client: fix send-after-close in broadcaster

client/structs/broadcaster.go

@@ -135,16 +135,16 @@ func (b *AllocBroadcaster) Listen() *AllocListener {
 
 	ch := make(chan *structs.Allocation, listenerCap)
 
-	// Broadcaster is already closed, close this listener
-	if b.closed {
-		close(ch)
-	}
-
 	// Send last update if there was one
 	if b.last != nil {
 		ch <- b.last
 	}
 
+	// Broadcaster is already closed, close this listener
+	if b.closed {
+		close(ch)
+	}
+
 	b.listeners[b.nextId] = ch
 
 	return &AllocListener{ch, b, b.nextId}

client/structs/broadcaster_test.go

@@ -184,3 +184,37 @@ func TestAllocBroadcaster_PrimeListener(t *testing.T) {
 		t.Fatalf("expected to recieve initial value")
 	}
 }
+
+// TestAllocBroadcaster_Closed asserts that newly created listeners are
+// primed with the last sent alloc even when the broadcaster is closed.
+func TestAllocBroadcaster_Closed(t *testing.T) {
+	t.Parallel()
+
+	b := NewAllocBroadcaster(testlog.HCLogger(t))
+
+	alloc := mock.Alloc()
+
+	// Send an update before creating a listener
+	require.NoError(t, b.Send(alloc))
+
+	// Close the broadcaster after sending a single update
+	b.Close()
+
+	// Create a listener and assert it immediately receives an update
+	l := b.Listen()
+	defer l.Close()
+	select {
+	case recv := <-l.Ch():
+		require.Equal(t, alloc, recv)
+	case <-time.After(10 * time.Millisecond):
+		t.Fatalf("expected to recieve initial value")
+	}
+
+	// Ch should now be closed.
+	select {
+	case _, ok := <-l.Ch():
+		require.False(t, ok)
+	case <-time.After(10 * time.Millisecond):
+		t.Fatalf("expected Ch() to be closed")
+	}
+}