1 files changed, 90 insertions, 0 deletions
diff --git a/pkg/com/cond.go b/pkg/com/cond.go
new file mode 100644
index 0000000..72ba347
--- /dev/null
+++ b/pkg/com/cond.go
@@ -0,0 +1,90 @@
+package com
+
+import (
+	"context"
+	"github.com/pkg/errors"
+)
+
+// Cond implements a channel-based synchronization for goroutines that wait for signals or send them.
+// Internally based on a controller loop that handles the synchronization of new listeners and signal propagation,
+// which is only started when NewCond is called. Thus the zero value cannot be used.
+type Cond struct {
+	broadcast chan struct{}
+	done      chan struct{}
+	cancel    context.CancelFunc
+	listeners chan chan struct{}
+}
+
+// NewCond returns a new Cond and starts the controller loop.
+func NewCond(ctx context.Context) *Cond {
+	ctx, cancel := context.WithCancel(ctx)
+
+	c := &Cond{
+		broadcast: make(chan struct{}),
+		cancel:    cancel,
+		done:      make(chan struct{}),
+		listeners: make(chan chan struct{}),
+	}
+
+	go c.controller(ctx)
+
+	return c
+}
+
+// Broadcast sends a signal to all current listeners by closing the previously returned channel from Wait.
+// Panics if the controller loop has already ended.
+func (c *Cond) Broadcast() {
+	select {
+	case c.broadcast <- struct{}{}:
+	case <-c.done:
+		panic(errors.New("condition closed"))
+	}
+}
+
+// Close stops the controller loop, waits for it to finish, and returns an error if any.
+// Implements the io.Closer interface.
+func (c *Cond) Close() error {
+	c.cancel()
+	<-c.done
+
+	return nil
+}
+
+// Done returns a channel that will be closed when the controller loop has ended.
+func (c *Cond) Done() <-chan struct{} {
+	return c.done
+}
+
+// Wait returns a channel that is closed with the next signal.
+// Panics if the controller loop has already ended.
+func (c *Cond) Wait() <-chan struct{} {
+	select {
+	case l := <-c.listeners:
+		return l
+	case <-c.done:
+		panic(errors.New("condition closed"))
+	}
+}
+
+// controller loop.
+func (c *Cond) controller(ctx context.Context) {
+	defer close(c.done)
+
+	// Note that the notify channel does not close when the controller loop ends
+	// in order not to notify pending listeners.
+	notify := make(chan struct{})
+
+	for {
+		select {
+		case <-c.broadcast:
+			// Close channel to notify all current listeners.
+			close(notify)
+			// Create a new channel for the next listeners.
+			notify = make(chan struct{})
+		case c.listeners <- notify:
+			// A new listener received the channel.
+		case <-ctx.Done():
+			return
+		}
+	}
+}