Fix bug in state handling where we incorrectly identified a missing pdu. Update tests to catch this case.

2 files changed, 49 insertions, 52 deletions

diff --git a/synapse/state.py b/synapse/state.py
index 5dcff27367..e69282860a 100644
--- a/synapse/state.py
+++ b/synapse/state.py
@@ -134,7 +134,9 @@ class StateHandler(object):
     @defer.inlineCallbacks
     @log_function
     def _handle_new_state(self, new_pdu):
-        tree = yield self.store.get_unresolved_state_tree(new_pdu)
+        tree, missing_branch = yield self.store.get_unresolved_state_tree(
+            new_pdu
+        )
         new_branch, current_branch = tree
 
         logger.debug(
@@ -142,6 +144,28 @@ class StateHandler(object):
             new_branch, current_branch
         )
 
+        if missing_branch is not None:
+            # We're missing some PDUs. Fetch them.
+            # TODO (erikj): Limit this.
+            missing_prev = tree[missing_branch][-1]
+
+            pdu_id = missing_prev.prev_state_id
+            origin = missing_prev.prev_state_origin
+
+            is_missing = yield self.store.get_pdu(pdu_id, origin) is None
+            if not is_missing:
+                raise Exception("Conflict resolution failed")
+
+            yield self._replication.get_pdu(
+                destination=missing_prev.origin,
+                pdu_origin=origin,
+                pdu_id=pdu_id,
+                outlier=True
+            )
+
+            updated_current = yield self._handle_new_state(new_pdu)
+            defer.returnValue(updated_current)
+
         if not current_branch:
             # There is no current state
             defer.returnValue(True)
@@ -151,65 +175,35 @@ class StateHandler(object):
         c = current_branch[-1]
 
         if n.pdu_id == c.pdu_id and n.origin == c.origin:
-            # We have all the PDUs we need, so we can just do the conflict
-            # resolution.
+            # We found a common ancestor!
 
             if len(current_branch) == 1:
                 # This is a direct clobber so we can just...
                 defer.returnValue(True)
 
-            conflict_res = [
-                self._do_power_level_conflict_res,
-                self._do_chain_length_conflict_res,
-                self._do_hash_conflict_res,
-            ]
-
-            for algo in conflict_res:
-                new_res, curr_res = algo(new_branch, current_branch)
-
-                if new_res < curr_res:
-                    defer.returnValue(False)
-                elif new_res > curr_res:
-                    defer.returnValue(True)
-
-            raise Exception("Conflict resolution failed.")
-
         else:
-            # We need to ask for PDUs.
-            missing_prev = max(
-                new_branch[-1], current_branch[-1],
-                key=lambda x: x.depth
-            )
-
-            if not hasattr(missing_prev, "prev_state_id"):
-                # FIXME Hmm
-                # temporary fallback
-                for algo in conflict_res:
-                    new_res, curr_res = algo(new_branch, current_branch)
-
-                    if new_res < curr_res:
-                        defer.returnValue(False)
-                    elif new_res > curr_res:
-                        defer.returnValue(True)
-                return
+            # We didn't find a common ancestor. This is probably fine.
+            pass
 
-            pdu_id = missing_prev.prev_state_id
-            origin = missing_prev.prev_state_origin
+        result = self._do_conflict_res(new_branch, current_branch)
+        defer.returnValue(result)
 
-            is_missing = yield self.store.get_pdu(pdu_id, origin) is None
+    def _do_conflict_res(self, new_branch, current_branch):
+        conflict_res = [
+            self._do_power_level_conflict_res,
+            self._do_chain_length_conflict_res,
+            self._do_hash_conflict_res,
+        ]
 
-            if not is_missing:
-                raise Exception("Conflict resolution failed.")
+        for algo in conflict_res:
+            new_res, curr_res = algo(new_branch, current_branch)
 
-            yield self._replication.get_pdu(
-                destination=missing_prev.origin,
-                pdu_origin=origin,
-                pdu_id=pdu_id,
-                outlier=True
-            )
+            if new_res < curr_res:
+                defer.returnValue(False)
+            elif new_res > curr_res:
+                defer.returnValue(True)
 
-            updated_current = yield self._handle_new_state(new_pdu)
-            defer.returnValue(updated_current)
+        raise Exception("Conflict resolution failed.")
 
     def _do_power_level_conflict_res(self, new_branch, current_branch):
         max_power_new = max(
diff --git a/synapse/storage/pdu.py b/synapse/storage/pdu.py
index 0bf97e37ee..3cbce2d0a1 100644
--- a/synapse/storage/pdu.py
+++ b/synapse/storage/pdu.py
@@ -308,8 +308,8 @@ class PduStore(SQLBaseStore):
 
     @defer.inlineCallbacks
     def get_oldest_pdus_in_context(self, context):
-        """Get a list of Pdus that we haven't backfilled beyond yet (and haven't    
-        seen). This list is used when we want to backfill backwards and is the 
+        """Get a list of Pdus that we haven't backfilled beyond yet (and havent
+        seen). This list is used when we want to backfill backwards and is the
         list we send to the remote server.
 
         Args:
@@ -524,13 +524,16 @@ class StatePduStore(SQLBaseStore):
             txn, new_pdu, current
         )
 
+        missing_branch = None
         for branch, prev_state, state in enum_branches:
             if state:
                 return_value[branch].append(state)
             else:
+                # We don't have prev_state :(
+                missing_branch = branch
                 break
 
-        return return_value
+        return (return_value, missing_branch)
 
     def update_current_state(self, pdu_id, origin, context, pdu_type,
                              state_key):