I have a CL which fixes the crash: https://chromium-review.googlesource.com/c/chromium/src/+/1195931

However, EasyUnlockServiceRegular should also be updated so that it waits until DeviceSyncClient is ready before trying to access its synced devices. Once the CL which fixes this crash lands, we should leave this bug open so that the correct fix can be implemented within EasyUnlock.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/6f452dfe25f06aad95744e175e6d292881c6a457

commit 6f452dfe25f06aad95744e175e6d292881c6a457
Author: Kyle Horimoto <khorimoto@google.com>
Date: Wed Aug 29 22:15:27 2018

[CrOS MultiDevice] Remove two unnecessary DCHECK()s.

These DCHECK()s cause a crash on login when debug mode is enabled (see
 https://crbug.com/878653 ).

Bug:  878653 
Change-Id: Iae62dc0bd75018a175299efbae3cb616ae21bc5c
Reviewed-on: https://chromium-review.googlesource.com/1195931
Reviewed-by: Alexander Alekseev <alemate@chromium.org>
Commit-Queue: Kyle Horimoto <khorimoto@chromium.org>
Cr-Commit-Position: refs/heads/master@{#587320}
[modify] https://crrev.com/6f452dfe25f06aad95744e175e6d292881c6a457/chromeos/services/device_sync/public/cpp/device_sync_client_impl.cc

Assigning to hansberry@ to fix the rest of the issue (see comment #5).

Kyle, does the fix need to be merged anywhere?

If this is what is causing the crash on Eve, then yes, it needs to be merged back to 69 at least.

These are DCHECKs that were removed. This crash is happening on user devices, which wouldn't have DCHECKs enabled.

So then maybe this isn't the crash that folks are seeing or that crash is an additional one?

Ah nevermind, this isn't the M-69 blocking crash. That crash is tracked here: https://bugs.chromium.org/p/chromium/issues/detail?id=878002

This is not a launch-blocker -- it was a crash only in debug mode, which no longer crashes after its culprit DCHECKs were removed. Downgrading priority.

As a sidenote, the correct way to resolve this issue will be to put the culprit DCHECKs back, and remove this line [1]. This line is actually not necessary, and was mistakenly placed there. I will send out this proposed fix once our current crunch to get launch blockers in 70 has passed.

1) https://cs.chromium.org/chromium/src/chrome/browser/chromeos/login/easy_unlock/easy_unlock_service_regular.cc?q=easyunlockserviceregular&sq=package:chromium&dr=CSs&l=519

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/196454e3fde49ffea3b580a520a8778e2133982c

commit 196454e3fde49ffea3b580a520a8778e2133982c
Author: Ryan Hansberry <hansberry@chromium.org>
Date: Mon Sep 17 21:58:03 2018

Smart Lock: Remove unnecessary and incorrect call to DeviceSyncClient.

Remove call to DeviceSyncClient::GetSyncedDevices() in
EasyUnlockServiceRegular. This call was incorrect because it didn't
wait for DeviceSyncClient::Observer::OnReady(), but is also
unnecessary because EasyUnlockServiceRegular's override of OnReady()
will call GetSyncedDevices() anyway.

This CL also puts back DCHECKs within DeviceSyncClient which were
removed in [1]. These DCHECKs should remain present to continue
catching incorrect calls such as this one.

1) https://chromium-review.googlesource.com/c/chromium/src/+/1195931

R=jhawkins@chromium.org, khorimoto@chromium.org

Bug:  878653 
Change-Id: Ib13668063be89b3856076c1476cb853215f12f22
Reviewed-on: https://chromium-review.googlesource.com/1218342
Reviewed-by: Kyle Horimoto <khorimoto@chromium.org>
Reviewed-by: James Hawkins <jhawkins@chromium.org>
Commit-Queue: Ryan Hansberry <hansberry@chromium.org>
Cr-Commit-Position: refs/heads/master@{#591835}
[modify] https://crrev.com/196454e3fde49ffea3b580a520a8778e2133982c/chrome/browser/chromeos/login/easy_unlock/easy_unlock_service_regular.cc
[modify] https://crrev.com/196454e3fde49ffea3b580a520a8778e2133982c/chromeos/services/device_sync/public/cpp/device_sync_client_impl.cc