[vpython] Allow default spec via enviornment.

vpython/application/application.go

Index: vpython/application/application.go
diff --git a/vpython/application/application.go b/vpython/application/application.go
index 0fcfcbfdb634419348e4f990ae75b560a3379ee6..0961460b411b986bb2e2ca2c3ffbb98dfcb83fb0 100644
--- a/vpython/application/application.go
+++ b/vpython/application/application.go
@@ -42,6 +42,10 @@ const (
 	// Like "-root", if this value is present but empty, a tempdir will be used
 	// for the VirtualEnv root.
 	VirtualEnvRootENV = "VPYTHON_VIRTUALENV_ROOT"
+
+	// DefaultSpecENV is an enviornment variable that, if set, will be used as the
+	// default VirtualEnv spec file if none is provided or found through probing.
+	DefaultSpecENV = "VPYTHON_DEFAULT_SPEC"
 )
 
 // ReturnCodeError is an error wrapping a return code value.
@@ -177,10 +181,19 @@ func (a *application) mainImpl(c context.Context, args []string) error {
 	if a.specPath != "" {
 		var err error
 		if a.opts.EnvConfig.Spec, err = spec.Load(a.specPath); err != nil {
-			return errors.Annotate(err).Reason("failed to load specification file (-spec) from: %(path)s").
+			return errors.Annotate(err).
+				Reason("failed to load specification file (%(source)s) from: %(path)s").
 				D("path", a.specPath).

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

no D for source?

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

oops, "source" is old.

 				Err()
 		}
+	} else if specPath := a.opts.Environ.GetEmpty(DefaultSpecENV); specPath != "" {
+		var err error
+		if a.opts.DefaultSpec, err = spec.Load(specPath); err != nil {
+			return errors.Annotate(err).
+				Reason("failed to load default specification file ("+DefaultSpecENV+"from: %(path)s").
+				D("path", specPath).
+				Err()
+		}
 	}
 
 	// If an empty BaseDir was specified, use a temporary directory and clean it