Some review comments

roomserver/internal/input/input.go

@@ -102,7 +102,7 @@ func (r *Inputer) Start() error {
 				_ = msg.InProgress() // resets the acknowledgement wait timer
 				defer eventsInProgress.Delete(index)
 				defer roomserverInputBackpressure.With(prometheus.Labels{"room_id": roomID}).Dec()
-				retry, err := r.processRoomEventUsingUpdater(context.Background(), roomID, &inputRoomEvent)
+				action, err := r.processRoomEventUsingUpdater(context.Background(), roomID, &inputRoomEvent)
 				if err != nil {
 					if !errors.Is(err, context.DeadlineExceeded) && !errors.Is(err, context.Canceled) {
 						sentry.CaptureException(err)
@@ -113,9 +113,10 @@ func (r *Inputer) Start() error {
 						"type":     inputRoomEvent.Event.Type(),
 					}).Warn("Roomserver failed to process async event")
 				}
-				if retry {
+				switch action {
+				case retryLater:
 					_ = msg.Nak()
-				} else {
+				case doNotRetry:
 					_ = msg.Ack()
 				}
 			})
@@ -137,35 +138,41 @@ func (r *Inputer) Start() error {
 	return err
 }
 
+type retryAction int
+
+const (
+	doNotRetry retryAction = iota
+	retryLater
+)
+
 // processRoomEventUsingUpdater opens up a room updater and tries to
-// process the event. It returns two values: the bool signifying whether
+// process the event. It returns whether or not we should positively
-// we should retry later if possible (i.e. using NATS, because we couldn't
+// or negatively acknowledge the event (i.e. for NATS) and an error
-// commit the transaction) and an error signifying anything else that may
+// if it occurred.
-// have gone wrong.
 func (r *Inputer) processRoomEventUsingUpdater(
 	ctx context.Context,
 	roomID string,
 	inputRoomEvent *api.InputRoomEvent,
-) (bool, error) {
+) (retryAction, error) {
 	roomInfo, err := r.DB.RoomInfo(ctx, roomID)
 	if err != nil {
-		return false, fmt.Errorf("r.DB.RoomInfo: %w", err)
+		return doNotRetry, fmt.Errorf("r.DB.RoomInfo: %w", err)
 	}
 	updater, err := r.DB.GetRoomUpdater(ctx, roomInfo)
 	if err != nil {
-		return true, fmt.Errorf("r.DB.GetRoomUpdater: %w", err)
+		return retryLater, fmt.Errorf("r.DB.GetRoomUpdater: %w", err)
 	}
 	commit, err := r.processRoomEvent(ctx, updater, inputRoomEvent)
 	if commit {
 		if cerr := updater.Commit(); cerr != nil {
-			return true, fmt.Errorf("updater.Commit: %w", cerr)
+			return retryLater, fmt.Errorf("updater.Commit: %w", cerr)
 		}
 	} else {
 		if rerr := updater.Rollback(); rerr != nil {
-			return true, fmt.Errorf("updater.Rollback: %w", rerr)
+			return retryLater, fmt.Errorf("updater.Rollback: %w", rerr)
 		}
 	}
-	return false, err
+	return doNotRetry, err
 }
 
 // InputRoomEvents implements api.RoomserverInternalAPI

roomserver/storage/interface.go

@@ -24,8 +24,8 @@ import (
 )
 
 type Database interface {
-	// Do we support processing input events for more than one room at a time?
+	// Do we support transactional isolation on this database engine?
-	SupportsConcurrentRoomInputs() bool
+	SupportsTransactionalIsolation() bool
 	// RoomInfo returns room information for the given room ID, or nil if there is no room.
 	RoomInfo(ctx context.Context, roomID string) (*types.RoomInfo, error)
 	// Store the room state at an event in the database

roomserver/storage/shared/room_updater.go

@@ -25,7 +25,16 @@ func rollback(txn *sql.Tx) {
 	txn.Rollback() // nolint: errcheck
 }
 
-func NewRoomUpdater(ctx context.Context, d *Database, txn *sql.Tx, roomInfo *types.RoomInfo) (*RoomUpdater, error) {
+func NewRoomUpdater(ctx context.Context, d *Database, roomInfo *types.RoomInfo) (*RoomUpdater, error) {
+	var txn *sql.Tx
+	if d.SupportsTransactionalIsolation() {
+		var err error
+		txn, err = d.DB.Begin()
+		if err != nil {
+			return nil, err
+		}
+	}
+
 	// If the roomInfo is nil then that means that the room doesn't exist
 	// yet, so we can't do `SelectLatestEventsNIDsForUpdate` because that
 	// would involve locking a row on the table that doesn't exist. Instead

roomserver/storage/shared/storage.go

@@ -45,7 +45,7 @@ type Database struct {
 	GetRoomUpdaterFn    func(ctx context.Context, roomInfo *types.RoomInfo) (*RoomUpdater, error)
 }
 
-func (d *Database) SupportsConcurrentRoomInputs() bool {
+func (d *Database) SupportsTransactionalIsolation() bool {
 	return true
 }
 
@@ -447,7 +447,7 @@ func (d *Database) events(
 	if err != nil {
 		return nil, err
 	}
-	eventIDs, _ := d.EventsTable.BulkSelectEventID(ctx, nil, eventNIDs)
+	eventIDs, _ := d.EventsTable.BulkSelectEventID(ctx, txn, eventNIDs)
 	if err != nil {
 		eventIDs = map[types.EventNID]string{}
 	}
@@ -501,16 +501,15 @@ func (d *Database) MembershipUpdater(
 	ctx context.Context, roomID, targetUserID string,
 	targetLocal bool, roomVersion gomatrixserverlib.RoomVersion,
 ) (*MembershipUpdater, error) {
-	txn, err := d.DB.Begin()
+	var txn *sql.Tx
+	var err error
+	if d.SupportsTransactionalIsolation() {
+		txn, err = d.DB.Begin()
 		if err != nil {
 			return nil, err
 		}
-	var updater *MembershipUpdater
+	}
-	_ = d.Writer.Do(d.DB, txn, func(txn *sql.Tx) error {
+	return NewMembershipUpdater(ctx, d, txn, roomID, targetUserID, targetLocal, roomVersion)
-		updater, err = NewMembershipUpdater(ctx, d, txn, roomID, targetUserID, targetLocal, roomVersion)
-		return err
-	})
-	return updater, err
 }
 
 func (d *Database) GetRoomUpdater(
@@ -519,16 +518,7 @@ func (d *Database) GetRoomUpdater(
 	if d.GetRoomUpdaterFn != nil {
 		return d.GetRoomUpdaterFn(ctx, roomInfo)
 	}
-	txn, err := d.DB.Begin()
+	return NewRoomUpdater(ctx, d, roomInfo)
-	if err != nil {
-		return nil, err
-	}
-	var updater *RoomUpdater
-	_ = d.Writer.Do(d.DB, txn, func(txn *sql.Tx) error {
-		updater, err = NewRoomUpdater(ctx, d, txn, roomInfo)
-		return err
-	})
-	return updater, err
 }
 
 func (d *Database) StoreEvent(

roomserver/storage/sqlite3/storage.go

@@ -23,7 +23,6 @@ import (
 	"github.com/matrix-org/dendrite/internal/sqlutil"
 	"github.com/matrix-org/dendrite/roomserver/storage/shared"
 	"github.com/matrix-org/dendrite/roomserver/storage/sqlite3/deltas"
-	"github.com/matrix-org/dendrite/roomserver/types"
 	"github.com/matrix-org/dendrite/setup/config"
 	"github.com/matrix-org/gomatrixserverlib"
 )
@@ -193,7 +192,7 @@ func (d *Database) prepare(db *sql.DB, cache caching.RoomServerCaches) error {
 	return nil
 }
 
-func (d *Database) SupportsConcurrentRoomInputs() bool {
+func (d *Database) SupportsTransactionalIsolation() bool {
 	// This isn't supported in SQLite mode yet because of issues with
 	// database locks.
 	// TODO: Look at this again - the problem is probably to do with
@@ -201,18 +200,6 @@ func (d *Database) SupportsConcurrentRoomInputs() bool {
 	return false
 }
 
-func (d *Database) GetRoomUpdater(
-	ctx context.Context, roomInfo *types.RoomInfo,
-) (*shared.RoomUpdater, error) {
-	// TODO: Do not use transactions. We should be holding open this transaction but we cannot have
-	// multiple write transactions on sqlite. The code will perform additional
-	// write transactions independent of this one which will consistently cause
-	// 'database is locked' errors. As sqlite doesn't support multi-process on the
-	// same DB anyway, and we only execute updates sequentially, the only worries
-	// are for rolling back when things go wrong. (atomicity)
-	return shared.NewRoomUpdater(ctx, &d.Database, nil, roomInfo)
-}
-
 func (d *Database) MembershipUpdater(
 	ctx context.Context, roomID, targetUserID string,
 	targetLocal bool, roomVersion gomatrixserverlib.RoomVersion,