Don't wait on handling unsent events during startup

src/github.com/matrix-org/dendrite/common/linearizer.go

@@ -43,14 +43,14 @@ func (l *Linearizer) Await(key string, callback func()) {
 // testing as any functions scheduled after hook has been closed are guaranteed
 // to be run after this callback has finished. If hook is nil then it is
 // ignored.
-func (l *Linearizer) AwaitWithHook(key string, callback func(), hook chan<- struct{}) {
+func (l *Linearizer) AwaitWithHook(key string, callback func(), hook func()) {
 	closeChannel := make(chan struct{})
 	defer close(closeChannel)
 
 	awaitChannel := l.getAndSetLastMutex(key, closeChannel)
 
 	if hook != nil {
-		close(hook)
+		hook()
 	}
 
 	if awaitChannel != nil {

src/github.com/matrix-org/dendrite/common/linearizer_test.go

@@ -96,7 +96,7 @@ func TestMultipleConcurrentLinearizer(t *testing.T) { // nolint: gocyclo
 			if numThirdCalls != 0 {
 				t.Fatalf("Expected third function to not be called, called %d times", numThirdCalls)
 			}
-		}, setupAwait2)
+		}, func() { close(setupAwait2) })
 
 		t.Log("Finished waiting on w1")
 		waitGroup.Done()
@@ -118,7 +118,7 @@ func TestMultipleConcurrentLinearizer(t *testing.T) { // nolint: gocyclo
 			if numThirdCalls != 0 {
 				t.Fatalf("Expected third function to not be called, called %d times", numThirdCalls)
 			}
-		}, setupAwait3)
+		}, func() { close(setupAwait3) })
 
 		t.Log("Finished waiting on w2")
 
@@ -141,7 +141,7 @@ func TestMultipleConcurrentLinearizer(t *testing.T) { // nolint: gocyclo
 			if numThirdCalls != 1 {
 				t.Fatalf("Expected third function to be called once, called %d times", numThirdCalls)
 			}
-		}, startSignal)
+		}, func() { close(startSignal) })
 
 		t.Log("Finished waiting on w3")
 		waitGroup.Done()

src/github.com/matrix-org/dendrite/roomserver/input/latest_events.go

@@ -17,6 +17,7 @@ package input
 import (
 	"bytes"
 	"context"
+	"sync"
 
 	"github.com/pkg/errors"
 
@@ -68,8 +69,13 @@ func (e *EventSender) Start(ctx context.Context) (err error) {
 		eventToRoomMap[roomNID] = append(eventToRoomMap[roomNID], entries[i])
 	}
 
+	// We only wait until we have registered all the rooms with the linearizer,
+	// as we don't want to block startup on handling them.
+	var wg sync.WaitGroup
+	wg.Add(len(eventToRoomMap))
+
 	for roomNID, roomEntries := range eventToRoomMap {
-		e.Linearizer.Await(roomEntries[0].Event.RoomID(), func() {
+		e.Linearizer.AwaitWithHook(roomEntries[0].Event.RoomID(), func() {
 			for i := range roomEntries {
 				err = updateLatestEvents(
 					ctx, e.DB, e.OutputWriter, roomNID,
@@ -82,9 +88,11 @@ func (e *EventSender) Start(ctx context.Context) (err error) {
 					return
 				}
 			}
-		})
+		}, wg.Done)
 	}
 
+	wg.Wait()
+
 	return
 }