builder/virtualbox: verify output dir is writable

CHANGELOG.md

@@ -31,6 +31,8 @@ IMPROVEMENTS:
 * builder/qemu: Next `run_once` option tells Qemu to run only once,
   which is useful for Windows installs that handle reboots for you.
   [GH-687]
+* builder/virtualbox: Nice errors if Packer can't write to
+  the output directory.
 * provisioner/puppet-masterless: Can now specify a `manifest_dir` to
   upload manifests to the remote machine for imports. [GH-655]
 

builder/virtualbox/step_prepare_output_dir.go

 package virtualbox
 
 import (
-	"github.com/mitchellh/multistep"
-	"github.com/mitchellh/packer/packer"
+	"fmt"
 	"log"
 	"os"
+	"path/filepath"
 	"time"
+
+	"github.com/mitchellh/multistep"
+	"github.com/mitchellh/packer/packer"
 )
 
 type stepPrepareOutputDir struct{}
@@ -19,11 +22,22 @@ func (stepPrepareOutputDir) Run(state multistep.StateBag) multistep.StepAction {
 		os.RemoveAll(config.OutputDir)
 	}
 
+	// Create the directory
 	if err := os.MkdirAll(config.OutputDir, 0755); err != nil {
 		state.Put("error", err)
 		return multistep.ActionHalt
 	}
 
+	// Make sure we can write in the directory
+	f, err := os.Create(filepath.Join(config.OutputDir, "_packer_perm_check"))
+	if err != nil {
+		err = fmt.Errorf("Couldn't write to output directory: %s", err)
+		state.Put("error", err)
+		return multistep.ActionHalt
+	}
+	f.Close()
+	os.Remove(f.Name())
+
 	return multistep.ActionContinue
 }