vpython: Leave "pip" and related tools.

vpython/spec/spec.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 spec
 
 import (
         "crypto/sha256"
         "encoding/hex"
         "fmt"
@@ skipping 27 matching lines @@
                                 D("name", pkg.Name).
                                 Err()
                 }
         }
 
         return nil
 }
 
 // Hash hashes the contents of the supplied "spec" and returns the result as
 // a hex-encoded string.
-func Hash(spec *vpython.Spec) string {
+//
+// If not empty, the contents of extra are prefixed to hash string. This can
+// be used to factor additional influences into the spec hash.
+func Hash(spec *vpython.Spec, extra string) string {
         data, err := proto.Marshal(spec)
         if err != nil {
                 panic(fmt.Errorf("failed to marshal proto: %v", err))
         }
 
         mustWrite := func(v int, err error) {
                 if err != nil {
                         panic(fmt.Errorf("impossible: %s", err))
                 }
         }
 
         hash := sha256.New()
+        if extra != "" {
+                mustWrite(fmt.Fprintf(hash, "%s:", extra))
+        }
         mustWrite(fmt.Fprintf(hash, "%s:", vpython.Version))
         mustWrite(hash.Write(data))
         return hex.EncodeToString(hash.Sum(nil))
 }
 
 type specPackageSlice []*vpython.Spec_Package
 
 func (s specPackageSlice) Len() int      { return len(s) }
 func (s specPackageSlice) Swap(i, j int) { s[i], s[j] = s[j], s[i] }
 
 func (s specPackageSlice) Less(i, j int) bool {
         return sortby.Chain{
                 func(i, j int) bool { return s[i].Name < s[j].Name },
                 func(i, j int) bool { return s[i].Version < s[j].Version },
         }.Use(i, j)
 }