Milo: Handle missing / transient LogDog failures.

milo/appengine/logdog/http.go

 // Copyright 2016 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 logdog
 
 import (
         "net/http"
         "strings"
 
@@ skipping 43 matching lines @@
         var err error
         if as.Client, err = NewClient(c, ""); err != nil {
                 log.WithError(err).Errorf(c, "Failed to generate LogDog client.")
                 return nil, &miloerror.Error{
                         Code: http.StatusInternalServerError,
                 }
         }
 
         // Load the Milo annotation protobuf from the annotation stream.
         if _, err := as.Fetch(c); err != nil {
-                return nil, err
+                switch errors.Unwrap(err) {
+                case coordinator.ErrNoSuchStream:
+                        return nil, &miloerror.Error{
+                                Message: "Stream does not exist",
+                                Code:    http.StatusNotFound,
+                        }
+
+                case coordinator.ErrNoAccess:
+                        return nil, &miloerror.Error{
+                                Message: "No access to stream",
+                                Code:    http.StatusForbidden,
+                        }
+
+                default:
+                        return nil, &miloerror.Error{
+                                Message: "Failed to load stream",
+                                Code:    http.StatusInternalServerError,
+                        }
+                }
         }
 
         // Convert the Milo Annotation protobuf to Milo objects.
         return &templates.Args{
                 "Build": as.toMiloBuild(c),
         }, nil
 }
 
 func resolveHost(host string) (string, error) {
         // Resolveour our Host, and validate it against a host whitelist.
@@ skipping 24 matching lines @@
         }
 
         // Setup our LogDog client.
         return coordinator.NewClient(&prpc.Client{
                 C: &http.Client{
                         Transport: t,
                 },
                 Host: host,
         }), nil
 }