add --ephemeral mode for persistent worker

When passing `--ephemeral` to `cirrus worker run`, the worker will
accept one task and then exit the process once the task completed.

This can be used inside ephemeral VMs which should be shutdown after
each task. A user has to take care of cleaning up after the worker
has finished.

PERSISTENT-WORKERS.md

@@ -28,6 +28,18 @@ Note that persistent worker's name should be unique within a pool.
 
 Note that by default a persistent worker has the privileges of the user that invoked it. Read more [about isolation](#isolation) below to learn how to limit or extend persistent worker privileges.
 
+### Ephemeral Mode
+
+The worker can be started in ephemeral mode with:
+
+```
+cirrus worker run --token <poll registration token> --ephemeral
+```
+
+After having completed a single task, the worker process will exit. This can be used in ephemeral setups where the enviroment should be cleaned after each run.
+
+Note that users of `--ephemeral` need to take care of cleaning the enviroment and caching build inputs themselves.
+
 ## Configuration
 
 Path to the YAML configuration can be specified via the `--file` (or `-f` for short version) command-line flag.
@@ -38,6 +50,7 @@ Example configuration:
 token: e3b0c44298fc1c149afbf4c8996fb92427ae41e4649b934ca495991b7852b855
 
 name: "MacMini-Rack-1-Slot-2"
+ephemeral: false
 
 labels:
   connected-device: iPhone12ProMax

internal/commands/worker/config.go

@@ -23,8 +23,9 @@ import (
 var ErrConfiguration = errors.New("configuration error")
 
 type Config struct {
-	Name  string `yaml:"name"`
-	Token string `yaml:"token"`
+	Name      string `yaml:"name"`
+	Token     string `yaml:"token"`
+	Ephemeral bool   `yaml:"ephemeral"`
 
 	Labels    map[string]string  `yaml:"labels"`
 	Resources map[string]float64 `yaml:"resources"`
@@ -67,6 +68,7 @@ var (
 	labels      map[string]string
 	resources   map[string]string
 	rpcEndpoint string
+	ephemeral   bool
 )
 
 func attachFlags(cmd *cobra.Command) {
@@ -88,13 +90,16 @@ func attachFlags(cmd *cobra.Command) {
 		"additional resources to use (e.g. --resources devices=2)")
 	cmd.PersistentFlags().StringVar(&rpcEndpoint, "rpc-endpoint", upstream.DefaultRPCEndpoint,
 		"RPC endpoint address")
+	cmd.PersistentFlags().BoolVar(&ephemeral, "ephemeral", false,
+		"run a single task and then exit the process")
 }
 
 func parseConfig(path string) (*Config, error) {
 	// Instantiate a default configuration
 	config := Config{
 		Name:      name,
 		Token:     token,
+		Ephemeral: ephemeral,
 		Labels:    labels,
 		Resources: map[string]float64{},
 		RPC: ConfigRPC{
@@ -231,6 +236,10 @@ func buildWorker(output io.Writer) (*worker.Worker, error) {
 		opts = append(opts, worker.WithTartPrePull(config.TartPrePull))
 	}
 
+	if ephemeral {
+		opts = append(opts, worker.WithEphemeral(ephemeral))
+	}
+
 	// Instantiate worker
 	return worker.New(opts...)
 }

internal/commands/worker/config_test.go

@@ -70,4 +70,13 @@ func TestRestrictForceSoftnet(t *testing.T) {
 	require.NoError(t, err)
 
 	require.True(t, config.Security.AllowedIsolations.Tart.ForceSoftnet)
+	// ephemeral is not set, it should default to false
+	require.False(t, config.Ephemeral)
+}
+
+func TestEphemeral(t *testing.T) {
+	config, err := parseConfig(filepath.Join("testdata", "ephemeral.yml"))
+	require.NoError(t, err)
+
+	require.True(t, config.Ephemeral)
 }

internal/commands/worker/testdata/ephemeral.yml

@@ -0,0 +1,4 @@
+token: e3b0c44298fc1c149afbf4c8996fb92427ae41e4649b934ca495991b7852b855
+
+name: "ephemeral-runner"
+ephemeral: true

internal/worker/options.go

@@ -56,3 +56,9 @@ func WithTartPrePull(tartPrePull []string) Option {
 		e.tartPrePull = tartPrePull
 	}
 }
+
+func WithEphemeral(ephemeral bool) Option {
+	return func(e *Worker) {
+		e.ephemeral = ephemeral
+	}
+}

internal/worker/task.go

@@ -35,8 +35,8 @@ func (worker *Worker) startTask(
 	ctx context.Context,
 	upstream *upstreampkg.Upstream,
 	agentAwareTask *api.PollResponse_AgentAwareTask,
-) {
-	taskID := agentAwareTask.TaskId
+) (taskID string) {
+	taskID = agentAwareTask.TaskId
 	if taskID == "" {
 		taskID = fmt.Sprintf("%d", agentAwareTask.OldTaskId)
 	}
@@ -75,6 +75,8 @@ func (worker *Worker) startTask(
 		taskID, agentAwareTask.ClientSecret, agentAwareTask.ServerSecret)
 
 	worker.logger.Infof("started task %s", taskID)
+
+	return
 }
 
 func (worker *Worker) getInstance(

internal/worker/worker.go

@@ -57,6 +57,10 @@ type Worker struct {
 	standbyInstance abstract.Instance
 
 	tartPrePull []string
+
+	// In ephemeral mode, only run a single task and then exit
+	ephemeral       bool
+	ephemeralTaskID string
 }
 
 func New(opts ...Option) (*Worker, error) {
@@ -73,6 +77,8 @@ func New(opts ...Option) (*Worker, error) {
 
 		logger:        logrus.New(),
 		echelonLogger: echelon.NewLogger(echelon.TraceLevel, renderers.NewSimpleRenderer(os.Stdout, nil)),
+
+		ephemeralTaskID: "",
 	}
 
 	// Apply options
@@ -309,7 +315,15 @@ func (worker *Worker) pollSingleUpstream(ctx context.Context, upstream *upstream
 	}
 
 	for _, taskToStart := range response.TasksToStart {
-		worker.startTask(ctx, upstream, taskToStart)
+		// don't start a new task in ephemeral mode if one is running
+		if worker.ephemeral && worker.ephemeralTaskID != "" {
+			break
+		}
+		taskID := worker.startTask(ctx, upstream, taskToStart)
+		if worker.ephemeral {
+			worker.ephemeralTaskID = taskID
+			break
+		}
 	}
 
 	if response.Shutdown {
@@ -319,6 +333,13 @@ func (worker *Worker) pollSingleUpstream(ctx context.Context, upstream *upstream
 		return ErrShutdown
 	}
 
+	if worker.ephemeral && worker.ephemeralTaskID != "" {
+		if _, ok := worker.tasks.Load(worker.ephemeralTaskID); !ok {
+			worker.logger.Infof("In ephemeral mode: task %s completed, terminating...", worker.ephemeralTaskID)
+			return ErrShutdown
+		}
+	}
+
 	return nil
 }