Implement incremental sync (#71)

This has the following known shortcomings:
 - It doesn't handle missing events not in the from/to range.
 - It doesn't order events in the timeline correctly.
 - It doesn't handle invited/left rooms at all.
 - There is no intelligent wakeup logic: events for Bob will wake up Alice's stream.

src/github.com/matrix-org/dendrite/syncserver/README.md

+# Sync Server
+
+This server is responsible for servicing `/sync` requests. It gets its data from the room server output log.
+
+## Internals
+
+When the server gets a `/sync` request, it needs to:
+ - Work out *which* rooms to return to the client.
+ - For each room, work out *which* events to return to the client.
+
+The logic for working out which rooms is based on [Synapse](https://github.com/matrix-org/synapse/blob/v0.19.3/synapse/handlers/sync.py#L821):
+  1) Get the CURRENT joined room list for this user.
+  2) Get membership list changes for this user between the provided stream position and now.
+  3) For each room which has membership list changes:
+     - Check if the room is 'newly joined' (insufficient to just check for a join event because we allow dupe joins).
+        If it is, then we need to send the full room state down (and 'limited' is always true).
+     - Check if user is still CURRENTLY invited to the room. If so, add room to 'invited' block.
+     - Check if the user is CURRENTLY left/banned. If so, add room to 'archived' block.
+  4) Add joined rooms (joined room list)
+
+For each room, the /sync response returns the most recent timeline events and the state of the room at the start of the timeline.
+The logic for working out *which* events is not based entirely on Synapse code, as it is known broken with respect to working out
+room state. In order to know which events to return, the server needs to calculate room state at various points in the history of
+the room. For example, imagine a room with the following 15 events (letters are state events (updated via `'`), numbers are timeline events):
+
+```
+index     0  1  2  3  4  5  6  7  8   9  10  11  12  13    14     15   (1-based indexing as StreamPosition(0) represents no event)
+timeline    [A, B, C, D, 1, 2, 3, D', 4, D'', 5, B', D''', D'''', 6]
+```
+
+The current state of this room is: `[A, B', C, D'''']`.
+
+If this room was requested with `?since=14&limit=5` then 1 timeline event would be returned, the most recent one:
+```
+    15
+   [ 6 ]
+```
+
+If this room was requested with `?since=9&limit=5` then 5 timeline events would be returned, the most recent ones:
+```
+    11 12  13    14     15
+   [5, B', D''', D'''', 6]
+```
+
+The state of the room at the START of the timeline can be represented in 2 ways:
+ - The `full_state` from index 0 : `[A, B, C, D'']` (aka the state between 0-11 exclusive)
+ - A partial state from index 9  : `[D'']`          (aka the state between 9-11 exclusive)
+
+Servers advance state events (e.g from `D'` to `D''`) based on the state conflict resolution algorithm.
+You might think that you could advance the current state by just updating the entry for the `(event type, state_key)` tuple
+for each state event, but this state can diverge from the state calculated using the state conflict resolution algorithm.
+For example, if there are two "simultaneous" updates to the same state key, that is two updates at the same depth in the
+event graph, then the final result of the state conflict resolution algorithm might not match the order the events appear
+in the timeline.
+
+The correct advancement for state events is represented by the `AddsStateEventIDs` and `RemovesStateEventIDs` that
+are in `OutputRoomEvents` from the room server.
+
+This version of the sync server uses very simple indexing to calculate room state at various points.
+This is inefficient when a very old `since` value is provided, or the `full_state` is requested, as the state delta becomes
+very large. This is mitigated slightly with indexes, but better data structures could be used in the future.

src/github.com/matrix-org/dendrite/syncserver/storage/output_room_events_table.go

@@ -4,7 +4,9 @@ import (
 	"database/sql"
 	"fmt"
 
+	log "github.com/Sirupsen/logrus"
 	"github.com/lib/pq"
+	"github.com/matrix-org/dendrite/syncserver/types"
 	"github.com/matrix-org/gomatrixserverlib"
 )
 
@@ -40,17 +42,24 @@ const selectEventsInRangeSQL = "" +
 	"SELECT event_json FROM output_room_events WHERE id > $1 AND id <= $2"
 
 const selectRecentEventsSQL = "" +
-	"SELECT event_json FROM output_room_events WHERE room_id = $1 ORDER BY id DESC LIMIT $2"
+	"SELECT event_json FROM output_room_events WHERE room_id = $1 AND id > $2 AND id <= $3 ORDER BY id DESC LIMIT $4"
 
 const selectMaxIDSQL = "" +
 	"SELECT MAX(id) FROM output_room_events"
 
+// In order for us to apply the state updates correctly, rows need to be ordered in the order they were received (id).
+const selectStateInRangeSQL = "" +
+	"SELECT event_json, add_state_ids, remove_state_ids FROM output_room_events" +
+	" WHERE (id > $1 AND id < $2) AND (add_state_ids IS NOT NULL OR remove_state_ids IS NOT NULL)" +
+	" ORDER BY id ASC"
+
 type outputRoomEventsStatements struct {
 	insertEventStmt         *sql.Stmt
 	selectEventsStmt        *sql.Stmt
 	selectMaxIDStmt         *sql.Stmt
 	selectEventsInRangeStmt *sql.Stmt
 	selectRecentEventsStmt  *sql.Stmt
+	selectStateInRangeStmt  *sql.Stmt
 }
 
 func (s *outputRoomEventsStatements) prepare(db *sql.DB) (err error) {
@@ -73,9 +82,102 @@ func (s *outputRoomEventsStatements) prepare(db *sql.DB) (err error) {
 	if s.selectRecentEventsStmt, err = db.Prepare(selectRecentEventsSQL); err != nil {
 		return
 	}
+	if s.selectStateInRangeStmt, err = db.Prepare(selectStateInRangeSQL); err != nil {
+		return
+	}
 	return
 }
 
+// StateBetween returns the state events between the two given stream positions, exclusive of both.
+// Results are bucketed based on the room ID. If the same state is overwritten multiple times between the
+// two positions, only the most recent state is returned.
+func (s *outputRoomEventsStatements) StateBetween(txn *sql.Tx, oldPos, newPos types.StreamPosition) (map[string][]gomatrixserverlib.Event, error) {
+	rows, err := txn.Stmt(s.selectStateInRangeStmt).Query(oldPos, newPos)
+	if err != nil {
+		return nil, err
+	}
+	// Fetch all the state change events for all rooms between the two positions then loop each event and:
+	//  - Keep a cache of the event by ID (99% of state change events are for the event itself)
+	//  - For each room ID, build up an array of event IDs which represents cumulative adds/removes
+	// For each room, map cumulative event IDs to events and return. This may need to a batch SELECT based on event ID
+	// if they aren't in the event ID cache. We don't handle state deletion yet.
+	eventIDToEvent := make(map[string]gomatrixserverlib.Event)
+
+	// RoomID => A set (map[string]bool) of state event IDs which are between the two positions
+	stateNeeded := make(map[string]map[string]bool)
+
+	for rows.Next() {
+		var (
+			eventBytes []byte
+			addIDs     pq.StringArray
+			delIDs     pq.StringArray
+		)
+		if err := rows.Scan(&eventBytes, &addIDs, &delIDs); err != nil {
+			return nil, err
+		}
+		// Sanity check for deleted state and whine if we see it. We don't need to do anything
+		// since it'll just mark the event as not being needed.
+		if len(addIDs) < len(delIDs) {
+			log.WithFields(log.Fields{
+				"since":   oldPos,
+				"current": newPos,
+				"adds":    addIDs,
+				"dels":    delIDs,
+			}).Warn("StateBetween: ignoring deleted state")
+		}
+
+		// TODO: Handle redacted events
+		ev, err := gomatrixserverlib.NewEventFromTrustedJSON(eventBytes, false)
+		if err != nil {
+			return nil, err
+		}
+		needSet := stateNeeded[ev.RoomID()]
+		if needSet == nil { // make set if required
+			needSet = make(map[string]bool)
+		}
+		for _, id := range delIDs {
+			needSet[id] = false
+		}
+		for _, id := range addIDs {
+			needSet[id] = true
+		}
+		stateNeeded[ev.RoomID()] = needSet
+
+		eventIDToEvent[ev.EventID()] = ev
+	}
+
+	stateBetween, missingEvents := mapEventIDsToEvents(eventIDToEvent, stateNeeded)
+
+	if len(missingEvents) > 0 {
+		return nil, fmt.Errorf("error StateBetween: TODO missing events")
+	}
+	return stateBetween, nil
+}
+
+// convert the set of event IDs into a set of events. Mark any which are missing.
+func mapEventIDsToEvents(eventIDToEvent map[string]gomatrixserverlib.Event, stateNeeded map[string]map[string]bool) (map[string][]gomatrixserverlib.Event, map[string][]string) {
+	stateBetween := make(map[string][]gomatrixserverlib.Event)
+	missingEvents := make(map[string][]string)
+	for roomID, ids := range stateNeeded {
+		events := stateBetween[roomID]
+		for id, need := range ids {
+			if !need {
+				continue // deleted state
+			}
+			e, ok := eventIDToEvent[id]
+			if ok {
+				events = append(events, e)
+			} else {
+				m := missingEvents[roomID]
+				m = append(m, id)
+				missingEvents[roomID] = m
+			}
+		}
+		stateBetween[roomID] = events
+	}
+	return stateBetween, missingEvents
+}
+
 // MaxID returns the ID of the last inserted event in this table. 'txn' is optional. If it is not supplied,
 // then this function should only ever be used at startup, as it will race with inserting events if it is
 // done afterwards. If there are no inserted events, 0 is returned.
@@ -92,27 +194,6 @@ func (s *outputRoomEventsStatements) MaxID(txn *sql.Tx) (id int64, err error) {
 	return
 }
 
-// InRange returns all the events in the range between oldPos exclusive and newPos inclusive. Returns an empty array if
-// there are no events between the provided range. Returns an error if events are missing in the range.
-func (s *outputRoomEventsStatements) InRange(oldPos, newPos int64) ([]gomatrixserverlib.Event, error) {
-	rows, err := s.selectEventsInRangeStmt.Query(oldPos, newPos)
-	if err != nil {
-		return nil, err
-	}
-	defer rows.Close()
-
-	result, err := rowsToEvents(rows)
-	if err != nil {
-		return nil, err
-	}
-	// Expect one event per position, exclusive of old. eg old=3, new=5, expect 4,5 so 2 events.
-	wantNum := int(newPos - oldPos)
-	if len(result) != wantNum {
-		return nil, fmt.Errorf("failed to map all positions to events: (got %d, wanted, %d)", len(result), wantNum)
-	}
-	return result, nil
-}
-
 // InsertEvent into the output_room_events table. addState and removeState are an optional list of state event IDs. Returns the position
 // of the inserted event.
 func (s *outputRoomEventsStatements) InsertEvent(txn *sql.Tx, event *gomatrixserverlib.Event, addState, removeState []string) (streamPos int64, err error) {
@@ -123,8 +204,8 @@ func (s *outputRoomEventsStatements) InsertEvent(txn *sql.Tx, event *gomatrixser
 }
 
 // RecentEventsInRoom returns the most recent events in the given room, up to a maximum of 'limit'.
-func (s *outputRoomEventsStatements) RecentEventsInRoom(txn *sql.Tx, roomID string, limit int) ([]gomatrixserverlib.Event, error) {
-	rows, err := s.selectRecentEventsStmt.Query(roomID, limit)
+func (s *outputRoomEventsStatements) RecentEventsInRoom(txn *sql.Tx, roomID string, fromPos, toPos types.StreamPosition, limit int) ([]gomatrixserverlib.Event, error) {
+	rows, err := s.selectRecentEventsStmt.Query(roomID, fromPos, toPos, limit)
 	if err != nil {
 		return nil, err
 	}

src/github.com/matrix-org/dendrite/syncserver/storage/syncserver.go

@@ -96,6 +96,36 @@ func (d *SyncServerDatabase) SyncStreamPosition() (types.StreamPosition, error)
 	return types.StreamPosition(id), nil
 }
 
+// IncrementalSync returns all the data needed in order to create an incremental sync response.
+func (d *SyncServerDatabase) IncrementalSync(userID string, fromPos, toPos types.StreamPosition, numRecentEventsPerRoom int) (data map[string]types.RoomData, returnErr error) {
+	data = make(map[string]types.RoomData)
+	returnErr = runTransaction(d.db, func(txn *sql.Tx) error {
+		roomIDs, err := d.roomstate.SelectRoomIDsWithMembership(txn, userID, "join")
+		if err != nil {
+			return err
+		}
+
+		state, err := d.events.StateBetween(txn, fromPos, toPos)
+		if err != nil {
+			return err
+		}
+
+		for _, roomID := range roomIDs {
+			recentEvents, err := d.events.RecentEventsInRoom(txn, roomID, fromPos, toPos, numRecentEventsPerRoom)
+			if err != nil {
+				return err
+			}
+			roomData := types.RoomData{
+				State:        state[roomID],
+				RecentEvents: recentEvents,
+			}
+			data[roomID] = roomData
+		}
+		return nil
+	})
+	return
+}
+
 // CompleteSync returns all the data needed in order to create a complete sync response.
 func (d *SyncServerDatabase) CompleteSync(userID string, numRecentEventsPerRoom int) (pos types.StreamPosition, data map[string]types.RoomData, returnErr error) {
 	data = make(map[string]types.RoomData)
@@ -121,7 +151,7 @@ func (d *SyncServerDatabase) CompleteSync(userID string, numRecentEventsPerRoom
 			}
 			// TODO: When filters are added, we may need to call this multiple times to get enough events.
 			//       See: https://github.com/matrix-org/synapse/blob/v0.19.3/synapse/handlers/sync.py#L316
-			recentEvents, err := d.events.RecentEventsInRoom(txn, roomID, numRecentEventsPerRoom)
+			recentEvents, err := d.events.RecentEventsInRoom(txn, roomID, types.StreamPosition(0), pos, numRecentEventsPerRoom)
 			if err != nil {
 				return err
 			}
@@ -135,11 +165,6 @@ func (d *SyncServerDatabase) CompleteSync(userID string, numRecentEventsPerRoom
 	return
 }
 
-// EventsInRange returns all events in the given range, exclusive of oldPos, inclusive of newPos.
-func (d *SyncServerDatabase) EventsInRange(oldPos, newPos types.StreamPosition) ([]gomatrixserverlib.Event, error) {
-	return d.events.InRange(int64(oldPos), int64(newPos))
-}
-
 func runTransaction(db *sql.DB, fn func(txn *sql.Tx) error) (err error) {
 	txn, err := db.Begin()
 	if err != nil {

src/github.com/matrix-org/dendrite/syncserver/sync/requestpool.go

@@ -139,31 +139,18 @@ func (rp *RequestPool) currentSyncForUser(req syncRequest) (*types.Response, err
 
 	// TODO: handle ignored users
 
-	// TODO: Implement https://github.com/matrix-org/synapse/blob/v0.19.3/synapse/handlers/sync.py#L821
-	// 1) Get the CURRENT joined room list for this user
-	// 2) Get membership list changes for this user between the provided stream position and now.
-	// 3) For each room which has membership list changes:
-	//   a) Check if the room is 'newly joined' (insufficient to just check for a join event because we allow dupe joins).
-	//      If it is, then we need to send the full room state down (and 'limited' is always true).
-	//   b) Check if user is still CURRENTLY invited to the room. If so, add room to 'invited' block.
-	//   c) Check if the user is CURRENTLY left/banned. If so, add room to 'archived' block. // Synapse has a TODO: How do we handle ban -> leave in same batch?
-	// 4) Add joined rooms (joined room list)
-
-	evs, err := rp.db.EventsInRange(req.since, currentPos)
+	data, err := rp.db.IncrementalSync(req.userID, req.since, currentPos, req.limit)
 	if err != nil {
 		return nil, err
 	}
 
 	res := types.NewResponse(currentPos)
-	// for now, dump everything as join timeline events
-	for _, ev := range evs {
-		roomData := res.Rooms.Join[ev.RoomID()]
-		roomData.Timeline.Events = append(roomData.Timeline.Events, gomatrixserverlib.ToClientEvent(ev, gomatrixserverlib.FormatSync))
-		res.Rooms.Join[ev.RoomID()] = roomData
+	for roomID, d := range data {
+		jr := types.NewJoinResponse()
+		jr.Timeline.Events = gomatrixserverlib.ToClientEvents(d.RecentEvents, gomatrixserverlib.FormatSync)
+		jr.Timeline.Limited = false // TODO: if len(events) >= numRecents + 1 and then set limited:true
+		jr.State.Events = gomatrixserverlib.ToClientEvents(d.State, gomatrixserverlib.FormatSync)
+		res.Rooms.Join[roomID] = *jr
 	}
-
-	// Make sure we send the next_batch as a string. We don't want to confuse clients by sending this
-	// as an integer even though (at the moment) it is.
-	res.NextBatch = currentPos.String()
 	return res, nil
 }

src/github.com/matrix-org/dendrite/syncserver/types/types.go

 package types
 
 import (
-	"github.com/matrix-org/gomatrixserverlib"
 	"strconv"
+
+	"github.com/matrix-org/gomatrixserverlib"
 )
 
 // StreamPosition represents the offset in the sync stream a client is at.
@@ -38,6 +39,8 @@ type Response struct {
 // NewResponse creates an empty response with initialised maps.
 func NewResponse(pos StreamPosition) *Response {
 	res := Response{}
+	// Make sure we send the next_batch as a string. We don't want to confuse clients by sending this
+	// as an integer even though (at the moment) it is.
 	res.NextBatch = pos.String()
 	// Pre-initalise the maps. Synapse will return {} even if there are no rooms under a specific section,
 	// so let's do the same thing. Bonus: this means we can't get dreaded 'assignment to entry in nil map' errors.