Collect the sent_to_us_directly code together

We can now properly separate the two arms of the `if sent_to_us_directly`
condition.

Author: richvdh

changelog.d/10615.misc

@@ -0,0 +1 @@
+Clean up some of the federation event authentication code for clarity.

synapse/handlers/federation.py

@@ -326,8 +326,32 @@ async def on_receive_pdu(
                     if not missing_prevs:
                         logger.info("Found all missing prev_events")
 
-            if missing_prevs:
-                # We've still not been able to get all of the prev_events for this event.
+                if missing_prevs:
+                    # since this event was pushed to us, it is possible for it to
+                    # become the only forward-extremity in the room, and we would then
+                    # trust its state to be the state for the whole room. This is very
+                    # bad. Further, if the event was pushed to us, there is no excuse
+                    # for us not to have all the prev_events. (XXX: apart from
+                    # min_depth?)
+                    #
+                    # We therefore reject any such events.
+                    logger.warning(
+                        "Rejecting: failed to fetch %d prev events: %s",
+                        len(missing_prevs),
+                        shortstr(missing_prevs),
+                    )
+                    raise FederationError(
+                        "ERROR",
+                        403,
+                        (
+                            "Your server isn't divulging details about prev_events "
+                            "referenced in this event."
+                        ),
+                        affected=pdu.event_id,
+                    )
+
+            else:
+                # We don't have all of the prev_events for this event.
                 #
                 # In this case, we need to fall back to asking another server in the
                 # federation for the state at this event. That's ok provided we then
@@ -336,37 +360,15 @@ async def on_receive_pdu(
                 # withholding its prev_events, and declaring yourself to be an admin in
                 # the subsequent state request).
                 #
-                # Now, if we're pulling this event as a missing prev_event, then clearly
+                # Since we're pulling this event as a missing prev_event, then clearly
                 # this event is not going to become the only forward-extremity and we are
                 # guaranteed to resolve its state against our existing forward
                 # extremities, so that should be fine.
                 #
-                # On the other hand, if this event was pushed to us, it is possible for
-                # it to become the only forward-extremity in the room, and we would then
-                # trust its state to be the state for the whole room. This is very bad.
-                # Further, if the event was pushed to us, there is no excuse for us not to
-                # have all the prev_events. We therefore reject any such events.
-                #
                 # XXX this really feels like it could/should be merged with the above,
                 # but there is an interaction with min_depth that I'm not really
                 # following.
 
-                if sent_to_us_directly:
-                    logger.warning(
-                        "Rejecting: failed to fetch %d prev events: %s",
-                        len(missing_prevs),
-                        shortstr(missing_prevs),
-                    )
-                    raise FederationError(
-                        "ERROR",
-                        403,
-                        (
-                            "Your server isn't divulging details about prev_events "
-                            "referenced in this event."
-                        ),
-                        affected=pdu.event_id,
-                    )
-
                 logger.info(
                     "Event %s is missing prev_events %s: calculating state for a "
                     "backwards extremity",