[vpython] Allow default spec via enviornment.

vpython/application/application.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 application
 
 import (
         "flag"
         "fmt"
         "io/ioutil"
@@ skipping 25 matching lines @@
 const (
         // VirtualEnvRootENV is an environment variable that, if set, will be used
         // as the default VirtualEnv root.
         //
         // This value overrides the default (~/.vpython), but can be overridden by the
         // "-root" flag.
         //
         // 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.
 type ReturnCodeError int
 
 func (err ReturnCodeError) Error() string {
         return fmt.Sprintf("python interpreter returned non-zero error: %d", err)
 }
 
 // VerificationFunc is a function used in environment verification.
@@ skipping 136 matching lines @@
         a.opts.EnvConfig.LookPathFunc = lp.look
 
         if a.help {
                 return a.showPythonHelp(c, fs, &lp)
         }
 
         c = a.logConfig.Set(c)
 
         // If an spec path was manually specified, load and use it.
         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").
-                                D("path", a.specPath).
+                var sp vpythonAPI.Spec
+                if err := spec.Load(a.specPath, &sp); err != nil {
+                        return err
+                }
+                a.opts.EnvConfig.Spec = &sp
+        } else if specPath := a.opts.Environ.GetEmpty(DefaultSpecENV); specPath != "" {
+                if err := spec.Load(specPath, &a.opts.DefaultSpec); 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
         // up on completion.
         if a.opts.EnvConfig.BaseDir == "" {
                 tdir, err := ioutil.TempDir("", "vpython")
                 if err != nil {
                         return errors.Annotate(err).Reason("failed to create temporary directory").Err()
@@ skipping 86 matching lines @@
                 },
                 logConfig: logging.Config{
                         Level: logging.Error,
                 },
         }
 
         return run(c, func(c context.Context) error {
                 return a.mainImpl(c, os.Args[0], os.Args[1:])
         })
 }