From 5e7dec014a1415fcb9eec8d8103b07e7efd28d7c Mon Sep 17 00:00:00 2001
From: Joachim Bauch <bauch@struktur.de>
Date: Thu, 19 Jan 2023 15:34:28 +0100
Subject: [PATCH] Add helper class for channel waiters.

---
 channel_waiter.go      | 62 +++++++++++++++++++++++++++++++++++++++
 channel_waiter_test.go | 66 ++++++++++++++++++++++++++++++++++++++++++
 2 files changed, 128 insertions(+)
 create mode 100644 channel_waiter.go
 create mode 100644 channel_waiter_test.go

diff --git a/channel_waiter.go b/channel_waiter.go
new file mode 100644
index 0000000..20b0883
--- /dev/null
+++ b/channel_waiter.go
@@ -0,0 +1,62 @@
+/**
+ * Standalone signaling server for the Nextcloud Spreed app.
+ * Copyright (C) 2023 struktur AG
+ *
+ * @author Joachim Bauch <bauch@struktur.de>
+ *
+ * @license GNU AGPL version 3 or any later version
+ *
+ * This program is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU Affero General Public License as published by
+ * the Free Software Foundation, either version 3 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU Affero General Public License for more details.
+ *
+ * You should have received a copy of the GNU Affero General Public License
+ * along with this program.  If not, see <http://www.gnu.org/licenses/>.
+ */
+package signaling
+
+import (
+	"sync"
+)
+
+type ChannelWaiters struct {
+	mu      sync.RWMutex
+	id      uint64
+	waiters map[uint64]chan struct{}
+}
+
+func (w *ChannelWaiters) Wakeup() {
+	w.mu.RLock()
+	defer w.mu.RUnlock()
+	for _, ch := range w.waiters {
+		select {
+		case ch <- struct{}{}:
+		default:
+			// Receiver is still processing previous wakeup.
+		}
+	}
+}
+
+func (w *ChannelWaiters) Add(ch chan struct{}) uint64 {
+	w.mu.Lock()
+	defer w.mu.Unlock()
+	if w.waiters == nil {
+		w.waiters = make(map[uint64]chan struct{})
+	}
+	id := w.id
+	w.id++
+	w.waiters[id] = ch
+	return id
+}
+
+func (w *ChannelWaiters) Remove(id uint64) {
+	w.mu.Lock()
+	defer w.mu.Unlock()
+	delete(w.waiters, id)
+}
diff --git a/channel_waiter_test.go b/channel_waiter_test.go
new file mode 100644
index 0000000..e401ae8
--- /dev/null
+++ b/channel_waiter_test.go
@@ -0,0 +1,66 @@
+/**
+ * Standalone signaling server for the Nextcloud Spreed app.
+ * Copyright (C) 2023 struktur AG
+ *
+ * @author Joachim Bauch <bauch@struktur.de>
+ *
+ * @license GNU AGPL version 3 or any later version
+ *
+ * This program is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU Affero General Public License as published by
+ * the Free Software Foundation, either version 3 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU Affero General Public License for more details.
+ *
+ * You should have received a copy of the GNU Affero General Public License
+ * along with this program.  If not, see <http://www.gnu.org/licenses/>.
+ */
+package signaling
+
+import (
+	"testing"
+)
+
+func TestChannelWaiters(t *testing.T) {
+	var waiters ChannelWaiters
+
+	ch1 := make(chan struct{}, 1)
+	id1 := waiters.Add(ch1)
+	defer waiters.Remove(id1)
+
+	ch2 := make(chan struct{}, 1)
+	id2 := waiters.Add(ch2)
+	defer waiters.Remove(id2)
+
+	waiters.Wakeup()
+	<-ch1
+	<-ch2
+
+	select {
+	case <-ch1:
+		t.Error("should have not received another event")
+	case <-ch2:
+		t.Error("should have not received another event")
+	default:
+	}
+
+	ch3 := make(chan struct{}, 1)
+	id3 := waiters.Add(ch3)
+	waiters.Remove(id3)
+
+	// Multiple wakeups work even without processing.
+	waiters.Wakeup()
+	waiters.Wakeup()
+	waiters.Wakeup()
+	<-ch1
+	<-ch2
+	select {
+	case <-ch3:
+		t.Error("should have not received another event")
+	default:
+	}
+}