[vpython] Implement smart probing.

vpython/venv/config.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 venv
 
 import (
         "fmt"
         "os"
         "path/filepath"
@@ skipping 31 matching lines @@
 
         // Package is the VirtualEnv package to install. It must be non-nil and
         // valid. It will be used if the environment specification doesn't supply an
         // overriding one.
         Package vpython.Spec_Package
 
         // Python is the Python interpreter to use. If empty, one will be resolved
         // based on the Spec and the current PATH.
         Python string
 
+        // LookPathFunc, if not nil, will be used instead of exec.LookPath to find the
+        // underlying Python interpreter.
+        LookPathFunc python.LookPathFunc
+
         // Spec is the specification file to use to construct the VirtualEnv. If
         // nil, or if fields are missing, they will be filled in by probing the system
         // PATH.
         Spec *vpython.Spec
 
         // PruneThreshold, if >0, is the maximum age of a VirtualEnv before it should
         // be pruned. If <= 0, there is no maximum age, so no pruning will be
         // performed.
         PruneThreshold time.Duration
         // MaxPrunesPerSweep applies a limit to the number of items to prune per
@@ skipping 185 matching lines @@
         specVers, err := python.ParseVersion(s.PythonVersion)
         if err != nil {
                 return errors.Annotate(err).Reason("failed to parse Python version from: %(value)q").
                         D("value", s.PythonVersion).
                         Err()
         }
 
         if cfg.Python == "" {
                 // No explicitly-specified Python path. Determine one based on the
                 // specification.
-                if cfg.si, err = python.Find(c, specVers); err != nil {
+                if cfg.si, err = python.Find(c, specVers, cfg.LookPathFunc); err != nil {
                         return errors.Annotate(err).Reason("could not find Python for: %(vers)s").
                                 D("vers", specVers).
                                 Err()
                 }
                 cfg.Python = cfg.si.Python
         } else {
                 cfg.si = &python.Interpreter{
                         Python: cfg.Python,
                 }
         }
@@ skipping 20 matching lines @@
         // Resolve to absolute path.
         if err := filesystem.AbsPath(&cfg.Python); err != nil {
                 return errors.Annotate(err).Reason("could not get absolute path for: %(python)s").
                         D("python", cfg.Python).
                         Err()
         }
         return nil
 }
 
 func (cfg *Config) systemInterpreter() *python.Interpreter { return cfg.si }