revlog: highlight current incompatibility in `rewrite_sidedata`

See comment for details. We will need to fix the test coverage when this
incompatibility is lifted.

Differential Revision: https://phab.mercurial-scm.org/D10544

mercurial/revlog.py

@@ -3082,6 +3082,18 b' class revlog(object):'
             # Nothing to generate or remove
             return
 
+        # changelog implement some "delayed" writing mechanism that assume that
+        # all index data is writen in append mode and is therefor incompatible
+        # with the seeked write done in this method. The use of such "delayed"
+        # writing will soon be removed for revlog version that support side
+        # data, so for now, we only keep this simple assert to highlight the
+        # situation.
+        delayed = getattr(self, '_delayed', False)
+        diverted = getattr(self, '_divert', False)
+        if delayed and not diverted:
+            msg = "cannot rewrite_sidedata of a delayed revlog"
+            raise error.ProgrammingError(msg)
+
         new_entries = []
         # append the new sidedata
         with self._datafp(b'a+') as fp: