docker: kill signal API should include timeout context

When the Docker driver kills as task, we send a request via the Docker API for
dockerd to fire the signal. We send that signal and then block for the
`kill_timeout` waiting for the container to exit. But if the Docker API
blocks, we will block indefinitely because we haven't configured the API call
with the same timeout.

This changeset is a minimal intervention to add the timeout to the Docker API
call _only_ when we have the `kill_timeout` set. Future work should examine
whether we should be threading contexts through other `go-dockerclient` API
calls.

drivers/docker/driver.go

@@ -1461,7 +1461,7 @@ func (d *Driver) SignalTask(taskID string, signal string) error {
 		return fmt.Errorf("failed to parse signal: %v", err)
 	}
 
-	return h.Signal(sig)
+	return h.Signal(context.Background(), sig)
 }
 
 func (d *Driver) ExecTask(taskID string, cmd []string, timeout time.Duration) (*drivers.ExecTaskResult, error) {

drivers/docker/handle.go

@@ -103,7 +103,7 @@ func (h *taskHandle) Exec(ctx context.Context, cmd string, args []string) (*driv
 	return execResult, nil
 }
 
-func (h *taskHandle) Signal(s os.Signal) error {
+func (h *taskHandle) Signal(ctx context.Context, s os.Signal) error {
 	// Convert types
 	sysSig, ok := s.(syscall.Signal)
 	if !ok {
@@ -116,8 +116,9 @@ func (h *taskHandle) Signal(s os.Signal) error {
 
 	dockerSignal := docker.Signal(sysSig)
 	opts := docker.KillContainerOptions{
-		ID:     h.containerID,
-		Signal: dockerSignal,
+		ID:      h.containerID,
+		Signal:  dockerSignal,
+		Context: ctx,
 	}
 	return h.client.KillContainer(opts)
 
@@ -127,7 +128,10 @@ func (h *taskHandle) Signal(s os.Signal) error {
 func (h *taskHandle) Kill(killTimeout time.Duration, signal os.Signal) error {
 	// Only send signal if killTimeout is set, otherwise stop container
 	if killTimeout > 0 {
-		if err := h.Signal(signal); err != nil {
+		ctx, cancel := context.WithTimeout(context.Background(), killTimeout)
+		defer cancel()
+
+		if err := h.Signal(ctx, signal); err != nil {
 			// Container has already been removed.
 			if strings.Contains(err.Error(), NoSuchContainerError) {
 				h.logger.Debug("attempted to signal nonexistent container")
@@ -146,7 +150,7 @@ func (h *taskHandle) Kill(killTimeout time.Duration, signal os.Signal) error {
 		select {
 		case <-h.waitCh:
 			return nil
-		case <-time.After(killTimeout):
+		case <-ctx.Done():
 		}
 	}