bundle2: track life cycle of parts

We introduce a ``_generated`` attribute on parts. Coming changesets will
make it easier to update a part's contents after its creation. We need a way to track
if the part is still open to modification or if it is currently being generated
and should not be touched anymore.

As a bonus, we can now detect and crash if someone manages to write bogus code
to get a part generated twice.

mercurial/bundle2.py

@@ -552,8 +552,17 b' class bundlepart(object):'
         self.data = data
         self.mandatoryparams = mandatoryparams
         self.advisoryparams = advisoryparams
+        # status of the part's generation:
+        # - None: not started,
+        # - False: currently generated,
+        # - True: generation done.
+        self._generated = None
 
+    # methods used to generates the bundle2 stream
     def getchunks(self):
+        if self._generated is not None:
+            raise RuntimeError('part can only be consumed once')
+        self._generated = False
         #### header
         ## parttype
         header = [_pack(_fparttypesize, len(self.type)),
@@ -591,6 +600,7 b' class bundlepart(object):'
             yield chunk
         # end of payload
         yield _pack(_fpayloadsize, 0)
+        self._generated = True
 
     def _payloadchunks(self):
         """yield chunks of a the part payload