[vpython] Allow default spec via enviornment.

vpython/options.go

Index: vpython/options.go
diff --git a/vpython/options.go b/vpython/options.go
index f24b3ff69f074305fef4d86ac515e30154b9ae90..dd026afd111fb80573941dce786f42adab7dba7e 100644
--- a/vpython/options.go
+++ b/vpython/options.go
@@ -26,6 +26,10 @@ type Options struct {
 	// EnvConfig is the VirtualEnv configuration to run from.
 	EnvConfig venv.Config
 
+	// DefaultSpec is the default specification to use, if no specification was
+	// supplied or probed.
+	DefaultSpec *vpython.Spec
+
 	// SpecLoader is the spec.Loader to use to load a specification file for a
 	// given script.
 	//
@@ -154,6 +158,12 @@ func (o *Options) ResolveSpec(c context.Context) error {
 		return nil
 	}
 
+	// If standard resolution doesn't yield a spec, fall back on our default spec.
+	if o.DefaultSpec != nil {
+		o.EnvConfig.Spec = o.DefaultSpec

[iannucci, 2017/06/05 19:46:36]

would it make sense to have this be by-value, then this case and the below case
collapse into one?

[dnj, 2017/06/06 13:40:39]

Yeah I suppose it would. Done.

+		return nil
+	}
+
 	// Unable to load a spec.
 	logging.Infof(c, "Unable to resolve specification path. Using empty specification.")
 	o.EnvConfig.Spec = &vpython.Spec{}