https://w3c.github.io/webrtc-pc/#rtcrtpreceiver-interface indeed seems not nullable.

Able to reproduce the issue and this is regression in M61, Please find bisect range below :

You are probably looking for a change made after 480501 (known good), but no lat
er than 480502 (first known bad).
CHANGELOG URL:
  https://chromium.googlesource.com/chromium/src/+log/fcbeaac2e4058876f6e76a8dc0215b460f1c36b8..1d05904bcc684ee1267bcef56496a9c3ebce2b61

@hbos: Friendly ping! Could you please provide any update on this issue as it has been marked as a stable blocker.

Thank You!

@guidou can you take a look while I'm on vacation? Back on monday. -From a tent in the middle of nowhere.

wrt to impact, I think there are very few people who use receivers yet, in particular together with renegotiation. adapter did which caused an exception in the ontrack shim but that has now been fixed and a new version has been published.

[Bulk Edit]
URGENT - PTAL.
Your bug is labelled as M61 Stable ReleaseBlock, pls make sure to land the fix and get it merged into the release branch ASAP.

Know that this issue shouldn't block the release?  Remove the ReleaseBlock-Stable label.

Thank you.

govind@: I prepared a CL (https://chromium-review.googlesource.com/c/610233) that fixes part of the bug, but not all of it. I think once this patch lands (and is merged to 61), we can remove the ReleaseBlock label.

hbos@ will prepare a complete fix for 62.

With guidou's fix getReceivers() there is no initialization race and tracks will be created for the receivers.

There is still a bug if a track is created by adding it a stream that already exists, in which case the upper layer will not be aware of the track. With guidou's fix getReceivers() will ignore such receivers instead of exposing a track-less receiver (or crashing in debug build). This will be tracked by  https://crbug.com/755166  instead.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/bed043e4d38f7e38c98bec358a5ef870358ba195

commit bed043e4d38f7e38c98bec358a5ef870358ba195
Author: Guido Urdaneta <guidou@chromium.org>
Date: Mon Aug 14 18:51:05 2017

Fix race during WebRtcMediaStreamTrackAdapter initialization.

This does not completely fix  bug 751785 , but it is a prerequisite.

Bug:  751785 
Change-Id: I3ece316c04f80cbafdb5959d9a19851086a1baf1
Reviewed-on: https://chromium-review.googlesource.com/610233
Commit-Queue: Guido Urdaneta <guidou@chromium.org>
Reviewed-by: Henrik Boström <hbos@chromium.org>
Cr-Commit-Position: refs/heads/master@{#494115}
[modify] https://crrev.com/bed043e4d38f7e38c98bec358a5ef870358ba195/chrome/browser/media/webrtc/webrtc_rtp_browsertest.cc
[modify] https://crrev.com/bed043e4d38f7e38c98bec358a5ef870358ba195/chrome/test/data/webrtc/peerconnection_rtp.js
[modify] https://crrev.com/bed043e4d38f7e38c98bec358a5ef870358ba195/content/renderer/media/webrtc/webrtc_media_stream_track_adapter.cc
[modify] https://crrev.com/bed043e4d38f7e38c98bec358a5ef870358ba195/content/renderer/media/webrtc/webrtc_media_stream_track_adapter.h
[modify] https://crrev.com/bed043e4d38f7e38c98bec358a5ef870358ba195/third_party/WebKit/Source/modules/peerconnection/RTCPeerConnection.cpp

Shall we bump this to M62?

This bug requires manual review: M61 has already been promoted to the beta branch, so this requires manual review
Please contact the milestone owner if you have questions.
Owners: amineer@(Android), cmasso@(iOS), ketakid@(ChromeOS), govind@(Desktop)

For more details visit https://www.chromium.org/issue-tracking/autotriage - Your friendly Sheriffbot

Let's swap the blocked on/ing around. With guidou's fix receivers with null tracks won't be exposed, fixing this bug.

@anatolid: No this is to be merged into M61. The remaining bug can be M62.

Before we approve merge to M61, please answer followings:
* Is the change well baked/verified in Canary, having enough automation tests coverage and safe to merge to M61?
* Any other important details to justify the merge.

Please note M61 is already in Beta, so merge bar is VERY high. Thank you.

govind@: I have verified the fix in Canary. The fix has enough coverage and it should be a clean merge to M61.
If approved, I would merge after a couple more days in Canary just as a precaution, unless you recommend otherwise.

Since this fixes a race that might potentially be exploited, I think we should really merge this to M61.

Approving merge to M61 branch 3163 based on comment #18. Please merge after few more days of Canary coverage. Thank you.

Please merge you change to M61 branch 3163 by 4:00 PM PT tomorrow, Thursday (08/17) so we can take it in for next week Beta release. Thank you.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/7acabd7d26a2d9229393f3596bde08a3eec7334e

commit 7acabd7d26a2d9229393f3596bde08a3eec7334e
Author: Guido Urdaneta <guidou@chromium.org>
Date: Thu Aug 17 08:58:58 2017

Fix race during WebRtcMediaStreamTrackAdapter initialization.

This does not completely fix  bug 751785 , but it is a prerequisite.

TBR=guidou@chromium.org

(cherry picked from commit bed043e4d38f7e38c98bec358a5ef870358ba195)

Bug:  751785 
Change-Id: I3ece316c04f80cbafdb5959d9a19851086a1baf1
Reviewed-on: https://chromium-review.googlesource.com/610233
Commit-Queue: Guido Urdaneta <guidou@chromium.org>
Reviewed-by: Henrik Boström <hbos@chromium.org>
Cr-Original-Commit-Position: refs/heads/master@{#494115}
Reviewed-on: https://chromium-review.googlesource.com/618332
Reviewed-by: Guido Urdaneta <guidou@chromium.org>
Cr-Commit-Position: refs/branch-heads/3163@{#627}
Cr-Branched-From: ff259bab28b35d242e10186cd63af7ed404fae0d-refs/heads/master@{#488528}
[modify] https://crrev.com/7acabd7d26a2d9229393f3596bde08a3eec7334e/chrome/browser/media/webrtc/webrtc_rtp_browsertest.cc
[modify] https://crrev.com/7acabd7d26a2d9229393f3596bde08a3eec7334e/chrome/test/data/webrtc/peerconnection_rtp.js
[modify] https://crrev.com/7acabd7d26a2d9229393f3596bde08a3eec7334e/content/renderer/media/webrtc/webrtc_media_stream_track_adapter.cc
[modify] https://crrev.com/7acabd7d26a2d9229393f3596bde08a3eec7334e/content/renderer/media/webrtc/webrtc_media_stream_track_adapter.h
[modify] https://crrev.com/7acabd7d26a2d9229393f3596bde08a3eec7334e/third_party/WebKit/Source/modules/peerconnection/RTCPeerConnection.cpp

The NextAction date has arrived: 2017-08-17

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/4bf60fd73e9e4f069d49b44a166988d45cf33aae

commit 4bf60fd73e9e4f069d49b44a166988d45cf33aae
Author: Henrik Boström <hbos@chromium.org>
Date: Thu Aug 17 18:08:32 2017

Added DCHECK and TODO in WebRtcMediaStreamTrackAdapter.

Documentation/TODO related to changes
https://chromium-review.googlesource.com/c/610233 which added the
EnsureTrackIsInitialized code.

Bug:  751785 
Change-Id: I69611e461f371c9f926b638c1dc8169716c8771a
Reviewed-on: https://chromium-review.googlesource.com/618715
Reviewed-by: Guido Urdaneta <guidou@chromium.org>
Commit-Queue: Henrik Boström <hbos@chromium.org>
Cr-Commit-Position: refs/heads/master@{#495223}
[modify] https://crrev.com/4bf60fd73e9e4f069d49b44a166988d45cf33aae/content/renderer/media/webrtc/webrtc_media_stream_track_adapter.cc
[modify] https://crrev.com/4bf60fd73e9e4f069d49b44a166988d45cf33aae/content/renderer/media/webrtc/webrtc_media_stream_track_adapter.h