I can add those, SimpleUrlLoader now expose the response headers. url_chain is added in Oct.2017, I probably missed that.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/5c9266c5f89cd6b808e1b00a417dba9bc8e7a535

commit 5c9266c5f89cd6b808e1b00a417dba9bc8e7a535
Author: Xing Liu <xingliu@chromium.org>
Date: Thu Jul 19 00:40:26 2018

Background download: Propagates URL chain and response headers.

This CL propagates URL chain and response headers for in memory download
backend that is used in incognito mode for background fetch.

Bug:  863949 
Change-Id: I07baff393111aaa2bd2ce2ef9965a78d299be125
Reviewed-on: https://chromium-review.googlesource.com/1142466
Reviewed-by: Min Qin <qinmin@chromium.org>
Commit-Queue: Xing Liu <xingliu@chromium.org>
Cr-Commit-Position: refs/heads/master@{#576282}
[modify] https://crrev.com/5c9266c5f89cd6b808e1b00a417dba9bc8e7a535/components/download/internal/background_service/in_memory_download.cc
[modify] https://crrev.com/5c9266c5f89cd6b808e1b00a417dba9bc8e7a535/components/download/internal/background_service/in_memory_download.h
[modify] https://crrev.com/5c9266c5f89cd6b808e1b00a417dba9bc8e7a535/components/download/internal/background_service/in_memory_download_driver_unittest.cc
[modify] https://crrev.com/5c9266c5f89cd6b808e1b00a417dba9bc8e7a535/components/download/internal/background_service/in_memory_download_unittest.cc

I don't think this is fixed, I'm still getting the same issue.

OnResponseStarted, which sets the headers (https://cs.chromium.org/chromium/src/components/download/internal/background_service/in_memory_download.cc?g=0&l=204)
is being called after the client is notified (https://cs.chromium.org/chromium/src/components/download/internal/background_service/in_memory_download_driver.cc?g=0&l=111).

Good catch, this should be called after getting server response to match non incognito download.

Hi, is there any URL that we can test background fetch?

Hey, you can use https://rayankans.github.io/bgf/bgf.html (but you will need to build chrome from head for it to work)

You will also need to enable this flag: chrome://flags/#enable-experimental-web-platform-features

Thanks.

Test website has been moved to https://kanslulz.co.uk/bgf

Currently I have a CL for this https://chromium-review.googlesource.com/c/chromium/src/+/1171594, but some webkit layout test for background fetch will fail and hit a DCHECK.

This is probably due to BackgroundFetchDelegateProxy::OnDownloadComplete can be called without BackgroundFetchDelegateProxy::Core::OnDownloadStarted if the request fails at the very beginning on Mac with this fix.

Having OnDownloadStarted being called before OnDownloadComplete seems like a reasonable contract to me. 

Are you planning to fix that or do you think some changes need to be made from our end?

Currently OnDownloadStarted is triggered by network::SimpleURLLoader(SimpleURLLoader::SetOnResponseStartedCallback), we just plumb this event.

Let me try to remove the DCHECK hit in BackgroundFetchCrossOriginFilter to see if that works.

OnDownloadStarted is supposed to call after we get the server response, so if the connection is dropped before response kicked in, the network layer will directly call OnResponseComplete with an error code. I feel this is slightly better than tweaking the call contract in download service layer or background fetch layer.

per #17, I just double checked the download code path for normal profile, it seems we actually tweaked the call contract in 
https://cs.chromium.org/chromium/src/content/browser/download/download_request_core.cc?type=cs&g=0&l=431
https://cs.chromium.org/chromium/src/components/download/internal/common/download_response_handler.cc?type=cs&q=download_response&g=0&l=218

Probably do the same thing in InMemoryDownload is the correct fix for this.

Just confirmed this is working. Thanks!

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/8d9f363ef54cf1debdade0ac5e84b497c0e814c6

commit 8d9f363ef54cf1debdade0ac5e84b497c0e814c6
Author: Xing Liu <xingliu@chromium.org>
Date: Wed Sep 05 17:27:42 2018

Background download: Propagate OnDownloadCreated after server response.

In the download backend used by incognito mode in background download
service, OnDownloadCreated is called before receiving the server
response, that the URL chain and the server response headers are not
correctly propagate to clients.

TBR=peter@chromium.org

Bug:  863949 
Change-Id: Ic5f4dabe1c312a8c949bfb5edbc4e17e3e1081be
Reviewed-on: https://chromium-review.googlesource.com/1171594
Commit-Queue: Xing Liu <xingliu@chromium.org>
Reviewed-by: Rayan Kanso <rayankans@chromium.org>
Reviewed-by: Shakti Sahu <shaktisahu@chromium.org>
Reviewed-by: Min Qin <qinmin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#588914}
[modify] https://crrev.com/8d9f363ef54cf1debdade0ac5e84b497c0e814c6/components/download/internal/background_service/in_memory_download.cc
[modify] https://crrev.com/8d9f363ef54cf1debdade0ac5e84b497c0e814c6/components/download/internal/background_service/in_memory_download.h
[modify] https://crrev.com/8d9f363ef54cf1debdade0ac5e84b497c0e814c6/components/download/internal/background_service/in_memory_download_driver.cc
[modify] https://crrev.com/8d9f363ef54cf1debdade0ac5e84b497c0e814c6/components/download/internal/background_service/in_memory_download_driver.h
[modify] https://crrev.com/8d9f363ef54cf1debdade0ac5e84b497c0e814c6/components/download/internal/background_service/in_memory_download_driver_unittest.cc
[modify] https://crrev.com/8d9f363ef54cf1debdade0ac5e84b497c0e814c6/components/download/internal/background_service/in_memory_download_unittest.cc
[modify] https://crrev.com/8d9f363ef54cf1debdade0ac5e84b497c0e814c6/content/shell/browser/layout_test/layout_test_background_fetch_delegate.cc