Milo buildbot builder page: Add pagnation with cursors.

milo/appengine/buildbot/datastore.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 buildbot
 
 import (
-        ds "github.com/luci/gae/service/datastore"
+        "github.com/luci/gae/service/datastore"
 
         "golang.org/x/net/context"
 )
 
 // buildQueryBatchSize is the batch size to use when querying build. It is
 // employed as an upper bound by getBuildQueryBatcher.
 //
 // This should be tuned to the observed query timeout for build loading. Since
 // loading is CPU-bound, this will probably be low. If build queries start
 // encountering datastore timeouts, reduce this value.
 const buildQueryBatchSize = 50
 
-// getBuildQueryBatcher returns a ds.Batcher tuned for executing queries on the
+// getBuildQueryBatcher returns a datastore.Batcher tuned for executing queries on the
 // "buildbotBuild" entity.
-func getBuildQueryBatcher(c context.Context) *ds.Batcher {
-        constraints := ds.Raw(c).Constraints()
+func getBuildQueryBatcher(c context.Context) *datastore.Batcher {
+        constraints := datastore.Raw(c).Constraints()
         if constraints.QueryBatchSize > buildQueryBatchSize {
                 constraints.QueryBatchSize = buildQueryBatchSize
         }
-        return &ds.Batcher{
+        return &datastore.Batcher{
                 Size: constraints.QueryBatchSize,
         }
 }
+
+// runBuildsQuery takes a buildbotBuild query and returns a list of builds
+// along with a cursor.  We pass the limit here and apply it to the query as
+// an optimization because then we can create a build container of that size.
+func runBuildsQuery(c context.Context, q *datastore.Query, limit int32) (
+        []*buildbotBuild, *datastore.Cursor, error) {
+
+        if limit != 0 {
+                q = q.Limit(limit)
+        }
+        builds := make([]*buildbotBuild, 0, limit)
+        var nextCursor *datastore.Cursor
+        err := getBuildQueryBatcher(c).Run(
+                c, q, func(build *buildbotBuild, getCursor datastore.CursorCB) error {
+                        builds = append(builds, build)
+                        tmpCursor, err := getCursor()
+                        if err != nil {
+                                return err
+                        }
+                        nextCursor = &tmpCursor
+                        return nil
+                })
+        return builds, nextCursor, err
+}