Actually I'm not totally sure this is a bug. We give the stream to the URLLoaderClient via OnStartLoadingResponseBody and maybe from then on it's the Client's job to read the stream and detect any failures?

Not sure what's the intended contract of URLLoaderClient. When talking to kinuko@ it sounded like OnComplete() should be called only after the response body finished loading completely. But I'm not sure that if currently we have an actual bug or if changing the OnComplete() call will affect real behavior (AFAICT, once  NavigationURLLoaderNetworkService::OnStartLoadingResponseBody() is called we commit the navigation to the renderer, so it might be too late for OnComplete() to have any effect).

OnComplete is supposed to provide some metrics like transferred bytes / timing info, and they can surface at Performance Timing API.

(Separately from that I also have wondered if we could loosen that part after OnStartLoadingResponseBody is called and data pipe is given to the client, which makes the use cases like this, i.e. just passing the given data pipe to the loader client, really easy)

(memo from offline chat) yhirano says the interface is intended to call OnComplete after all data has finished being written. Since SWUrlLoaderJob doesn't know when that is, either we need to add a new signal from the service worker to the loader, or have the loader create an intermediate data pipe and mediate data transfer from SW to the URL loader client. yhirano suggests asking Mojo team for a native solution and says this has been discussed before.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/19a781d5c693db99f8c1dcc4e4cd7c3844796e58

commit 19a781d5c693db99f8c1dcc4e4cd7c3844796e58
Author: Emi Morikawa <emim@google.com>
Date: Fri Aug 25 07:53:39 2017

Add more ServiceWorkerURLLoaderJob tests: stream response abort.

Bug:  748416 , 758455 
Change-Id: I588f541fde654339e73c01852419992911ac6af9
Reviewed-on: https://chromium-review.googlesource.com/631080
Reviewed-by: Kinuko Yasuda <kinuko@chromium.org>
Reviewed-by: Makoto Shimazu <shimazu@chromium.org>
Reviewed-by: Matt Falkenhagen <falken@chromium.org>
Commit-Queue: Emi Morikawa <emim@google.com>
Cr-Commit-Position: refs/heads/master@{#497350}
[modify] https://crrev.com/19a781d5c693db99f8c1dcc4e4cd7c3844796e58/content/browser/service_worker/service_worker_url_loader_job.cc
[modify] https://crrev.com/19a781d5c693db99f8c1dcc4e4cd7c3844796e58/content/browser/service_worker/service_worker_url_loader_job_unittest.cc

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/fd0efa6a9192b26f257bb2aa3c4135dbec38ce37

commit fd0efa6a9192b26f257bb2aa3c4135dbec38ce37
Author: Emi Morikawa <emim@google.com>
Date: Mon Aug 28 10:37:00 2017

Add more ServiceWorkerURLLoaderJob tests: stream response and cancel.

Bug:  748416 ,  758455 
Change-Id: Ie6f103f9bc6de2a59192648f4b7526a5d3a13076
Reviewed-on: https://chromium-review.googlesource.com/637060
Commit-Queue: Emi Morikawa <emim@google.com>
Reviewed-by: Kinuko Yasuda <kinuko@chromium.org>
Reviewed-by: Matt Falkenhagen <falken@chromium.org>
Cr-Commit-Position: refs/heads/master@{#497729}
[modify] https://crrev.com/fd0efa6a9192b26f257bb2aa3c4135dbec38ce37/content/browser/service_worker/service_worker_url_loader_job_unittest.cc

I learned we already have the mechanism set up via ServiceWorkerStreamHandle and ServiceWorkerStreamCallback interfaces. The SWStreamCallback::OnCompleted() is called when the response body finishes, and OnAborted() is called on error.

We should port what ServiceWorkerURLRequestJob and ServiceWorkerDataPipeReader do to ServiceWorkerURLLoaderJob.

I think we don't need all of SWDataPipeReader, just need to make a mojom::ServiceWorkerStreamCallback implementation in SWURLLoaderJob so that when OnCompleted() is called we CommitCompleted(OK) and when OnAborted is called we CommitCompleted(ERR).

We also need to do what SWDataPipeReader does with streaming_version->AddStreamingURLRequestJob(). This lets everyone know the service worker is still doing work, even though the fetch event may have already finished. 

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/8b58011311a0bba4701d72aa26718fc45ea424bf

commit 8b58011311a0bba4701d72aa26718fc45ea424bf
Author: Emi Morikawa <emim@google.com>
Date: Tue Sep 12 04:53:41 2017

S13nServiceWorker: Wait for the stream response to finish in ServiceWorkerURLLoaderJob.

Make a new inner class. This calls OnComplete() with appropriate error code.

Bug:  758455 
Change-Id: I2589dc740a7338bffaf6a059526775f620ee1a1c
Reviewed-on: https://chromium-review.googlesource.com/656880
Reviewed-by: Matt Falkenhagen <falken@chromium.org>
Reviewed-by: Kinuko Yasuda <kinuko@chromium.org>
Commit-Queue: Emi Morikawa <emim@google.com>
Cr-Commit-Position: refs/heads/master@{#501179}
[modify] https://crrev.com/8b58011311a0bba4701d72aa26718fc45ea424bf/content/browser/service_worker/service_worker_url_loader_job.cc
[modify] https://crrev.com/8b58011311a0bba4701d72aa26718fc45ea424bf/content/browser/service_worker/service_worker_url_loader_job.h
[modify] https://crrev.com/8b58011311a0bba4701d72aa26718fc45ea424bf/content/browser/service_worker/service_worker_url_loader_job_unittest.cc
[modify] https://crrev.com/8b58011311a0bba4701d72aa26718fc45ea424bf/content/browser/service_worker/service_worker_version.cc
[modify] https://crrev.com/8b58011311a0bba4701d72aa26718fc45ea424bf/content/browser/service_worker/service_worker_version.h

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/9f173b376d00e590375290b8aba3f205e8f74c9e

commit 9f173b376d00e590375290b8aba3f205e8f74c9e
Author: Emi Morikawa <emim@google.com>
Date: Tue Sep 12 09:18:38 2017

ServiceWorker: Use counter for monitoring the number of ongoing stream responses.

Use counter instead of using pointers in ServiceWorkerVersion.

Bug:  758455 
Change-Id: I4fc5205940e18b130be17c52fbcea3e56246fd0e
Reviewed-on: https://chromium-review.googlesource.com/662004
Commit-Queue: Emi Morikawa <emim@google.com>
Reviewed-by: Matt Falkenhagen <falken@chromium.org>
Reviewed-by: Kinuko Yasuda <kinuko@chromium.org>
Cr-Commit-Position: refs/heads/master@{#501223}
[modify] https://crrev.com/9f173b376d00e590375290b8aba3f205e8f74c9e/content/browser/service_worker/service_worker_data_pipe_reader.cc
[modify] https://crrev.com/9f173b376d00e590375290b8aba3f205e8f74c9e/content/browser/service_worker/service_worker_url_loader_job.cc
[modify] https://crrev.com/9f173b376d00e590375290b8aba3f205e8f74c9e/content/browser/service_worker/service_worker_version.cc
[modify] https://crrev.com/9f173b376d00e590375290b8aba3f205e8f74c9e/content/browser/service_worker/service_worker_version.h

Apologies, applied the wrong component in bulk.