Correctly generate backpagination tokens for events which have the same depth (#996)

* Correctly generate backpagination tokens for events which have the same depth

With tests. Unfortunately the code around here is hard to understand.
There will be a subsequent PR which fixes this up now that we have a test
harness in place.

* Add postgres impl

* More linting

* Fix psql statement so it actually works

syncapi/storage/storage_test.go

@@ -182,7 +182,7 @@ func TestSyncResponse(t *testing.T) {
 				// limit set to 5
 				return db.CompleteSync(ctx, testUserIDA, 5)
 			},
-			// want the last 5 events, NOT the last 10.
+			// want the last 5 events
 			WantTimeline: events[len(events)-5:],
 			// want all state for the room
 			WantState: state,
@@ -248,11 +248,11 @@ func TestGetEventsInRangeWithTopologyToken(t *testing.T) {
 	db := MustCreateDatabase(t)
 	events, _ := SimpleRoom(t, testRoomID, testUserIDA, testUserIDB)
 	MustWriteEvents(t, db, events)
-	latest, err := db.MaxTopologicalPosition(ctx, testRoomID)
+	latest, latestStream, err := db.MaxTopologicalPosition(ctx, testRoomID)
 	if err != nil {
 		t.Fatalf("failed to get MaxTopologicalPosition: %s", err)
 	}
-	from := types.NewPaginationTokenFromTypeAndPosition(types.PaginationTokenTypeTopology, latest, 0)
+	from := types.NewPaginationTokenFromTypeAndPosition(types.PaginationTokenTypeTopology, latest, latestStream)
 	// head towards the beginning of time
 	to := types.NewPaginationTokenFromTypeAndPosition(types.PaginationTokenTypeTopology, 0, 0)
 
@@ -265,6 +265,105 @@ func TestGetEventsInRangeWithTopologyToken(t *testing.T) {
 	assertEventsEqual(t, "", true, gots, reversed(events[len(events)-5:]))
 }
 
+// The purpose of this test is to make sure that backpagination returns all events, even if some events have the same depth.
+// For cases where events have the same depth, the streaming token should be used to tie break so events written via WriteEvent
+// will appear FIRST when going backwards. This test creates a DAG like:
+//                            .-----> Message ---.
+//     Create -> Membership --------> Message -------> Message
+//                            `-----> Message ---`
+// depth  1          2                   3                 4
+//
+// With a total depth of 4. It tests that:
+// - Backpagination over the whole fork should include all messages and not leave any out.
+// - Backpagination from the middle of the fork should not return duplicates (things later than the token).
+func TestGetEventsInRangeWithEventsSameDepth(t *testing.T) {
+	t.Parallel()
+	db := MustCreateDatabase(t)
+
+	var events []gomatrixserverlib.HeaderedEvent
+	events = append(events, MustCreateEvent(t, testRoomID, nil, &gomatrixserverlib.EventBuilder{
+		Content:  []byte(fmt.Sprintf(`{"room_version":"4","creator":"%s"}`, testUserIDA)),
+		Type:     "m.room.create",
+		StateKey: &emptyStateKey,
+		Sender:   testUserIDA,
+		Depth:    int64(len(events) + 1),
+	}))
+	events = append(events, MustCreateEvent(t, testRoomID, []gomatrixserverlib.HeaderedEvent{events[len(events)-1]}, &gomatrixserverlib.EventBuilder{
+		Content:  []byte(fmt.Sprintf(`{"membership":"join"}`)),
+		Type:     "m.room.member",
+		StateKey: &testUserIDA,
+		Sender:   testUserIDA,
+		Depth:    int64(len(events) + 1),
+	}))
+	// fork the dag into three, same prev_events and depth
+	parent := []gomatrixserverlib.HeaderedEvent{events[len(events)-1]}
+	depth := int64(len(events) + 1)
+	for i := 0; i < 3; i++ {
+		events = append(events, MustCreateEvent(t, testRoomID, parent, &gomatrixserverlib.EventBuilder{
+			Content: []byte(fmt.Sprintf(`{"body":"Message A %d"}`, i+1)),
+			Type:    "m.room.message",
+			Sender:  testUserIDA,
+			Depth:   depth,
+		}))
+	}
+	// merge the fork, prev_events are all 3 messages, depth is increased by 1.
+	events = append(events, MustCreateEvent(t, testRoomID, events[len(events)-3:], &gomatrixserverlib.EventBuilder{
+		Content: []byte(fmt.Sprintf(`{"body":"Message merge"}`)),
+		Type:    "m.room.message",
+		Sender:  testUserIDA,
+		Depth:   depth + 1,
+	}))
+	MustWriteEvents(t, db, events)
+	latestPos, latestStreamPos, err := db.EventPositionInTopology(ctx, events[len(events)-1].EventID())
+	if err != nil {
+		t.Fatalf("failed to get EventPositionInTopology: %s", err)
+	}
+	topoPos, streamPos, err := db.EventPositionInTopology(ctx, events[len(events)-3].EventID()) // Message 2
+	if err != nil {
+		t.Fatalf("failed to get EventPositionInTopology for event: %s", err)
+	}
+	fromLatest := types.NewPaginationTokenFromTypeAndPosition(types.PaginationTokenTypeTopology, latestPos, latestStreamPos)
+	fromFork := types.NewPaginationTokenFromTypeAndPosition(types.PaginationTokenTypeTopology, topoPos, streamPos)
+	// head towards the beginning of time
+	to := types.NewPaginationTokenFromTypeAndPosition(types.PaginationTokenTypeTopology, 0, 0)
+
+	testCases := []struct {
+		Name  string
+		From  *types.PaginationToken
+		Limit int
+		Wants []gomatrixserverlib.HeaderedEvent
+	}{
+		{
+			Name:  "Pagination over the whole fork",
+			From:  fromLatest,
+			Limit: 5,
+			Wants: reversed(events[len(events)-5:]),
+		},
+		{
+			Name:  "Paginating to the middle of the fork",
+			From:  fromLatest,
+			Limit: 2,
+			Wants: reversed(events[len(events)-2:]),
+		},
+		{
+			Name:  "Pagination FROM the middle of the fork",
+			From:  fromFork,
+			Limit: 3,
+			Wants: reversed(events[len(events)-5 : len(events)-2]),
+		},
+	}
+
+	for _, tc := range testCases {
+		// backpaginate messages starting at the latest position.
+		paginatedEvents, err := db.GetEventsInRange(ctx, tc.From, to, testRoomID, tc.Limit, true)
+		if err != nil {
+			t.Fatalf("%s GetEventsInRange returned an error: %s", tc.Name, err)
+		}
+		gots := gomatrixserverlib.HeaderedToClientEvents(db.StreamEventsToEvents(&testUserDeviceA, paginatedEvents), gomatrixserverlib.FormatAll)
+		assertEventsEqual(t, tc.Name, true, gots, tc.Wants)
+	}
+}
+
 func assertEventsEqual(t *testing.T, msg string, checkRoomID bool, gots []gomatrixserverlib.ClientEvent, wants []gomatrixserverlib.HeaderedEvent) {
 	if len(gots) != len(wants) {
 		t.Fatalf("%s response returned %d events, want %d", msg, len(gots), len(wants))
@@ -294,7 +393,8 @@ func assertEventsEqual(t *testing.T, msg string, checkRoomID bool, gots []gomatr
 			t.Errorf("%s event[%d] unsigned mismatch: got %s want %s", msg, i, string(g.Unsigned), string(w.Unsigned()))
 		}
 		if (g.StateKey == nil && w.StateKey() != nil) || (g.StateKey != nil && w.StateKey() == nil) {
-			t.Fatalf("%s event[%d] state_key [not] missing: got %v want %v", msg, i, g.StateKey, w.StateKey())
+			t.Errorf("%s event[%d] state_key [not] missing: got %v want %v", msg, i, g.StateKey, w.StateKey())
+			continue
 		}
 		if g.StateKey != nil {
 			if !w.StateKeyEquals(*g.StateKey) {