Milo: Prefetch logs that are failures

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 afd1745f145a70d84d6db9685d48ea1466cd3795..2e7dc50da911a8a75040aa09dc4eb5ff614136dd 100644
--- a/milo/appengine/swarming/build.go
+++ b/milo/appengine/swarming/build.go
@@ -397,11 +397,14 @@ func streamsFromAnnotatedLog(ctx context.Context, log string) (*logdog.Streams,
 	return c.ToLogDogStreams()
 }
 
-func swarmingBuildImpl(c context.Context, linkBase, server, taskID string) (*resp.MiloBuild, error) {
+// swarmingBuildImpl fetches a swarming task from a swarming server, parses
+// the logs via Annotee, and returns a build response, along with a list of
+// URLs to prefetch.

[agable, 2016/08/18 21:44:11]

Any reason the list of urls isn't part of an "interestingURLs" parameter on the
MiloBuild object, or something like that? Maybe that wouldn't be very Go-ish, I
dunno, just figured I'd ask.

[Ryan Tseng, 2016/08/18 22:09:45]

Originally the thought was that this would manifest itself differently in the
html version and the grpc version, so an html-only hack was good enough.  After
thinking about it a little more, it makes more sense to mark each URL as
interesting or not interesting, and have the templates take care of adding the
prefetch hints.

The downside is the go template engine isn't powerful enough to stop at some
predefined number, so all failures will now be prefetched.  But that's probably
fine too, because they're hints after all.  If everything were failures, then
they're probably all interesting, and it's up to the web browser to decide
wither or not to follow those hints.

+func swarmingBuildImpl(c context.Context, linkBase, server, taskID string) (*resp.MiloBuild, []string, error) {
 	// Fetch the data from Swarming
 	sr, body, err := getSwarming(c, server, taskID)
 	if err != nil {
-		return nil, err
+		return nil, nil, err
 	}
 
 	allowMilo := false
@@ -412,7 +415,7 @@ func swarmingBuildImpl(c context.Context, linkBase, server, taskID string) (*res
 		}
 	}
 	if !allowMilo {
-		return nil, fmt.Errorf("Not A Milo Job")
+		return nil, nil, fmt.Errorf("Not A Milo Job")
 	}
 
 	var build resp.MiloBuild
@@ -439,17 +442,29 @@ func swarmingBuildImpl(c context.Context, linkBase, server, taskID string) (*res
 			}
 
 			if err := addTaskToMiloStep(c, server, sr, lds.MainStream.Data); err != nil {
-				return nil, err
+				return nil, nil, err
 			}
 			logdog.AddLogDogToBuild(c, swarmingURLBuilder(linkBase), lds, &build)
 		}
 	}
 
 	if err := addTaskToBuild(c, server, sr, &build); err != nil {
-		return nil, err
+		return nil, nil, err
+	}
+
+	// Extract links that are failures or exceptions for prefetching.
+	prefetch := []string{}
+	for _, step := range build.Components {
+		if len(prefetch) > 3 {
+			break // Don't prefetch more than 3 things.
+		}
+		switch step.Status {
+		case resp.Failure, resp.InfraFailure:
+			prefetch = append(prefetch, step.MainLink.URL)
+		}
 	}
 
-	return &build, nil
+	return &build, prefetch, nil
 }
 
 // taskPageURL returns a URL to a human-consumable page of a swarming task.