builder/docker: use exec for v1.4+

builder/docker/builder.go

 package docker
 
 import (
+	"log"
+
 	"github.com/mitchellh/multistep"
 	"github.com/mitchellh/packer/common"
 	"github.com/mitchellh/packer/packer"
-	"log"
 )
 
 const BuilderId = "packer.docker"
@@ -31,6 +32,12 @@ func (b *Builder) Run(ui packer.Ui, hook packer.Hook, cache packer.Cache) (packe
 		return nil, err
 	}
 
+	version, err := driver.Version()
+	if err != nil {
+		return nil, err
+	}
+	log.Printf("[DEBUG] Docker version: %s", version.String())
+
 	steps := []multistep.Step{
 		&StepTempDir{},
 		&StepPull{},

builder/docker/communicator.go

@@ -9,7 +9,6 @@ import (
 	"os"
 	"os/exec"
 	"path/filepath"
-	"regexp"
 	"strconv"
 	"sync"
 	"syscall"
@@ -24,36 +23,20 @@ type Communicator struct {
 	ContainerId  string
 	HostDir      string
 	ContainerDir string
+	Version      *version.Version
 
 	lock sync.Mutex
 }
 
-var dockerVersion *version.Version
-var useDockerExec bool
-
-func init() {
-	execConstraint, _ := version.NewConstraint(">= 1.4.0")
-
-	versionExtractor := regexp.MustCompile(version.VersionRegexpRaw)
-	dockerVersionOutput, err := exec.Command("docker", "-v").Output()
-	extractedVersion := versionExtractor.FindSubmatch(dockerVersionOutput)
-
-	if extractedVersion != nil {
-		dockerVersionString := string(extractedVersion[0])
-		dockerVersion, err = version.NewVersion(dockerVersionString)
-	}
-
-	if dockerVersion == nil {
-		log.Printf("Could not determine docker version: %v", err)
-		log.Printf("Assuming no `exec` capability, using `attatch`")
-		useDockerExec = false
-	} else {
-		log.Printf("Docker version detected as %s", dockerVersion)
-		useDockerExec = execConstraint.Check(dockerVersion)
+func (c *Communicator) Start(remote *packer.RemoteCmd) error {
+	// Determine if we're using docker exec or not
+	useExec := false
+	execConstraint, err := version.NewConstraint(">= 1.4.0")
+	if err != nil {
+		return err
 	}
-}
+	useExec = execConstraint.Check(c.Version)
 
-func (c *Communicator) Start(remote *packer.RemoteCmd) error {
 	// Create a temporary file to store the output. Because of a bug in
 	// Docker, sometimes all the output doesn't properly show up. This
 	// file will capture ALL of the output, and we'll read that.
@@ -69,7 +52,7 @@ func (c *Communicator) Start(remote *packer.RemoteCmd) error {
 	exitCodePath := outputFile.Name() + "-exit"
 
 	var cmd *exec.Cmd
-	if useDockerExec {
+	if useExec {
 		cmd = exec.Command("docker", "exec", "-i", c.ContainerId, "/bin/sh")
 	} else {
 		cmd = exec.Command("docker", "attach", c.ContainerId)
@@ -150,7 +133,7 @@ func (c *Communicator) UploadDir(dst string, src string, exclude []string) error
 			return os.MkdirAll(hostpath, info.Mode())
 		}
 
-		if info.Mode() & os.ModeSymlink == os.ModeSymlink {
+		if info.Mode()&os.ModeSymlink == os.ModeSymlink {
 			dest, err := os.Readlink(path)
 
 			if err != nil {

builder/docker/driver.go

@@ -2,6 +2,8 @@ package docker
 
 import (
 	"io"
+
+	"github.com/hashicorp/go-version"
 )
 
 // Driver is the interface that has to be implemented to communicate with
@@ -48,6 +50,9 @@ type Driver interface {
 
 	// Verify verifies that the driver can run
 	Verify() error
+
+	// Version reads the Docker version
+	Version() (*version.Version, error)
 }
 
 // ContainerConfig is the configuration used to start a container.

builder/docker/driver_docker.go

@@ -7,9 +7,11 @@ import (
 	"log"
 	"os"
 	"os/exec"
+	"regexp"
 	"strings"
 	"sync"
 
+	"github.com/hashicorp/go-version"
 	"github.com/mitchellh/packer/packer"
 	"github.com/mitchellh/packer/template/interpolate"
 )
@@ -263,3 +265,17 @@ func (d *DockerDriver) Verify() error {
 
 	return nil
 }
+
+func (d *DockerDriver) Version() (*version.Version, error) {
+	output, err := exec.Command("docker", "-v").Output()
+	if err != nil {
+		return nil, err
+	}
+
+	match := regexp.MustCompile(version.VersionRegexpRaw).FindSubmatch(output)
+	if match == nil {
+		return nil, fmt.Errorf("unknown version: %s", output)
+	}
+
+	return version.NewVersion(string(match[0]))
+}

builder/docker/driver_mock.go

@@ -2,6 +2,8 @@ package docker
 
 import (
 	"io"
+
+	"github.com/hashicorp/go-version"
 )
 
 // MockDriver is a driver implementation that can be used for tests.
@@ -63,6 +65,9 @@ type MockDriver struct {
 	StopCalled   bool
 	StopID       string
 	VerifyCalled bool
+
+	VersionCalled  bool
+	VersionVersion string
 }
 
 func (d *MockDriver) Commit(id string) (string, error) {
@@ -162,3 +167,8 @@ func (d *MockDriver) Verify() error {
 	d.VerifyCalled = true
 	return d.VerifyError
 }
+
+func (d *MockDriver) Version() (*version.Version, error) {
+	d.VersionCalled = true
+	return version.NewVersion(d.VersionVersion)
+}

builder/docker/step_provision.go

@@ -9,14 +9,23 @@ type StepProvision struct{}
 
 func (s *StepProvision) Run(state multistep.StateBag) multistep.StepAction {
 	containerId := state.Get("container_id").(string)
+	driver := state.Get("driver").(Driver)
 	tempDir := state.Get("temp_dir").(string)
 
+	// Get the version so we can pass it to the communicator
+	version, err := driver.Version()
+	if err != nil {
+		state.Put("error", err)
+		return multistep.ActionHalt
+	}
+
 	// Create the communicator that talks to Docker via various
 	// os/exec tricks.
 	comm := &Communicator{
 		ContainerId:  containerId,
 		HostDir:      tempDir,
 		ContainerDir: "/packer-files",
+		Version:      version,
 	}
 
 	prov := common.StepProvision{Comm: comm}