Use a custom FIFO queue for the RS input API (#1888)

* Use a FIFO queue instead of a channel to reduce backpressure

* Make sure someone wakes up

* Tweaks

* Add comments

roomserver/internal/input/input.go

@@ -38,8 +38,7 @@ type Inputer struct {
 	ServerName           gomatrixserverlib.ServerName
 	ACLs                 *acls.ServerACLs
 	OutputRoomEventTopic string
-
-	workers sync.Map // room ID -> *inputWorker
+	workers              sync.Map // room ID -> *inputWorker
 }
 
 type inputTask struct {
@@ -52,15 +51,19 @@ type inputTask struct {
 type inputWorker struct {
 	r       *Inputer
 	running atomic.Bool
-	input   chan *inputTask
+	input   *fifoQueue
 }
 
 // Guarded by a CAS on w.running
 func (w *inputWorker) start() {
 	defer w.running.Store(false)
 	for {
 		select {
-		case task := <-w.input:
+		case <-w.input.wait():
+			task, ok := w.input.pop()
+			if !ok {
+				continue
+			}
 			hooks.Run(hooks.KindNewEventReceived, task.event.Event)
 			_, task.err = w.r.processRoomEvent(task.ctx, task.event)
 			if task.err == nil {
@@ -143,7 +146,7 @@ func (r *Inputer) InputRoomEvents(
 		// room - the channel will be quite small as it's just pointer types.
 		w, _ := r.workers.LoadOrStore(roomID, &inputWorker{
 			r:     r,
-			input: make(chan *inputTask, 32),
+			input: newFIFOQueue(),
 		})
 		worker := w.(*inputWorker)
 
@@ -160,7 +163,7 @@ func (r *Inputer) InputRoomEvents(
 		if worker.running.CAS(false, true) {
 			go worker.start()
 		}
-		worker.input <- tasks[i]
+		worker.input.push(tasks[i])
 	}
 
 	// Wait for all of the workers to return results about our tasks.

roomserver/internal/input/input_fifo.go

@@ -0,0 +1,64 @@
+package input
+
+import (
+	"sync"
+)
+
+type fifoQueue struct {
+	tasks  []*inputTask
+	count  int
+	mutex  sync.Mutex
+	notifs chan struct{}
+}
+
+func newFIFOQueue() *fifoQueue {
+	q := &fifoQueue{
+		notifs: make(chan struct{}, 1),
+	}
+	return q
+}
+
+func (q *fifoQueue) push(frame *inputTask) {
+	q.mutex.Lock()
+	defer q.mutex.Unlock()
+	q.tasks = append(q.tasks, frame)
+	q.count++
+	select {
+	case q.notifs <- struct{}{}:
+	default:
+	}
+}
+
+// pop returns the first item of the queue, if there is one.
+// The second return value will indicate if a task was returned.
+// You must check this value, even after calling wait().
+func (q *fifoQueue) pop() (*inputTask, bool) {
+	q.mutex.Lock()
+	defer q.mutex.Unlock()
+	if q.count == 0 {
+		return nil, false
+	}
+	frame := q.tasks[0]
+	q.tasks[0] = nil
+	q.tasks = q.tasks[1:]
+	q.count--
+	if q.count == 0 {
+		// Force a GC of the underlying array, since it might have
+		// grown significantly if the queue was hammered for some reason
+		q.tasks = nil
+	}
+	return frame, true
+}
+
+// wait returns a channel which can be used to detect when an
+// item is waiting in the queue.
+func (q *fifoQueue) wait() <-chan struct{} {
+	q.mutex.Lock()
+	defer q.mutex.Unlock()
+	if q.count > 0 && len(q.notifs) == 0 {
+		ch := make(chan struct{})
+		close(ch)
+		return ch
+	}
+	return q.notifs
+}