auth: Make luci-go services trust signatures produced by the token server.

server/auth/internal/fetch.go

 // Copyright 2015 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 internal
 
 import (
         "bytes"
         "encoding/json"
         "fmt"
@@ skipping 53 matching lines @@
 func (r *Request) Do(c context.Context) error {
         // Grab a client first. Use same client for all retry attempts.
         var client *http.Client
         if clientFactory != nil {
                 var err error
                 if client, err = clientFactory(c, r.Scopes); err != nil {
                         return err
                 }
         } else {
                 client = http.DefaultClient
+                if testTransport := c.Value(&testTransportKey); testTransport != nil {
+                        client = &http.Client{Transport: testTransport.(http.RoundTripper)}
+                }
         }
 
         // Prepare a blob with the request body. Marshal it once, to avoid
         // remarshaling on retries.
         isJSON := false
         var bodyBlob []byte
         if blob, ok := r.Body.([]byte); ok {
                 bodyBlob = blob
         } else if r.Body != nil {
                 var err error
@@ skipping 51 matching lines @@
                 return err
         }
         if val != nil {
                 if err = json.NewDecoder(resp.Body).Decode(val); err != nil {
                         logging.Errorf(c, "auth: URL fetch failed, bad JSON - %s", err)
                         return fmt.Errorf("auth: can't deserialize JSON at %q - %s", r.URL, err)
                 }
         }
         return nil
 }