feature: add early unflock step for chroot builder

builder/osc/chroot/builder.go

@@ -248,6 +248,7 @@ func (b *Builder) Run(ui packer.Ui, hook packer.Hook, cache packer.Cache) (packe
 			Ctx:            b.config.ctx,
 		},
 		&StepLinkVolume{},
+		&StepEarlyUnflock{},
 	)
 
 	// Run!

builder/osc/chroot/cleanup.go

@@ -0,0 +1,10 @@
+package chroot
+
+import (
+	"github.com/hashicorp/packer/helper/multistep"
+)
+
+// Cleanup is an interface that some steps implement for early cleanup.
+type Cleanup interface {
+	CleanupFunc(multistep.StateBag) error
+}

builder/osc/chroot/step_early_unflock.go

@@ -0,0 +1,30 @@
+package chroot
+
+import (
+	"context"
+	"fmt"
+	"log"
+
+	"github.com/hashicorp/packer/helper/multistep"
+	"github.com/hashicorp/packer/packer"
+)
+
+// StepEarlyUnflock unlocks the flock.
+type StepEarlyUnflock struct{}
+
+func (s *StepEarlyUnflock) Run(_ context.Context, state multistep.StateBag) multistep.StepAction {
+	cleanup := state.Get("flock_cleanup").(Cleanup)
+	ui := state.Get("ui").(packer.Ui)
+
+	log.Println("Unlocking file lock...")
+	if err := cleanup.CleanupFunc(state); err != nil {
+		err := fmt.Errorf("Error unlocking file lock: %s", err)
+		state.Put("error", err)
+		ui.Error(err.Error())
+		return multistep.ActionHalt
+	}
+
+	return multistep.ActionContinue
+}
+
+func (s *StepEarlyUnflock) Cleanup(state multistep.StateBag) {}