[vpython] Allow default spec via enviornment.

vpython/options.go

 // Copyright 2017 The LUCI Authors. All rights reserved.
 // Use of this source code is governed under the Apache License, Version 2.0
 // that can be found in the LICENSE file.
 
 package vpython
 
 import (
         "os"
 
         "golang.org/x/net/context"
 
         "github.com/luci/luci-go/vpython/api/vpython"
         "github.com/luci/luci-go/vpython/python"
         "github.com/luci/luci-go/vpython/spec"
         "github.com/luci/luci-go/vpython/venv"
 
         "github.com/luci/luci-go/common/errors"
         "github.com/luci/luci-go/common/logging"
         "github.com/luci/luci-go/common/system/environ"
         "github.com/luci/luci-go/common/system/filesystem"
 )
 
 // Options is the set of options to use to construct and execute a VirtualEnv
 // Python application.
 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.
         //
         // The empty value is a valid default spec.Loader.
         SpecLoader spec.Loader
 
         // Args are the arguments to forward to the Python process.
         Args []string
 
         // WaitForEnv, if true, means that if another agent holds a lock on the target
@@ skipping 99 matching lines @@
                                 Err()
                 }
                 if spec != nil {
                         o.EnvConfig.Spec = spec
                         return nil
                 }
         }
 
         // Do we have a spec file in the environment?
         if v, ok := o.Environ.Get(EnvironmentStampPathENV); ok {
-                if o.EnvConfig.Spec, err = spec.Load(v); err != nil {
+                var sp vpython.Spec
+                if err := spec.Load(v, &sp); err != nil {
                         return errors.Annotate(err).Reason("failed to load environment-supplied spec from: %(path)s").
                                 D("path", v).
                                 Err()
                 }
+
                 logging.Infof(c, "Loaded spec from environment: %s", v)
+                o.EnvConfig.Spec = &sp
                 return nil
         }
 
-        // Unable to load a spec.
-        logging.Infof(c, "Unable to resolve specification path. Using empty specification.")
-        o.EnvConfig.Spec = &vpython.Spec{}
+        // If standard resolution doesn't yield a spec, fall back on our default spec.
+        logging.Infof(c, "Unable to resolve specification path. Using default specification.")
+        o.EnvConfig.Spec = &o.DefaultSpec
         return nil
 }