dbus/methods: observe context for Cancellation to remove job listener

Oberserve the given context from the function e.g. StartUnitContext,
which are using the internal function startJob. If this context is done
remove the job Listener if existing.

Fixes coreos#428

dbus/methods.go

@@ -67,12 +67,27 @@ func (c *Conn) startJob(ctx context.Context, ch chan<- string, job string, args
 		c.jobListener.jobs[p] = ch
 	}
 
+	go c.observeContextCancellation(ctx, p)
+
 	// ignore error since 0 is fine if conversion fails
 	jobID, _ := strconv.Atoi(path.Base(string(p)))
 
 	return jobID, nil
 }
 
+func (c *Conn) observeContextCancellation(ctx context.Context, jobId dbus.ObjectPath) {
+	<-ctx.Done()
+
+	// remove job listener if one exists
+	c.jobListener.Lock()
+	defer c.jobListener.Unlock()
+
+	_, ok := c.jobListener.jobs[jobId]
+	if ok {
+		delete(c.jobListener.jobs, jobId)
+	}
+}
+
 // Deprecated: use StartUnitContext instead.
 func (c *Conn) StartUnit(name string, mode string, ch chan<- string) (int, error) {
 	return c.StartUnitContext(context.Background(), name, mode, ch)