refactor some things, add back one mutex for each subscription for dispatching events, but in a cleaner way.

relay.go

@@ -186,9 +186,6 @@ func (r *Relay) Connect(ctx context.Context) error {
 	// ping every 29 seconds
 	ticker := time.NewTicker(29 * time.Second)
 
-	// this ensures we don't send an event to the Events channel after closing it
-	eventsChannelCloserMutex := &sync.Mutex{}
-
 	// to be used when the connection is closed
 	go func() {
 		<-r.connectionContext.Done()
@@ -230,16 +227,6 @@ func (r *Relay) Connect(ctx context.Context) error {
 		}
 	}()
 
-	// every time a subscription ends we use this queue to close its .Events channel
-	go func() {
-		for toClose := range r.subscriptionChannelCloseQueue {
-			eventsChannelCloserMutex.Lock()
-			close(toClose.Events)
-			toClose.Events = make(chan *Event)
-			eventsChannelCloserMutex.Unlock()
-		}
-	}()
-
 	// general message reader loop
 	go func() {
 		for {
@@ -298,19 +285,19 @@ func (r *Relay) Connect(ctx context.Context) error {
 						}
 					}
 
-					go func() {
-						eventsChannelCloserMutex.Lock()
-						if subscription.live {
-							subscription.Events <- &env.Event
-						}
-						eventsChannelCloserMutex.Unlock()
-					}()
+					// dispatch this to the internal .events channel of the subscription
+					subscription.events <- &env.Event
 				}
 			case *EOSEEnvelope:
 				if subscription, ok := r.Subscriptions.Load(string(*env)); ok {
-					subscription.emitEose.Do(func() {
+					// implementation adapted from the naïve/incorrect implementation of sync.Once
+					// (which is ok for this use case)
+					if subscription.eosed.CompareAndSwap(false, true) {
+						go func() {
+							time.Sleep(time.Millisecond) // this basically ensures the EndOfStoredEvents call happens after the last EVENT
 							close(subscription.EndOfStoredEvents)
-					})
+						}()
+					}
 				}
 			case *OKEnvelope:
 				if okCallback, exist := r.okCallbacks.Load(env.EventID); exist {
@@ -475,8 +462,10 @@ func (r *Relay) PrepareSubscription(ctx context.Context, filters Filters, opts .
 		cancel:             cancel,
 		counter:            int(current),
 		Events:             make(chan *Event),
+		events:             make(chan *Event),
 		EndOfStoredEvents:  make(chan struct{}),
 		Filters:            filters,
+		closeEventsChannel: make(chan struct{}),
 	}
 
 	for _, opt := range opts {
@@ -487,14 +476,10 @@ func (r *Relay) PrepareSubscription(ctx context.Context, filters Filters, opts .
 	}
 
 	id := sub.GetID()
-
 	r.Subscriptions.Store(id, sub)
 
-	// the subscription ends once the context is canceled
-	go func() {
-		<-sub.Context.Done()
-		sub.Unsub()
-	}()
+	// start handling events, eose, unsub etc:
+	go sub.start()
 
 	return sub
 }

subscription.go

@@ -5,6 +5,7 @@ import (
 	"fmt"
 	"strconv"
 	"sync"
+	"sync/atomic"
 )
 
 type Subscription struct {
@@ -17,6 +18,7 @@ type Subscription struct {
 	// the Events channel emits all EVENTs that come in a Subscription
 	// will be closed when the subscription ends
 	Events chan *Event
+	events chan *Event // underlines the above, this one is never closed
 
 	// the EndOfStoredEvents channel gets closed when an EOSE comes for that subscription
 	EndOfStoredEvents chan struct{}
@@ -24,9 +26,10 @@ type Subscription struct {
 	// Context will be .Done() when the subscription ends
 	Context context.Context
 
-	live     bool
+	live               atomic.Bool
+	eosed              atomic.Bool
+	closeEventsChannel chan struct{}
 	cancel             context.CancelFunc
-	emitEose sync.Once
 }
 
 type EventMessage struct {
@@ -54,17 +57,48 @@ func (sub *Subscription) GetID() string {
 	return sub.label + ":" + strconv.Itoa(sub.counter)
 }
 
+func (sub *Subscription) start() {
+	var mu sync.Mutex
+
+	for {
+		select {
+		case event := <-sub.events:
+			// this is guarded such that it will only fire until the .Events channel is closed
+			go func() {
+				mu.Lock()
+				if sub.live.Load() {
+					sub.Events <- event
+				}
+				mu.Unlock()
+			}()
+		case <-sub.Context.Done():
+			// the subscription ends once the context is canceled
+			sub.Unsub()
+			return
+		case <-sub.closeEventsChannel:
+			// this is called only once on .Unsub() and closes the .Events channel
+			mu.Lock()
+			close(sub.Events)
+			mu.Unlock()
+			return
+		}
+	}
+}
+
 // Unsub closes the subscription, sending "CLOSE" to relay as in NIP-01.
 // Unsub() also closes the channel sub.Events and makes a new one.
 func (sub *Subscription) Unsub() {
-	go sub.Close()
+	sub.cancel()
 
-	sub.live = false
+	// naïve sync.Once implementation:
+	if sub.live.CompareAndSwap(true, false) {
+		go sub.Close()
 		id := sub.GetID()
 		sub.Relay.Subscriptions.Delete(id)
 
 		// do this so we don't have the possibility of closing the Events channel and then trying to send to it
-	sub.Relay.subscriptionChannelCloseQueue <- sub
+		close(sub.closeEventsChannel)
+	}
 }
 
 // Close just sends a CLOSE message. You probably want Unsub() instead.
@@ -92,7 +126,7 @@ func (sub *Subscription) Fire() error {
 	reqb, _ := ReqEnvelope{id, sub.Filters}.MarshalJSON()
 	debugLog("{%s} sending %v", sub.Relay.URL, reqb)
 
-	sub.live = true
+	sub.live.Store(true)
 	if err := <-sub.Relay.Write(reqb); err != nil {
 		sub.cancel()
 		return fmt.Errorf("failed to write: %w", err)