Add Driver.Prestart method

The Driver.Prestart method currently does very little but lays the
foundation for where lifecycle plugins can interleave execution _after_
task environment setup but _before_ the task starts.

Currently Prestart does two things:

* Any driver specific task environment building
* Download Docker images

This change also attaches a TaskEvent emitter to Drivers, so they can
emit events during task initialization.

api/tasks.go

@@ -252,6 +252,7 @@ const (
 	TaskSiblingFailed          = "Sibling task failed"
 	TaskSignaling              = "Signaling"
 	TaskRestartSignal          = "Restart Signaled"
+	TaskInitializing           = "Initializing"
 )
 
 // TaskEvent is an event that effects the state of a task and contains meta-data
@@ -278,4 +279,5 @@ type TaskEvent struct {
 	VaultError            string
 	TaskSignalReason      string
 	TaskSignal            string
+	InitializationMessage string
 }

client/client.go

@@ -792,7 +792,7 @@ func (c *Client) setupDrivers() error {
 
 	var avail []string
 	var skipped []string
-	driverCtx := driver.NewDriverContext("", c.config, c.config.Node, c.logger, nil)
+	driverCtx := driver.NewDriverContext("", c.config, c.config.Node, c.logger, nil, nil)
 	for name := range driver.BuiltinDrivers {
 		// Skip fingerprinting drivers that are not in the whitelist if it is
 		// enabled.

client/driver/docker.go

@@ -91,6 +91,10 @@ const (
 
 type DockerDriver struct {
 	DriverContext
+
+	imageID      string
+	waitClient   *docker.Client
+	driverConfig *DockerDriverConfig
 }
 
 type DockerDriverAuth struct {
@@ -339,31 +343,29 @@ func (d *DockerDriver) Abilities() DriverAbilities {
 	}
 }
 
-func (d *DockerDriver) Start(ctx *ExecContext, task *structs.Task) (DriverHandle, error) {
+func (d *DockerDriver) Prestart(ctx *ExecContext, task *structs.Task) error {
 	// Set environment variables.
 	d.taskEnv.SetAllocDir(allocdir.SharedAllocContainerPath).
 		SetTaskLocalDir(allocdir.TaskLocalContainerPath).SetSecretsDir(allocdir.TaskSecretsContainerPath).Build()
 
 	driverConfig, err := NewDockerDriverConfig(task, d.taskEnv)
 	if err != nil {
-		return nil, err
+		return err
 	}
 
-	cleanupImage := d.config.ReadBoolDefault("docker.cleanup.image", true)
-
 	taskDir, ok := ctx.AllocDir.TaskDirs[d.DriverContext.taskName]
 	if !ok {
-		return nil, fmt.Errorf("Could not find task directory for task: %v", d.DriverContext.taskName)
+		return fmt.Errorf("Could not find task directory for task: %v", d.DriverContext.taskName)
 	}
 
 	// Initialize docker API clients
 	client, waitClient, err := d.dockerClients()
 	if err != nil {
-		return nil, fmt.Errorf("Failed to connect to docker daemon: %s", err)
+		return fmt.Errorf("Failed to connect to docker daemon: %s", err)
 	}
 
 	if err := d.createImage(driverConfig, client, taskDir); err != nil {
-		return nil, err
+		return err
 	}
 
 	image := driverConfig.ImageName
@@ -371,14 +373,27 @@ func (d *DockerDriver) Start(ctx *ExecContext, task *structs.Task) (DriverHandle
 	dockerImage, err := client.InspectImage(image)
 	if err != nil {
 		d.logger.Printf("[ERR] driver.docker: failed getting image id for %s: %s", image, err)
-		return nil, fmt.Errorf("Failed to determine image id for `%s`: %s", image, err)
+		return fmt.Errorf("Failed to determine image id for `%s`: %s", image, err)
 	}
 	d.logger.Printf("[DEBUG] driver.docker: identified image %s as %s", image, dockerImage.ID)
 
+	// Set state needed by Start()
+	d.imageID = dockerImage.ID
+	d.waitClient = waitClient
+	d.driverConfig = driverConfig
+	return nil
+}
+
+func (d *DockerDriver) Start(ctx *ExecContext, task *structs.Task) (DriverHandle, error) {
 	bin, err := discover.NomadExecutable()
 	if err != nil {
 		return nil, fmt.Errorf("unable to find the nomad binary: %v", err)
 	}
+
+	taskDir, ok := ctx.AllocDir.TaskDirs[d.DriverContext.taskName]
+	if !ok {
+		return nil, fmt.Errorf("Could not find task directory for task: %v", d.DriverContext.taskName)
+	}
 	pluginLogFile := filepath.Join(taskDir, fmt.Sprintf("%s-executor.out", task.Name))
 	pluginConfig := &plugin.ClientConfig{
 		Cmd: exec.Command(bin, "executor", pluginLogFile),
@@ -404,9 +419,9 @@ func (d *DockerDriver) Start(ctx *ExecContext, task *structs.Task) (DriverHandle
 
 	// Only launch syslog server if we're going to use it!
 	syslogAddr := ""
-	if runtime.GOOS == "darwin" && len(driverConfig.Logging) == 0 {
+	if runtime.GOOS == "darwin" && len(d.driverConfig.Logging) == 0 {
 		d.logger.Printf("[DEBUG] driver.docker: disabling syslog driver as Docker for Mac workaround")
-	} else if len(driverConfig.Logging) == 0 || driverConfig.Logging[0].Type == "syslog" {
+	} else if len(d.driverConfig.Logging) == 0 || d.driverConfig.Logging[0].Type == "syslog" {
 		ss, err := exec.LaunchSyslogServer()
 		if err != nil {
 			pluginClient.Kill()
@@ -415,11 +430,11 @@ func (d *DockerDriver) Start(ctx *ExecContext, task *structs.Task) (DriverHandle
 		syslogAddr = ss.Addr
 	}
 
-	config, err := d.createContainerConfig(ctx, task, driverConfig, syslogAddr)
+	config, err := d.createContainerConfig(ctx, task, d.driverConfig, syslogAddr)
 	if err != nil {
-		d.logger.Printf("[ERR] driver.docker: failed to create container configuration for image %s: %s", image, err)
+		d.logger.Printf("[ERR] driver.docker: failed to create container configuration for image %s: %s", d.imageID, err)
 		pluginClient.Kill()
-		return nil, fmt.Errorf("Failed to create container configuration for image %s: %s", image, err)
+		return nil, fmt.Errorf("Failed to create container configuration for image %s: %s", d.imageID, err)
 	}
 
 	container, rerr := d.createContainer(config)
@@ -432,17 +447,17 @@ func (d *DockerDriver) Start(ctx *ExecContext, task *structs.Task) (DriverHandle
 
 	d.logger.Printf("[INFO] driver.docker: created container %s", container.ID)
 
+	cleanupImage := d.config.ReadBoolDefault("docker.cleanup.image", true)
+
 	// We don't need to start the container if the container is already running
 	// since we don't create containers which are already present on the host
 	// and are running
 	if !container.State.Running {
 		// Start the container
-		err := d.startContainer(container)
+		if err := client.StartContainer(container.ID, container.HostConfig); err != nil {
-		if err != nil {
 			d.logger.Printf("[ERR] driver.docker: failed to start container %s: %s", container.ID, err)
 			pluginClient.Kill()
-			err.Err = fmt.Sprintf("Failed to start container %s: %s", container.ID, err)
+			return nil, fmt.Errorf("Failed to start container %s: %s", container.ID, err)
-			return nil, err
 		}
 		d.logger.Printf("[INFO] driver.docker: started container %s", container.ID)
 	} else {
@@ -454,12 +469,12 @@ func (d *DockerDriver) Start(ctx *ExecContext, task *structs.Task) (DriverHandle
 	maxKill := d.DriverContext.config.MaxKillTimeout
 	h := &DockerHandle{
 		client:         client,
-		waitClient:     waitClient,
+		waitClient:     d.waitClient,
 		executor:       exec,
 		pluginClient:   pluginClient,
 		cleanupImage:   cleanupImage,
 		logger:         d.logger,
-		imageID:        dockerImage.ID,
+		imageID:        d.imageID,
 		containerID:    container.ID,
 		version:        d.config.Version,
 		killTimeout:    GetKillTimeout(task.KillTimeout, maxKill),

client/driver/docker_test.go

@@ -101,6 +101,10 @@ func dockerSetup(t *testing.T, task *structs.Task) (*docker.Client, DriverHandle
 	driver := NewDockerDriver(driverCtx)
 	copyImage(execCtx, task, "busybox.tar", t)
 
+	if err := driver.Prestart(execCtx, task); err != nil {
+		execCtx.AllocDir.Destroy()
+		t.Fatalf("error in prestart: %v", err)
+	}
 	handle, err := driver.Start(execCtx, task)
 	if err != nil {
 		execCtx.AllocDir.Destroy()
@@ -167,6 +171,9 @@ func TestDockerDriver_StartOpen_Wait(t *testing.T) {
 	d := NewDockerDriver(driverCtx)
 	copyImage(execCtx, task, "busybox.tar", t)
 
+	if err := d.Prestart(execCtx, task); err != nil {
+		t.Fatalf("error in prestart: %v", err)
+	}
 	handle, err := d.Start(execCtx, task)
 	if err != nil {
 		t.Fatalf("err: %v", err)
@@ -256,6 +263,9 @@ func TestDockerDriver_Start_LoadImage(t *testing.T) {
 	// Copy the image into the task's directory
 	copyImage(execCtx, task, "busybox.tar", t)
 
+	if err := d.Prestart(execCtx, task); err != nil {
+		t.Fatalf("error in prestart: %v", err)
+	}
 	handle, err := d.Start(execCtx, task)
 	if err != nil {
 		t.Fatalf("err: %v", err)
@@ -316,9 +326,9 @@ func TestDockerDriver_Start_BadPull_Recoverable(t *testing.T) {
 	defer execCtx.AllocDir.Destroy()
 	d := NewDockerDriver(driverCtx)
 
-	_, err := d.Start(execCtx, task)
+	err := d.Prestart(execCtx, task)
 	if err == nil {
-		t.Fatalf("want err: %v", err)
+		t.Fatalf("want error in prestart: %v", err)
 	}
 
 	if rerr, ok := err.(*structs.RecoverableError); !ok {
@@ -366,6 +376,9 @@ func TestDockerDriver_Start_Wait_AllocDir(t *testing.T) {
 	d := NewDockerDriver(driverCtx)
 	copyImage(execCtx, task, "busybox.tar", t)
 
+	if err := d.Prestart(execCtx, task); err != nil {
+		t.Fatalf("error in prestart: %v", err)
+	}
 	handle, err := d.Start(execCtx, task)
 	if err != nil {
 		t.Fatalf("err: %v", err)
@@ -456,6 +469,9 @@ func TestDockerDriver_StartN(t *testing.T) {
 		d := NewDockerDriver(driverCtx)
 		copyImage(execCtx, task, "busybox.tar", t)
 
+		if err := d.Prestart(execCtx, task); err != nil {
+			t.Fatalf("error in prestart #%d: %v", idx+1, err)
+		}
 		handles[idx], err = d.Start(execCtx, task)
 		if err != nil {
 			t.Errorf("Failed starting task #%d: %s", idx+1, err)
@@ -513,6 +529,9 @@ func TestDockerDriver_StartNVersions(t *testing.T) {
 		copyImage(execCtx, task, "busybox_musl.tar", t)
 		copyImage(execCtx, task, "busybox_glibc.tar", t)
 
+		if err := d.Prestart(execCtx, task); err != nil {
+			t.Fatalf("error in prestart #%d: %v", idx+1, err)
+		}
 		handles[idx], err = d.Start(execCtx, task)
 		if err != nil {
 			t.Errorf("Failed starting task #%d: %s", idx+1, err)
@@ -804,6 +823,10 @@ func TestDockerDriver_User(t *testing.T) {
 	defer execCtx.AllocDir.Destroy()
 	copyImage(execCtx, task, "busybox.tar", t)
 
+	if err := driver.Prestart(execCtx, task); err != nil {
+		t.Fatalf("error in prestart: %v", err)
+	}
+
 	// It should fail because the user "alice" does not exist on the given
 	// image.
 	handle, err := driver.Start(execCtx, task)
@@ -953,6 +976,9 @@ done
 		fmt.Errorf("Failed to write data")
 	}
 
+	if err := d.Prestart(execCtx, task); err != nil {
+		t.Fatalf("error in prestart: %v", err)
+	}
 	handle, err := d.Start(execCtx, task)
 	if err != nil {
 		t.Fatalf("err: %v", err)
@@ -1035,7 +1061,11 @@ func setupDockerVolumes(t *testing.T, cfg *config.Config, hostpath string) (*str
 		t.Fatalf("Failed to get task env: %v", err)
 	}
 
-	driverCtx := NewDriverContext(task.Name, cfg, cfg.Node, testLogger(), taskEnv)
+	logger := testLogger()
+	emitter := func(m string, args ...interface{}) {
+		logger.Printf("[EVENT] "+m, args...)
+	}
+	driverCtx := NewDriverContext(task.Name, cfg, cfg.Node, testLogger(), taskEnv, emitter)
 	driver := NewDockerDriver(driverCtx)
 	copyImage(execCtx, task, "busybox.tar", t)
 
@@ -1058,6 +1088,9 @@ func TestDockerDriver_VolumesDisabled(t *testing.T) {
 		task, driver, execCtx, _, cleanup := setupDockerVolumes(t, cfg, tmpvol)
 		defer cleanup()
 
+		if err := driver.Prestart(execCtx, task); err != nil {
+			t.Fatalf("error in prestart: %v", err)
+		}
 		if _, err := driver.Start(execCtx, task); err == nil {
 			t.Fatalf("Started driver successfully when volumes should have been disabled.")
 		}
@@ -1068,6 +1101,9 @@ func TestDockerDriver_VolumesDisabled(t *testing.T) {
 		task, driver, execCtx, fn, cleanup := setupDockerVolumes(t, cfg, ".")
 		defer cleanup()
 
+		if err := driver.Prestart(execCtx, task); err != nil {
+			t.Fatalf("error in prestart: %v", err)
+		}
 		handle, err := driver.Start(execCtx, task)
 		if err != nil {
 			t.Fatalf("err: %v", err)
@@ -1106,6 +1142,9 @@ func TestDockerDriver_VolumesEnabled(t *testing.T) {
 	task, driver, execCtx, hostpath, cleanup := setupDockerVolumes(t, cfg, tmpvol)
 	defer cleanup()
 
+	if err := driver.Prestart(execCtx, task); err != nil {
+		t.Fatalf("error in prestart: %v", err)
+	}
 	handle, err := driver.Start(execCtx, task)
 	if err != nil {
 		t.Fatalf("Failed to start docker driver: %v", err)

client/driver/driver.go

@@ -51,6 +51,10 @@ type Driver interface {
 	// Drivers must support the fingerprint interface for detection
 	fingerprint.Fingerprint
 
+	// Prestart prepares the task environment and performs expensive
+	// intialization steps like downloading images.
+	Prestart(*ExecContext, *structs.Task) error
+
 	// Start is used to being task execution
 	Start(ctx *ExecContext, task *structs.Task) (DriverHandle, error)
 
@@ -70,6 +74,9 @@ type DriverAbilities struct {
 	SendSignals bool
 }
 
+// LogEventFn is a callback which allows Drivers to emit task events.
+type LogEventFn func(message string, args ...interface{})
+
 // DriverContext is a means to inject dependencies such as loggers, configs, and
 // node attributes into a Driver without having to change the Driver interface
 // each time we do it. Used in conjection with Factory, above.
@@ -79,18 +86,14 @@ type DriverContext struct {
 	logger   *log.Logger
 	node     *structs.Node
 	taskEnv  *env.TaskEnvironment
+
+	emitEvent LogEventFn
 }
 
 // NewEmptyDriverContext returns a DriverContext with all fields set to their
 // zero value.
 func NewEmptyDriverContext() *DriverContext {
-	return &DriverContext{
+	return &DriverContext{}
-		taskName: "",
-		config:   nil,
-		node:     nil,
-		logger:   nil,
-		taskEnv:  nil,
-	}
 }
 
 // NewDriverContext initializes a new DriverContext with the specified fields.
@@ -98,13 +101,14 @@ func NewEmptyDriverContext() *DriverContext {
 // private to the driver. If we want to change this later we can gorename all of
 // the fields in DriverContext.
 func NewDriverContext(taskName string, config *config.Config, node *structs.Node,
-	logger *log.Logger, taskEnv *env.TaskEnvironment) *DriverContext {
+	logger *log.Logger, taskEnv *env.TaskEnvironment, eventEmitter LogEventFn) *DriverContext {
 	return &DriverContext{
 		taskName:  taskName,
 		config:    config,
 		node:      node,
 		logger:    logger,
 		taskEnv:   taskEnv,
+		emitEvent: eventEmitter,
 	}
 }
 

client/driver/driver_test.go

@@ -89,7 +89,11 @@ func testDriverContexts(task *structs.Task) (*DriverContext, *ExecContext) {
 		return nil, nil
 	}
 
-	driverCtx := NewDriverContext(task.Name, cfg, cfg.Node, testLogger(), taskEnv)
+	logger := testLogger()
+	emitter := func(m string, args ...interface{}) {
+		logger.Printf("[EVENT] "+m, args...)
+	}
+	driverCtx := NewDriverContext(task.Name, cfg, cfg.Node, logger, taskEnv, emitter)
 	return driverCtx, execCtx
 }
 

client/driver/exec.go

@@ -92,6 +92,13 @@ func (d *ExecDriver) Periodic() (bool, time.Duration) {
 	return true, 15 * time.Second
 }
 
+func (d *ExecDriver) Prestart(execctx *ExecContext, task *structs.Task) error {
+	// Set the host environment variables.
+	filter := strings.Split(d.config.ReadDefault("env.blacklist", config.DefaultEnvBlacklist), ",")
+	d.taskEnv.AppendHostEnvvars(filter)
+	return nil
+}
+
 func (d *ExecDriver) Start(ctx *ExecContext, task *structs.Task) (DriverHandle, error) {
 	var driverConfig ExecDriverConfig
 	if err := mapstructure.WeakDecode(task.Config, &driverConfig); err != nil {
@@ -104,10 +111,6 @@ func (d *ExecDriver) Start(ctx *ExecContext, task *structs.Task) (DriverHandle,
 		return nil, err
 	}
 
-	// Set the host environment variables.
-	filter := strings.Split(d.config.ReadDefault("env.blacklist", config.DefaultEnvBlacklist), ",")
-	d.taskEnv.AppendHostEnvvars(filter)
-
 	// Get the task directory for storing the executor logs.
 	taskDir, ok := ctx.AllocDir.TaskDirs[d.DriverContext.taskName]
 	if !ok {

client/driver/exec_test.go

@@ -65,6 +65,9 @@ func TestExecDriver_StartOpen_Wait(t *testing.T) {
 	defer execCtx.AllocDir.Destroy()
 	d := NewExecDriver(driverCtx)
 
+	if err := d.Prestart(execCtx, task); err != nil {
+		t.Fatalf("prestart err: %v", err)
+	}
 	handle, err := d.Start(execCtx, task)
 	if err != nil {
 		t.Fatalf("err: %v", err)
@@ -105,6 +108,9 @@ func TestExecDriver_KillUserPid_OnPluginReconnectFailure(t *testing.T) {
 	defer execCtx.AllocDir.Destroy()
 	d := NewExecDriver(driverCtx)
 
+	if err := d.Prestart(execCtx, task); err != nil {
+		t.Fatalf("prestart err: %v", err)
+	}
 	handle, err := d.Start(execCtx, task)
 	if err != nil {
 		t.Fatalf("err: %v", err)
@@ -165,6 +171,9 @@ func TestExecDriver_Start_Wait(t *testing.T) {
 	defer execCtx.AllocDir.Destroy()
 	d := NewExecDriver(driverCtx)
 
+	if err := d.Prestart(execCtx, task); err != nil {
+		t.Fatalf("prestart err: %v", err)
+	}
 	handle, err := d.Start(execCtx, task)
 	if err != nil {
 		t.Fatalf("err: %v", err)
@@ -215,6 +224,9 @@ func TestExecDriver_Start_Wait_AllocDir(t *testing.T) {
 	defer execCtx.AllocDir.Destroy()
 	d := NewExecDriver(driverCtx)
 
+	if err := d.Prestart(execCtx, task); err != nil {
+		t.Fatalf("prestart err: %v", err)
+	}
 	handle, err := d.Start(execCtx, task)
 	if err != nil {
 		t.Fatalf("err: %v", err)
@@ -265,6 +277,9 @@ func TestExecDriver_Start_Kill_Wait(t *testing.T) {
 	defer execCtx.AllocDir.Destroy()
 	d := NewExecDriver(driverCtx)
 
+	if err := d.Prestart(execCtx, task); err != nil {
+		t.Fatalf("prestart err: %v", err)
+	}
 	handle, err := d.Start(execCtx, task)
 	if err != nil {
 		t.Fatalf("err: %v", err)
@@ -327,6 +342,9 @@ done
 		fmt.Errorf("Failed to write data")
 	}
 
+	if err := d.Prestart(execCtx, task); err != nil {
+		t.Fatalf("prestart err: %v", err)
+	}
 	handle, err := d.Start(execCtx, task)
 	if err != nil {
 		t.Fatalf("err: %v", err)
@@ -388,6 +406,9 @@ func TestExecDriverUser(t *testing.T) {
 	defer execCtx.AllocDir.Destroy()
 	d := NewExecDriver(driverCtx)
 
+	if err := d.Prestart(execCtx, task); err != nil {
+		t.Fatalf("prestart err: %v", err)
+	}
 	handle, err := d.Start(execCtx, task)
 	if err == nil {
 		handle.Kill()

client/driver/java.go

@@ -163,16 +163,19 @@ func (d *JavaDriver) Fingerprint(cfg *config.Config, node *structs.Node) (bool,
 	return true, nil
 }
 
+func (d *JavaDriver) Prestart(ctx *ExecContext, task *structs.Task) error {
+	// Set the host environment variables.
+	filter := strings.Split(d.config.ReadDefault("env.blacklist", config.DefaultEnvBlacklist), ",")
+	d.taskEnv.AppendHostEnvvars(filter)
+	return nil
+}
+
 func (d *JavaDriver) Start(ctx *ExecContext, task *structs.Task) (DriverHandle, error) {
 	var driverConfig JavaDriverConfig
 	if err := mapstructure.WeakDecode(task.Config, &driverConfig); err != nil {
 		return nil, err
 	}
 
-	// Set the host environment variables.
-	filter := strings.Split(d.config.ReadDefault("env.blacklist", config.DefaultEnvBlacklist), ",")
-	d.taskEnv.AppendHostEnvvars(filter)
-
 	taskDir, ok := ctx.AllocDir.TaskDirs[d.DriverContext.taskName]
 	if !ok {
 		return nil, fmt.Errorf("Could not find task directory for task: %v", d.DriverContext.taskName)

client/driver/java_test.go

@@ -92,6 +92,9 @@ func TestJavaDriver_StartOpen_Wait(t *testing.T) {
 	dst, _ := execCtx.AllocDir.TaskDirs[task.Name]
 	copyFile("./test-resources/java/demoapp.jar", filepath.Join(dst, "demoapp.jar"), t)
 
+	if err := d.Prestart(execCtx, task); err != nil {
+		t.Fatalf("prestart err: %v", err)
+	}
 	handle, err := d.Start(execCtx, task)
 	if err != nil {
 		t.Fatalf("err: %v", err)
@@ -142,6 +145,9 @@ func TestJavaDriver_Start_Wait(t *testing.T) {
 	dst, _ := execCtx.AllocDir.TaskDirs[task.Name]
 	copyFile("./test-resources/java/demoapp.jar", filepath.Join(dst, "demoapp.jar"), t)
 
+	if err := d.Prestart(execCtx, task); err != nil {
+		t.Fatalf("prestart err: %v", err)
+	}
 	handle, err := d.Start(execCtx, task)
 	if err != nil {
 		t.Fatalf("err: %v", err)
@@ -204,6 +210,9 @@ func TestJavaDriver_Start_Kill_Wait(t *testing.T) {
 	dst, _ := execCtx.AllocDir.TaskDirs[task.Name]
 	copyFile("./test-resources/java/demoapp.jar", filepath.Join(dst, "demoapp.jar"), t)
 
+	if err := d.Prestart(execCtx, task); err != nil {
+		t.Fatalf("prestart err: %v", err)
+	}
 	handle, err := d.Start(execCtx, task)
 	if err != nil {
 		t.Fatalf("err: %v", err)
@@ -262,6 +271,9 @@ func TestJavaDriver_Signal(t *testing.T) {
 	dst, _ := execCtx.AllocDir.TaskDirs[task.Name]
 	copyFile("./test-resources/java/demoapp.jar", filepath.Join(dst, "demoapp.jar"), t)
 
+	if err := d.Prestart(execCtx, task); err != nil {
+		t.Fatalf("prestart err: %v", err)
+	}
 	handle, err := d.Start(execCtx, task)
 	if err != nil {
 		t.Fatalf("err: %v", err)
@@ -317,6 +329,9 @@ func TestJavaDriverUser(t *testing.T) {
 	defer execCtx.AllocDir.Destroy()
 	d := NewJavaDriver(driverCtx)
 
+	if err := d.Prestart(execCtx, task); err != nil {
+		t.Fatalf("prestart err: %v", err)
+	}
 	handle, err := d.Start(execCtx, task)
 	if err == nil {
 		handle.Kill()

client/driver/lxc.go

@@ -168,6 +168,10 @@ func (d *LxcDriver) Fingerprint(cfg *config.Config, node *structs.Node) (bool, e
 	return true, nil
 }
 
+func (d *LxcDriver) Prestart(ctx *ExecContext, task *structs.Task) error {
+	return nil
+}
+
 // Start starts the LXC Driver
 func (d *LxcDriver) Start(ctx *ExecContext, task *structs.Task) (DriverHandle, error) {
 	var driverConfig LxcDriverConfig

client/driver/lxc_test.go

@@ -69,6 +69,9 @@ func TestLxcDriver_Start_Wait(t *testing.T) {
 	defer execCtx.AllocDir.Destroy()
 	d := NewLxcDriver(driverCtx)
 
+	if err := d.Prestart(execCtx, task); err != nil {
+		t.Fatalf("prestart err: %v", err)
+	}
 	handle, err := d.Start(execCtx, task)
 	if err != nil {
 		t.Fatalf("err: %v", err)
@@ -141,6 +144,9 @@ func TestLxcDriver_Open_Wait(t *testing.T) {
 	defer execCtx.AllocDir.Destroy()
 	d := NewLxcDriver(driverCtx)
 
+	if err := d.Prestart(execCtx, task); err != nil {
+		t.Fatalf("prestart err: %v", err)
+	}
 	handle, err := d.Start(execCtx, task)
 	if err != nil {
 		t.Fatalf("err: %v", err)

client/driver/mock_driver.go

@@ -75,6 +75,10 @@ func (d *MockDriver) Abilities() DriverAbilities {
 	}
 }
 
+func (d *MockDriver) Prestart(ctx *ExecContext, task *structs.Task) error {
+	return nil
+}
+
 // Start starts the mock driver
 func (m *MockDriver) Start(ctx *ExecContext, task *structs.Task) (DriverHandle, error) {
 	var driverConfig MockDriverConfig

client/driver/qemu.go

@@ -135,6 +135,10 @@ func (d *QemuDriver) Fingerprint(cfg *config.Config, node *structs.Node) (bool,
 	return true, nil
 }
 
+func (d *QemuDriver) Prestart(ctx *ExecContext, task *structs.Task) error {
+	return nil
+}
+
 // Run an existing Qemu image. Start() will pull down an existing, valid Qemu
 // image and save it to the Drivers Allocation Dir
 func (d *QemuDriver) Start(ctx *ExecContext, task *structs.Task) (DriverHandle, error) {

client/driver/raw_exec.go

@@ -107,6 +107,13 @@ func (d *RawExecDriver) Fingerprint(cfg *config.Config, node *structs.Node) (boo
 	return false, nil
 }
 
+func (d *RawExecDriver) Prestart(ctx *ExecContext, task *structs.Task) error {
+	// Set the host environment variables.
+	filter := strings.Split(d.config.ReadDefault("env.blacklist", config.DefaultEnvBlacklist), ",")
+	d.taskEnv.AppendHostEnvvars(filter)
+	return nil
+}
+
 func (d *RawExecDriver) Start(ctx *ExecContext, task *structs.Task) (DriverHandle, error) {
 	var driverConfig ExecDriverConfig
 	if err := mapstructure.WeakDecode(task.Config, &driverConfig); err != nil {
@@ -125,10 +132,6 @@ func (d *RawExecDriver) Start(ctx *ExecContext, task *structs.Task) (DriverHandl
 		return nil, err
 	}
 
-	// Set the host environment variables.
-	filter := strings.Split(d.config.ReadDefault("env.blacklist", config.DefaultEnvBlacklist), ",")
-	d.taskEnv.AppendHostEnvvars(filter)
-
 	bin, err := discover.NomadExecutable()
 	if err != nil {
 		return nil, fmt.Errorf("unable to find the nomad binary: %v", err)

client/driver/raw_exec_test.go

@@ -75,6 +75,9 @@ func TestRawExecDriver_StartOpen_Wait(t *testing.T) {
 	defer execCtx.AllocDir.Destroy()
 	d := NewRawExecDriver(driverCtx)
 
+	if err := d.Prestart(execCtx, task); err != nil {
+		t.Fatalf("prestart err: %v", err)
+	}
 	handle, err := d.Start(execCtx, task)
 	if err != nil {
 		t.Fatalf("err: %v", err)
@@ -120,6 +123,9 @@ func TestRawExecDriver_Start_Wait(t *testing.T) {
 	defer execCtx.AllocDir.Destroy()
 	d := NewRawExecDriver(driverCtx)
 
+	if err := d.Prestart(execCtx, task); err != nil {
+		t.Fatalf("prestart err: %v", err)
+	}
 	handle, err := d.Start(execCtx, task)
 	if err != nil {
 		t.Fatalf("err: %v", err)
@@ -170,6 +176,9 @@ func TestRawExecDriver_Start_Wait_AllocDir(t *testing.T) {
 	defer execCtx.AllocDir.Destroy()
 	d := NewRawExecDriver(driverCtx)
 
+	if err := d.Prestart(execCtx, task); err != nil {
+		t.Fatalf("prestart err: %v", err)
+	}
 	handle, err := d.Start(execCtx, task)
 	if err != nil {
 		t.Fatalf("err: %v", err)
@@ -219,6 +228,9 @@ func TestRawExecDriver_Start_Kill_Wait(t *testing.T) {
 	defer execCtx.AllocDir.Destroy()
 	d := NewRawExecDriver(driverCtx)
 
+	if err := d.Prestart(execCtx, task); err != nil {
+		t.Fatalf("prestart err: %v", err)
+	}
 	handle, err := d.Start(execCtx, task)
 	if err != nil {
 		t.Fatalf("err: %v", err)
@@ -268,6 +280,9 @@ func TestRawExecDriverUser(t *testing.T) {
 	defer execCtx.AllocDir.Destroy()
 	d := NewRawExecDriver(driverCtx)
 
+	if err := d.Prestart(execCtx, task); err != nil {
+		t.Fatalf("prestart err: %v", err)
+	}
 	handle, err := d.Start(execCtx, task)
 	if err == nil {
 		handle.Kill()
@@ -313,6 +328,9 @@ done
 		fmt.Errorf("Failed to write data")
 	}
 
+	if err := d.Prestart(execCtx, task); err != nil {
+		t.Fatalf("prestart err: %v", err)
+	}
 	handle, err := d.Start(execCtx, task)
 	if err != nil {
 		t.Fatalf("err: %v", err)

client/driver/rkt.go

@@ -206,6 +206,13 @@ func (d *RktDriver) Periodic() (bool, time.Duration) {
 	return true, 15 * time.Second
 }
 
+func (d *RktDriver) Prestart(ctx *ExecContext, task *structs.Task) error {
+	d.taskEnv.SetAllocDir(allocdir.SharedAllocContainerPath)
+	d.taskEnv.SetTaskLocalDir(allocdir.TaskLocalContainerPath)
+	d.taskEnv.SetSecretsDir(allocdir.TaskSecretsContainerPath)
+	return nil
+}
+
 // Run an existing Rkt image.
 func (d *RktDriver) Start(ctx *ExecContext, task *structs.Task) (DriverHandle, error) {
 	var driverConfig RktDriverConfig
@@ -289,10 +296,6 @@ func (d *RktDriver) Start(ctx *ExecContext, task *structs.Task) (DriverHandle, e
 	cmdArgs = append(cmdArgs, fmt.Sprintf("--debug=%t", debug))
 
 	// Inject environment variables
-	d.taskEnv.SetAllocDir(allocdir.SharedAllocContainerPath)
-	d.taskEnv.SetTaskLocalDir(allocdir.TaskLocalContainerPath)
-	d.taskEnv.SetSecretsDir(allocdir.TaskSecretsContainerPath)
-	d.taskEnv.Build()
 	for k, v := range d.taskEnv.EnvMap() {
 		cmdArgs = append(cmdArgs, fmt.Sprintf("--set-env=%v=%v", k, v))
 	}

client/driver/rkt_test.go

@@ -98,6 +98,9 @@ func TestRktDriver_Start_DNS(t *testing.T) {
 
 	d := NewRktDriver(driverCtx)
 
+	if err := d.Prestart(execCtx, task); err != nil {
+		t.Fatalf("error in prestart: %v", err)
+	}
 	handle, err := d.Start(execCtx, task)
 	if err != nil {
 		t.Fatalf("err: %v", err)
@@ -145,6 +148,9 @@ func TestRktDriver_Start_Wait(t *testing.T) {
 	defer execCtx.AllocDir.Destroy()
 	d := NewRktDriver(driverCtx)
 
+	if err := d.Prestart(execCtx, task); err != nil {
+		t.Fatalf("error in prestart: %v", err)
+	}
 	handle, err := d.Start(execCtx, task)
 	if err != nil {
 		t.Fatalf("err: %v", err)
@@ -202,6 +208,9 @@ func TestRktDriver_Start_Wait_Skip_Trust(t *testing.T) {
 	defer execCtx.AllocDir.Destroy()
 	d := NewRktDriver(driverCtx)
 
+	if err := d.Prestart(execCtx, task); err != nil {
+		t.Fatalf("error in prestart: %v", err)
+	}
 	handle, err := d.Start(execCtx, task)
 	if err != nil {
 		t.Fatalf("err: %v", err)
@@ -252,6 +261,7 @@ func TestRktDriver_Start_Wait_AllocDir(t *testing.T) {
 				"-c",
 				fmt.Sprintf(`echo -n %s > foo/%s`, string(exp), file),
 			},
+			"net":     []string{"none"},
 			"volumes": []string{fmt.Sprintf("%s:/foo", tmpvol)},
 		},
 		LogConfig: &structs.LogConfig{
@@ -268,6 +278,9 @@ func TestRktDriver_Start_Wait_AllocDir(t *testing.T) {
 	defer execCtx.AllocDir.Destroy()
 	d := NewRktDriver(driverCtx)
 
+	if err := d.Prestart(execCtx, task); err != nil {
+		t.Fatalf("error in prestart: %v", err)
+	}
 	handle, err := d.Start(execCtx, task)
 	if err != nil {
 		t.Fatalf("err: %v", err)
@@ -326,6 +339,9 @@ func TestRktDriverUser(t *testing.T) {
 	defer execCtx.AllocDir.Destroy()
 	d := NewRktDriver(driverCtx)
 
+	if err := d.Prestart(execCtx, task); err != nil {
+		t.Fatalf("error in prestart: %v", err)
+	}
 	handle, err := d.Start(execCtx, task)
 	if err == nil {
 		handle.Kill()
@@ -364,6 +380,9 @@ func TestRktTrustPrefix(t *testing.T) {
 
 	d := NewRktDriver(driverCtx)
 
+	if err := d.Prestart(execCtx, task); err != nil {
+		t.Fatalf("error in prestart: %v", err)
+	}
 	handle, err := d.Start(execCtx, task)
 	if err == nil {
 		handle.Kill()
@@ -438,6 +457,9 @@ func TestRktDriver_PortsMapping(t *testing.T) {
 
 	d := NewRktDriver(driverCtx)
 
+	if err := d.Prestart(execCtx, task); err != nil {
+		t.Fatalf("error in prestart: %v", err)
+	}
 	handle, err := d.Start(execCtx, task)
 	if err != nil {
 		t.Fatalf("err: %v", err)

client/task_runner.go

@@ -346,7 +346,15 @@ func (r *TaskRunner) createDriver() (driver.Driver, error) {
 		return nil, fmt.Errorf("task environment not made for task %q in allocation %q", r.task.Name, r.alloc.ID)
 	}
 
-	driverCtx := driver.NewDriverContext(r.task.Name, r.config, r.config.Node, r.logger, env)
+	// Create a task-specific event emitter callback to expose minimal
+	// state to drivers
+	eventEmitter := func(m string, args ...interface{}) {
+		msg := fmt.Sprintf(m, args...)
+		r.logger.Printf("[DEBUG] client: initialization event for alloc %q: %s", r.alloc.ID, msg)
+		r.setState("", structs.NewTaskEvent(structs.TaskInitializing).SetInitializationMessage(msg))
+	}
+
+	driverCtx := driver.NewDriverContext(r.task.Name, r.config, r.config.Node, r.logger, env, eventEmitter)
 	driver, err := driver.NewDriver(r.task.Driver, driverCtx)
 	if err != nil {
 		return nil, fmt.Errorf("failed to create driver '%s' for alloc %s: %v",
@@ -1019,17 +1027,31 @@ func (r *TaskRunner) startTask() error {
 	// Create a driver
 	driver, err := r.createDriver()
 	if err != nil {
-		return fmt.Errorf("failed to create driver of task '%s' for alloc '%s': %v",
+		return fmt.Errorf("failed to create driver of task %q for alloc %q: %v",
 			r.task.Name, r.alloc.ID, err)
 	}
 
+	// Run prestart
+	if err := driver.Prestart(r.ctx, r.task); err != nil {
+		wrapped := fmt.Errorf("failed to initialize task %q for alloc %q: %v",
+			r.task.Name, r.alloc.ID, err)
+
+		r.logger.Printf("[WARN] client: %v", wrapped)
+
+		if rerr, ok := err.(*structs.RecoverableError); ok {
+			return structs.NewRecoverableError(wrapped, rerr.Recoverable)
+		}
+
+		return wrapped
+	}
+
 	// Start the job
 	handle, err := driver.Start(r.ctx, r.task)
 	if err != nil {
-		wrapped := fmt.Errorf("failed to start task '%s' for alloc '%s': %v",
+		wrapped := fmt.Errorf("failed to start task %q for alloc %q: %v",
 			r.task.Name, r.alloc.ID, err)
 
-		r.logger.Printf("[INFO] client: %v", wrapped)
+		r.logger.Printf("[WARN] client: %v", wrapped)
 
 		if rerr, ok := err.(*structs.RecoverableError); ok {
 			return structs.NewRecoverableError(wrapped, rerr.Recoverable)

command/alloc_status.go

@@ -378,6 +378,8 @@ func (c *AllocStatusCommand) outputTaskStatus(state *api.TaskState) {
 			} else {
 				desc = "Task signaled to restart"
 			}
+		case api.TaskInitializing:
+			desc = event.InitializationMessage
 		}
 
 		// Reverse order so we are sorted by time

nomad/structs/structs.go

@@ -2555,6 +2555,10 @@ const (
 	// TaskSiblingFailed indicates that a sibling task in the task group has
 	// failed.
 	TaskSiblingFailed = "Sibling task failed"
+
+	// TaskInitializing indicates that a task is performing a potentially
+	// slow initialization action such as downloading a Docker image.
+	TaskInitializing = "Initializing"
 )
 
 // TaskEvent is an event that effects the state of a task and contains meta-data
@@ -2613,6 +2617,9 @@ type TaskEvent struct {
 
 	// TaskSignal is the signal that was sent to the task
 	TaskSignal string
+
+	// InitializationMessage indicates the initialization step being executed.
+	InitializationMessage string
 }
 
 func (te *TaskEvent) GoString() string {
@@ -2741,6 +2748,11 @@ func (e *TaskEvent) SetVaultRenewalError(err error) *TaskEvent {
 	return e
 }
 
+func (e *TaskEvent) SetInitializationMessage(m string) *TaskEvent {
+	e.InitializationMessage = m
+	return e
+}
+
 // TaskArtifact is an artifact to download before running the task.
 type TaskArtifact struct {
 	// GetterSource is the source to download an artifact using go-getter