milo: Add a build info Swarming implementation.

milo/appengine/swarming/build.go

Index: milo/appengine/swarming/build.go
diff --git a/milo/appengine/swarming/build.go b/milo/appengine/swarming/build.go
index 7c41c8d89694e610ea5782c0bd661f9d40810223..cf38a550233ece5fe750306111654c9429871fed 100644
--- a/milo/appengine/swarming/build.go
+++ b/milo/appengine/swarming/build.go
@@ -73,6 +73,7 @@ func getSwarmingClient(c context.Context, server string) (*swarming.Service, err
 type swarmingService interface {
 	getHost() string
 	getSwarmingResult(c context.Context, taskID string) (*swarming.SwarmingRpcsTaskResult, error)
+	getSwarmingRequest(c context.Context, taskID string) (*swarming.SwarmingRpcsTaskRequest, error)
 	getTaskOutput(c context.Context, taskID string) (string, error)
 }
 
@@ -106,7 +107,14 @@ func (svc *prodSwarmingService) getTaskOutput(c context.Context, taskID string)
 	return stdout.Output, nil
 }
 
+func (svc *prodSwarmingService) getSwarmingRequest(c context.Context, taskID string) (*swarming.SwarmingRpcsTaskRequest, error) {
+	return svc.client.Task.Request(taskID).Context(c).Do()
+}
+
 type swarmingFetch struct {
+	fetchReq bool
+	req      *swarming.SwarmingRpcsTaskRequest
+
 	fetchRes bool
 	res      *swarming.SwarmingRpcsTaskResult
 
@@ -122,6 +130,13 @@ type swarmingFetch struct {
 // errNotMiloJob.
 func (f *swarmingFetch) get(c context.Context, svc swarmingService, taskID string) error {
 	err := parallel.FanOutIn(func(workC chan<- func() error) {
+		if f.fetchReq {
+			workC <- func() (err error) {
+				f.req, err = svc.getSwarmingRequest(c, taskID)
+				return
+			}
+		}
+
 		if f.fetchRes {
 			workC <- func() (err error) {
 				f.res, err = svc.getSwarmingResult(c, taskID)
@@ -142,6 +157,10 @@ func (f *swarmingFetch) get(c context.Context, svc swarmingService, taskID strin
 
 	// Current ACL implementation: error if this is not a Milo job.
 	switch {
+	case f.fetchReq:
+		if !isMiloJob(f.req.Tags) {
+			return errNotMiloJob
+		}
 	case f.fetchRes:
 		if !isMiloJob(f.res.Tags) {
 			return errNotMiloJob