post-processor/vagrant: validate the template

post-processor/vagrant/post-processor.go

@@ -8,6 +8,7 @@ import (
 	"github.com/mitchellh/mapstructure"
 	"github.com/mitchellh/packer/packer"
 	"log"
+	"text/template"
 )
 
 var builtins = map[string]string{
@@ -31,7 +32,12 @@ func (p *PostProcessor) Configure(raw interface{}) error {
 	}
 
 	if p.config.OutputPath == "" {
-		return fmt.Errorf("`output` must be specified.")
+		p.config.OutputPath = "packer_{{.Provider}}.box"
+	}
+
+	_, err = template.New("output").Parse(p.config.OutputPath)
+	if err != nil {
+		return fmt.Errorf("output invalid template: %s", err)
 	}
 
 	mapConfig, ok := raw.(map[string]interface{})

post-processor/vagrant/post-processor_test.go

@@ -20,10 +20,18 @@ func TestPostProcessor_ImplementsPostProcessor(t *testing.T) {
 func TestBuilderPrepare_OutputPath(t *testing.T) {
 	var p PostProcessor
 
+	// Default
 	c := testConfig()
 	delete(c, "output")
 	err := p.Configure(c)
+	if err != nil {
+		t.Fatalf("err: %s", err)
+	}
+
+	// Bad template
+	c["output"] = "bad {{{{.Template}}}}"
+	err = p.Configure(c)
 	if err == nil {
-		t.Fatalf("configure should fail")
+		t.Fatal("should have error")
 	}
 }