Fix historical messages backfilling in random order on remote homeservers (MSC2716) (#11114)

Fix https://github.com/matrix-org/synapse/issues/11091
Fix https://github.com/matrix-org/synapse/issues/10764 (side-stepping the issue because we no longer have to deal with `fake_prev_event_id`)

 1. Made the `/backfill` response return messages in `(depth, stream_ordering)` order (previously only sorted by `depth`)
    - Technically, it shouldn't really matter how `/backfill` returns things but I'm just trying to make the `stream_ordering` a little more consistent from the origin to the remote homeservers in order to get the order of messages from `/messages` consistent ([sorted by `(topological_ordering, stream_ordering)`](https://github.com/matrix-org/synapse/blob/develop/docs/development/room-dag-concepts.md#depth-and-stream-ordering)).
    - Even now that we return backfilled messages in order, it still doesn't guarantee the same `stream_ordering` (and more importantly the [`/messages` order](https://github.com/matrix-org/synapse/blob/develop/docs/development/room-dag-concepts.md#depth-and-stream-ordering)) on the other server. For example, if a room has a bunch of history imported and someone visits a permalink to a historical message back in time, their homeserver will skip over the historical messages in between and insert the permalink as the next message in the `stream_order` and totally throw off the sort.
       - This will be even more the case when we add the [MSC3030 jump to date API endpoint](https://github.com/matrix-org/matrix-doc/pull/3030) so the static archives can navigate and jump to a certain date.
       - We're solving this in the future by switching to [online topological ordering](https://github.com/matrix-org/gomatrixserverlib/issues/187) and [chunking](https://github.com/matrix-org/synapse/issues/3785) which by its nature will apply retroactively to fix any inconsistencies introduced by people permalinking
 2. As we're navigating `prev_events` to return in `/backfill`, we order by `depth` first (newest -> oldest) and now also tie-break based on the `stream_ordering` (newest -> oldest). This is technically important because MSC2716 inserts a bunch of historical messages at the same `depth` so it's best to be prescriptive about which ones we should process first. In reality, I think the code already looped over the historical messages as expected because the database is already in order.
 3. Making the historical state chain and historical event chain float on their own by having no `prev_events` instead of a fake `prev_event` which caused backfill to get clogged with an unresolvable event. Fixes https://github.com/matrix-org/synapse/issues/11091 and https://github.com/matrix-org/synapse/issues/10764
 4. We no longer find connected insertion events by finding a potential `prev_event` connection to the current event we're iterating over. We now solely rely on marker events which when processed, add the insertion event as an extremity and the federating homeserver can ask about it when time calls.
    - Related discussion, https://github.com/matrix-org/synapse/pull/11114#discussion_r741514793


Before | After
--- | ---
![](https://user-images.githubusercontent.com/558581/139218681-b465c862-5c49-4702-a59e-466733b0cf45.png) | ![](https://user-images.githubusercontent.com/558581/146453159-a1609e0a-8324-439d-ae44-e4bce43ac6d1.png)



#### Why aren't we sorting topologically when receiving backfill events?

> The main reason we're going to opt to not sort topologically when receiving backfill events is because it's probably best to do whatever is easiest to make it just work. People will probably have opinions once they look at [MSC2716](https://github.com/matrix-org/matrix-doc/pull/2716) which could change whatever implementation anyway.
> 
> As mentioned, ideally we would do this but code necessary to make the fake edges but it gets confusing and gives an impression of “just whyyyy” (feels icky). This problem also dissolves with online topological ordering.
>
> -- https://github.com/matrix-org/synapse/pull/11114#discussion_r741517138

See https://github.com/matrix-org/synapse/pull/11114#discussion_r739610091 for the technical difficulties

1 files changed, 22 insertions, 22 deletions

diff --git a/synapse/handlers/room_batch.py b/synapse/handlers/room_batch.py
index f880aa93d2..f8137ec04c 100644
--- a/synapse/handlers/room_batch.py
+++ b/synapse/handlers/room_batch.py
@@ -13,10 +13,6 @@ if TYPE_CHECKING:
 logger = logging.getLogger(__name__)
 
 
-def generate_fake_event_id() -> str:
-    return "$fake_" + random_string(43)
-
-
 class RoomBatchHandler:
     def __init__(self, hs: "HomeServer"):
         self.hs = hs
@@ -182,11 +178,12 @@ class RoomBatchHandler:
         state_event_ids_at_start = []
         auth_event_ids = initial_auth_event_ids.copy()
 
-        # Make the state events float off on their own so we don't have a
-        # bunch of `@mxid joined the room` noise between each batch
-        prev_event_id_for_state_chain = generate_fake_event_id()
+        # Make the state events float off on their own by specifying no
+        # prev_events for the first one in the chain so we don't have a bunch of
+        # `@mxid joined the room` noise between each batch.
+        prev_event_ids_for_state_chain: List[str] = []
 
-        for state_event in state_events_at_start:
+        for index, state_event in enumerate(state_events_at_start):
             assert_params_in_dict(
                 state_event, ["type", "origin_server_ts", "content", "sender"]
             )
@@ -222,7 +219,10 @@ class RoomBatchHandler:
                     content=event_dict["content"],
                     outlier=True,
                     historical=True,
-                    prev_event_ids=[prev_event_id_for_state_chain],
+                    # Only the first event in the chain should be floating.
+                    # The rest should hang off each other in a chain.
+                    allow_no_prev_events=index == 0,
+                    prev_event_ids=prev_event_ids_for_state_chain,
                     # Make sure to use a copy of this list because we modify it
                     # later in the loop here. Otherwise it will be the same
                     # reference and also update in the event when we append later.
@@ -242,7 +242,10 @@ class RoomBatchHandler:
                     event_dict,
                     outlier=True,
                     historical=True,
-                    prev_event_ids=[prev_event_id_for_state_chain],
+                    # Only the first event in the chain should be floating.
+                    # The rest should hang off each other in a chain.
+                    allow_no_prev_events=index == 0,
+                    prev_event_ids=prev_event_ids_for_state_chain,
                     # Make sure to use a copy of this list because we modify it
                     # later in the loop here. Otherwise it will be the same
                     # reference and also update in the event when we append later.
@@ -253,7 +256,7 @@ class RoomBatchHandler:
             state_event_ids_at_start.append(event_id)
             auth_event_ids.append(event_id)
             # Connect all the state in a floating chain
-            prev_event_id_for_state_chain = event_id
+            prev_event_ids_for_state_chain = [event_id]
 
         return state_event_ids_at_start
 
@@ -261,7 +264,6 @@ class RoomBatchHandler:
         self,
         events_to_create: List[JsonDict],
         room_id: str,
-        initial_prev_event_ids: List[str],
         inherited_depth: int,
         auth_event_ids: List[str],
         app_service_requester: Requester,
@@ -277,9 +279,6 @@ class RoomBatchHandler:
             events_to_create: List of historical events to create in JSON
                 dictionary format.
             room_id: Room where you want the events persisted in.
-            initial_prev_event_ids: These will be the prev_events for the first
-                event created. Each event created afterwards will point to the
-                previous event created.
             inherited_depth: The depth to create the events at (you will
                 probably by calling inherit_depth_from_prev_ids(...)).
             auth_event_ids: Define which events allow you to create the given
@@ -291,11 +290,14 @@ class RoomBatchHandler:
         """
         assert app_service_requester.app_service
 
-        prev_event_ids = initial_prev_event_ids.copy()
+        # Make the historical event chain float off on its own by specifying no
+        # prev_events for the first event in the chain which causes the HS to
+        # ask for the state at the start of the batch later.
+        prev_event_ids: List[str] = []
 
         event_ids = []
         events_to_persist = []
-        for ev in events_to_create:
+        for index, ev in enumerate(events_to_create):
             assert_params_in_dict(ev, ["type", "origin_server_ts", "content", "sender"])
 
             assert self.hs.is_mine_id(ev["sender"]), "User must be our own: %s" % (
@@ -319,6 +321,9 @@ class RoomBatchHandler:
                     ev["sender"], app_service_requester.app_service
                 ),
                 event_dict,
+                # Only the first event in the chain should be floating.
+                # The rest should hang off each other in a chain.
+                allow_no_prev_events=index == 0,
                 prev_event_ids=event_dict.get("prev_events"),
                 auth_event_ids=auth_event_ids,
                 historical=True,
@@ -370,7 +375,6 @@ class RoomBatchHandler:
         events_to_create: List[JsonDict],
         room_id: str,
         batch_id_to_connect_to: str,
-        initial_prev_event_ids: List[str],
         inherited_depth: int,
         auth_event_ids: List[str],
         app_service_requester: Requester,
@@ -385,9 +389,6 @@ class RoomBatchHandler:
             room_id: Room where you want the events created in.
             batch_id_to_connect_to: The batch_id from the insertion event you
                 want this batch to connect to.
-            initial_prev_event_ids: These will be the prev_events for the first
-                event created. Each event created afterwards will point to the
-                previous event created.
             inherited_depth: The depth to create the events at (you will
                 probably by calling inherit_depth_from_prev_ids(...)).
             auth_event_ids: Define which events allow you to create the given
@@ -436,7 +437,6 @@ class RoomBatchHandler:
         event_ids = await self.persist_historical_events(
             events_to_create=events_to_create,
             room_id=room_id,
-            initial_prev_event_ids=initial_prev_event_ids,
             inherited_depth=inherited_depth,
             auth_event_ids=auth_event_ids,
             app_service_requester=app_service_requester,