Check if image already exists before doing anything else on GCE.

This fixes #1729.

builder/googlecompute/builder.go

@@ -49,6 +49,7 @@ func (b *Builder) Run(ui packer.Ui, hook packer.Hook, cache packer.Cache) (packe
 
 	// Build the steps.
 	steps := []multistep.Step{
+		new(StepCheckExistingImage),
 		&StepCreateSSHKey{
 			Debug:        b.config.PackerDebug,
 			DebugKeyPath: fmt.Sprintf("gce_%s.pem", b.config.PackerBuildName),

builder/googlecompute/driver.go

@@ -4,6 +4,10 @@ package googlecompute
 // with GCE. The Driver interface exists mostly to allow a mock implementation
 // to be used to test the steps.
 type Driver interface {
+	// ImageExists returns true if the specified image exists. If an error
+	// occurs calling the API, this method returns false.
+	ImageExists(name string) bool
+
 	// CreateImage creates an image from the given disk in Google Compute
 	// Engine.
 	CreateImage(name, description, zone, disk string) <-chan error

builder/googlecompute/driver_gce.go

@@ -7,9 +7,9 @@ import (
 	"time"
 
 	"code.google.com/p/google-api-go-client/compute/v1"
+	"github.com/mitchellh/packer/packer"
 	"golang.org/x/oauth2"
 	"golang.org/x/oauth2/google"
-	"github.com/mitchellh/packer/packer"
 )
 
 // driverGCE is a Driver implementation that actually talks to GCE.
@@ -60,6 +60,13 @@ func NewDriverGCE(ui packer.Ui, p string, a *accountFile) (Driver, error) {
 	}, nil
 }
 
+func (d *driverGCE) ImageExists(name string) bool {
+	_, err := d.service.Images.Get(d.projectId, name).Do()
+	// The API may return an error for reasons other than the image not
+	// existing, but this heuristic is sufficient for now.
+	return err == nil
+}
+
 func (d *driverGCE) CreateImage(name, description, zone, disk string) <-chan error {
 	image := &compute.Image{
 		Description: description,

builder/googlecompute/driver_mock.go

@@ -3,6 +3,9 @@ package googlecompute
 // DriverMock is a Driver implementation that is a mocked out so that
 // it can be used for tests.
 type DriverMock struct {
+	ImageExistsName   string
+	ImageExistsResult bool
+
 	CreateImageName  string
 	CreateImageDesc  string
 	CreateImageZone  string
@@ -37,6 +40,11 @@ type DriverMock struct {
 	WaitForInstanceErrCh <-chan error
 }
 
+func (d *DriverMock) ImageExists(name string) bool {
+	d.ImageExistsName = name
+	return d.ImageExistsResult
+}
+
 func (d *DriverMock) CreateImage(name, description, zone, disk string) <-chan error {
 	d.CreateImageName = name
 	d.CreateImageDesc = description

builder/googlecompute/step_check_existing_image.go

@@ -0,0 +1,33 @@
+package googlecompute
+
+import (
+	"fmt"
+
+	"github.com/mitchellh/multistep"
+	"github.com/mitchellh/packer/packer"
+)
+
+// StepCheckExistingImage represents a Packer build step that checks if the
+// target image already exists, and aborts immediately if so.
+type StepCheckExistingImage int
+
+// Run executes the Packer build step that checks if the image already exists.
+func (s *StepCheckExistingImage) Run(state multistep.StateBag) multistep.StepAction {
+	config := state.Get("config").(*Config)
+	driver := state.Get("driver").(Driver)
+	ui := state.Get("ui").(packer.Ui)
+
+	ui.Say("Checking image does not exist...")
+	exists := driver.ImageExists(config.ImageName)
+	if exists {
+		err := fmt.Errorf("Image %s already exists", config.ImageName)
+		state.Put("error", err)
+		ui.Error(err.Error())
+		return multistep.ActionHalt
+	}
+
+	return multistep.ActionContinue
+}
+
+// Cleanup.
+func (s *StepCheckExistingImage) Cleanup(state multistep.StateBag) {}

builder/googlecompute/step_check_existing_image_test.go

@@ -0,0 +1,32 @@
+package googlecompute
+
+import (
+	"github.com/mitchellh/multistep"
+	"testing"
+)
+
+func TestStepCheckExistingImage_impl(t *testing.T) {
+	var _ multistep.Step = new(StepCheckExistingImage)
+}
+
+func TestStepCheckExistingImage(t *testing.T) {
+	state := testState(t)
+	step := new(StepCheckExistingImage)
+	defer step.Cleanup(state)
+
+	state.Put("instance_name", "foo")
+
+	config := state.Get("config").(*Config)
+	driver := state.Get("driver").(*DriverMock)
+	driver.ImageExistsResult = true
+
+	// run the step
+	if action := step.Run(state); action != multistep.ActionHalt {
+		t.Fatalf("bad action: %#v", action)
+	}
+
+	// Verify state
+	if driver.ImageExistsName != config.ImageName {
+		t.Fatalf("bad: %#v", driver.ImageExistsName)
+	}
+}