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_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
+}