builder/amazon/chroot: flock so that device searching is safe

builder/amazon/chroot/builder.go

@@ -123,6 +123,7 @@ func (b *Builder) Run(ui packer.Ui, hook packer.Hook, cache packer.Cache) (packe
 	steps := []multistep.Step{
 		&StepInstanceInfo{},
 		&StepSourceAMIInfo{},
+		&StepFlock{},
 		&StepPrepareDevice{},
 		&StepCreateVolume{},
 		&StepAttachVolume{},

builder/amazon/chroot/lockfile.go

@@ -0,0 +1,13 @@
+// +build windows
+
+package chroot
+
+import "errors"
+
+func lockFile(*os.File) error {
+	return errors.New("not supported on Windows")
+}
+
+func unlockFile(f *os.File) error {
+	return nil
+}

builder/amazon/chroot/lockfile_unix.go

@@ -0,0 +1,32 @@
+// +build !windows
+
+package chroot
+
+import (
+	"errors"
+	"os"
+	"syscall"
+)
+
+// See: http://linux.die.net/include/sys/file.h
+const LOCK_EX = 2
+const LOCK_NB = 4
+const LOCK_UN = 8
+
+func lockFile(f *os.File) error {
+	err := syscall.Flock(int(f.Fd()), LOCK_EX|LOCK_NB)
+	if err != nil {
+		errno, ok := err.(syscall.Errno)
+		if ok && errno == syscall.EWOULDBLOCK {
+			return errors.New("file already locked")
+		}
+
+		return err
+	}
+
+	return nil
+}
+
+func unlockFile(f *os.File) error {
+	return syscall.Flock(int(f.Fd()), LOCK_UN)
+}

builder/amazon/chroot/step_flock.go

@@ -0,0 +1,59 @@
+package chroot
+
+import (
+	"fmt"
+	"github.com/mitchellh/multistep"
+	"github.com/mitchellh/packer/packer"
+	"log"
+	"os"
+	"path/filepath"
+)
+
+// StepFlock provisions the instance within a chroot.
+type StepFlock struct {
+	fh *os.File
+}
+
+func (s *StepFlock) Run(state map[string]interface{}) multistep.StepAction {
+	ui := state["ui"].(packer.Ui)
+
+	lockfile := "/var/lock/packer-chroot/lock"
+	if err := os.MkdirAll(filepath.Dir(lockfile), 0755); err != nil {
+		err := fmt.Errorf("Error creating lock: %s", err)
+		state["error"] = err
+		ui.Error(err.Error())
+		return multistep.ActionHalt
+	}
+
+	log.Printf("Obtaining lock: %s", lockfile)
+	f, err := os.Create(lockfile)
+	if err != nil {
+		err := fmt.Errorf("Error creating lock: %s", err)
+		state["error"] = err
+		ui.Error(err.Error())
+		return multistep.ActionHalt
+	}
+
+	// LOCK!
+	if err := lockFile(f); err != nil {
+		err := fmt.Errorf("Error creating lock: %s", err)
+		state["error"] = err
+		ui.Error(err.Error())
+		return multistep.ActionHalt
+	}
+
+	// Set the file handle, we can't close it because we need to hold
+	// the lock.
+	s.fh = f
+
+	return multistep.ActionContinue
+}
+
+func (s *StepFlock) Cleanup(state map[string]interface{}) {
+	if s.fh == nil {
+		return
+	}
+
+	log.Printf("Unlocking: %s", s.fh.Name())
+	unlockFile(s.fh)
+}