feature: add file lock step for chroot builder

builder/osc/chroot/lockfile.go

@@ -0,0 +1,16 @@
+// +build windows
+
+package chroot
+
+import (
+	"errors"
+	"os"
+)
+
+func lockFile(*os.File) error {
+	return errors.New("not supported on Windows")
+}
+
+func unlockFile(f *os.File) error {
+	return nil
+}

builder/osc/chroot/lockfile_unix.go

@@ -0,0 +1,27 @@
+// +build !windows
+
+package chroot
+
+import (
+	"os"
+
+	"golang.org/x/sys/unix"
+)
+
+// 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 := unix.Flock(int(f.Fd()), LOCK_EX)
+	if err != nil {
+		return err
+	}
+
+	return nil
+}
+
+func unlockFile(f *os.File) error {
+	return unix.Flock(int(f.Fd()), LOCK_UN)
+}

builder/osc/chroot/step_flock.go

@@ -0,0 +1,74 @@
+package chroot
+
+import (
+	"context"
+	"fmt"
+	"log"
+	"os"
+	"path/filepath"
+
+	"github.com/hashicorp/packer/helper/multistep"
+	"github.com/hashicorp/packer/packer"
+)
+
+// StepFlock provisions the vm within a chroot.
+//
+// Produces:
+//   flock_cleanup Cleanup - To perform early cleanup
+type StepFlock struct {
+	fh *os.File
+}
+
+func (s *StepFlock) Run(_ context.Context, state multistep.StateBag) multistep.StepAction {
+	ui := state.Get("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.Put("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.Put("error", err)
+		ui.Error(err.Error())
+		return multistep.ActionHalt
+	}
+
+	// LOCK!
+	if err := lockFile(f); err != nil {
+		err := fmt.Errorf("Error obtaining lock: %s", err)
+		state.Put("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
+
+	state.Put("flock_cleanup", s)
+	return multistep.ActionContinue
+}
+
+func (s *StepFlock) Cleanup(state multistep.StateBag) {
+	s.CleanupFunc(state)
+}
+
+func (s *StepFlock) CleanupFunc(state multistep.StateBag) error {
+	if s.fh == nil {
+		return nil
+	}
+
+	log.Printf("Unlocking: %s", s.fh.Name())
+	if err := unlockFile(s.fh); err != nil {
+		return err
+	}
+
+	s.fh = nil
+	return nil
+}