bundle2: the ability to set ``data`` attribute of the part is now official

We make it safe to set the data attribute after part creation. It is an allowed
operation as long as the part has not started to be generated.

mercurial/bundle2.py

@@ -554,13 +554,17 b' class bundlepart(object):'
 
     The part `type` is used to route the part to the application level
     handler.
+
+    The part payload is contained in ``part.data``. It could be raw bytes or a
+    generator of byte chunks. The data attribute cannot be modified after the
+    generation has begun.
     """
 
     def __init__(self, parttype, mandatoryparams=(), advisoryparams=(),
                  data=''):
         self.id = None
         self.type = parttype
-        self.data = data
+        self._data = data
         self.mandatoryparams = mandatoryparams
         self.advisoryparams = advisoryparams
         # status of the part's generation:
@@ -569,6 +573,15 b' class bundlepart(object):'
         # - True: generation done.
         self._generated = None
 
+    # methods used to defines the part content
+    def __setdata(self, data):
+        if self._generated is not None:
+            raise ReadOnlyPartError('part is being generated')
+        self._data = data
+    def __getdata(self):
+        return self._data
+    data = property(__getdata, __setdata)
+
     # methods used to generates the bundle2 stream
     def getchunks(self):
         if self._generated is not None:

tests/test-bundle2.t

@@ -84,8 +84,9 b' Create an extension to test bundle2 API'
   >         bundler.newpart('b2x:replycaps', data=capsstring)
   > 
   >     if opts['pushrace']:
-  >         dummynode = '01234567890123456789'
-  >         bundler.newpart('b2x:check:heads', data=dummynode)
+  >         # also serve to test the assignement of data outside of init
+  >         part = bundler.newpart('b2x:check:heads')
+  >         part.data = '01234567890123456789'
   > 
   >     revs = opts['rev']
   >     if 'rev' in opts: