bundle2: safely read unpack data from part header

We use the same approach that the other unpack, as function is given the struct
format and his both responsible for reading the right amount of data from the
header and unpack the struct.

This give use flexibility if we decide to change the size of something in the
format before the release.

mercurial/bundle2.py

@@ -292,16 +292,22 b' class unbundle20(object):'
             data = headerblock[offset:(offset + size)]
             self._offset = offset + size
             return data
-        typesize = _unpack(_fparttypesize, fromheader(1))[0]
+        def unpackheader(format):
+            """read given format from header
+
+            This automatically compute the size of the format to read."""
+            data = fromheader(struct.calcsize(format))
+            return _unpack(format, data)
+
+        typesize = unpackheader(_fparttypesize)[0]
         parttype = fromheader(typesize)
         self.ui.debug('part type: "%s"\n' % parttype)
         ## reading parameters
         # param count
-        mancount, advcount = _unpack(_fpartparamcount, fromheader(2))
+        mancount, advcount = unpackheader(_fpartparamcount)
         self.ui.debug('part parameters: %i\n' % (mancount + advcount))
         # param size
-        paramsizes = _unpack(_makefpartparamsizes(mancount + advcount),
-                             fromheader(2*(mancount + advcount)))
+        paramsizes = unpackheader(_makefpartparamsizes(mancount + advcount))
         # make it a list of couple again
         paramsizes = zip(paramsizes[::2], paramsizes[1::2])
         # split mandatory from advisory