Milo: Create build summaries for buildbot in the pubsub pipeline

milo/buildsource/buildbot/build.go

 // Copyright 2016 The LUCI Authors.
 //
 // Licensed under the Apache License, Version 2.0 (the "License");
 // you may not use this file except in compliance with the License.
 // You may obtain a copy of the License at
 //
 //      http://www.apache.org/licenses/LICENSE-2.0
 //
 // Unless required by applicable law or agreed to in writing, software
 // distributed under the License is distributed on an "AS IS" BASIS,
@@ skipping 370 matching lines @@
                 // Also take this oppertunity to sort the properties within the groups.
                 sort.Sort(group)
                 result = append(result, group)
         }
         return
 }
 
 // blame extracts the commit and blame information from a buildbot build and
 // returns it as a list of Commits.
 func blame(b *buildbotBuild) (result []*resp.Commit) {
-        for _, c := range b.Sourcestamp.Changes {
-                files := c.GetFiles()
-                result = append(result, &resp.Commit{
-                        AuthorEmail: c.Who,
-                        Repo:        c.Repository,
-                        CommitTime:  time.Unix(int64(c.When), 0).UTC(),
-                        Revision:    resp.NewLink(c.Revision, c.Revlink),
-                        Description: c.Comments,
-                        Title:       strings.Split(c.Comments, "\n")[0],
-                        File:        files,
-                })
+        if b.Sourcestamp != nil {
+                for _, c := range b.Sourcestamp.Changes {
+                        files := c.GetFiles()
+                        result = append(result, &resp.Commit{
+                                AuthorEmail: c.Who,
+                                Repo:        c.Repository,
+                                CommitTime:  time.Unix(int64(c.When), 0).UTC(),
+                                Revision:    resp.NewLink(c.Revision, c.Revlink),
+                                Description: c.Comments,
+                                Title:       strings.Split(c.Comments, "\n")[0],
+                                File:        files,
+                        })
+                }
         }
         return
 }
 
 // sourcestamp extracts the source stamp from various parts of a buildbot build,
 // including the properties.
 func sourcestamp(c context.Context, b *buildbotBuild) *resp.SourceStamp {
         ss := &resp.SourceStamp{}
         rietveld := ""
         gerrit := ""
@@ skipping 284 matching lines @@
 
         if b.Master == "" {
                 return nil, errors.New("Master name is required", common.CodeParameterError)
         }
         if b.BuilderName == "" {
                 return nil, errors.New("BuilderName name is required", common.CodeParameterError)
         }
 
         return Build(c, b.Master, b.BuilderName, int(num))
 }