TUN-1562: Refactor connectedSignal to be safe to close multiple times

signal/safe_signal.go

@@ -0,0 +1,33 @@
+package signal
+
+import (
+	"sync"
+)
+
+// Signal lets goroutines signal that some event has occurred. Other goroutines can wait for the signal.
+type Signal struct {
+	ch   chan struct{}
+	once sync.Once
+}
+
+// New wraps a channel and turns it into a signal for a one-time event.
+func New(ch chan struct{}) *Signal {
+	return &Signal{
+		ch:   ch,
+		once: sync.Once{},
+	}
+}
+
+// Notify alerts any goroutines waiting on this signal that the event has occurred.
+// After the first call to Notify(), future calls are no-op.
+func (s *Signal) Notify() {
+	s.once.Do(func() {
+		close(s.ch)
+	})
+}
+
+// Wait returns a channel which will be written to when Notify() is called for the first time.
+// This channel will never be written to a second time.
+func (s *Signal) Wait() <-chan struct{} {
+	return s.ch
+}

signal/safe_signal_test.go

@@ -0,0 +1,25 @@
+package signal
+
+import (
+	"testing"
+)
+
+func TestMultiNotifyDoesntCrash(t *testing.T) {
+	sig := New(make(chan struct{}))
+	sig.Notify()
+	sig.Notify()
+	// If code has reached here without crashing, the test has passed.
+}
+
+func TestWait(t *testing.T) {
+	sig := New(make(chan struct{}))
+	sig.Notify()
+	select {
+	case <-sig.Wait():
+		// Test succeeds
+		return
+	default:
+		// sig.Wait() should have been read from, because sig.Notify() wrote to it.
+		t.Fail()
+	}
+}