commit the transaction'''
+        if self.count == 1 and self.onclose is not None:
+            self.onclose()
+
         self.count -= 1
         if self.count != 0:
             return
@@ -149,6 +167,9 b' class transaction(object):'
         self.usages = 0
         self.file.close()
 
+        if self.onabort is not None:
+            self.onabort()
+
         try:
             if not self.entries:
                 if self.journal: