Filed by: rmaries [at] mozilla.com
Parsed log: https://treeherder.mozilla.org/logviewer?job_id=323538283&repo=autoland
Full log: https://firefox-ci-tc.services.mozilla.com/api/queue/v1/task/Y38Nfp4mQoamo62ynJIzqg/runs/0/artifacts/public/logs/live_backing.log

[task 2020-12-04T04:53:03.299Z] 04:53:03     INFO - TEST-START | browser/modules/test/browser/browser_PermissionUI.js
[task 2020-12-04T04:53:11.518Z] 04:53:11     INFO - TEST-PASS | browser/modules/test/browser/browser_PermissionUI.js | Should be showing the URI of the requesting page - true == true - 
[task 2020-12-04T04:53:11.518Z] 04:53:11     INFO - Buffered messages finished
[task 2020-12-04T04:53:11.519Z] 04:53:11     INFO - TEST-UNEXPECTED-FAIL | browser/modules/test/browser/browser_PermissionUI.js | A promise chain failed to handle a rejection: can't access property "isAppProvided", engine is null - stack: _updateURLBarPlaceholder@chrome://browser/content/browser.js:4136:9
[task 2020-12-04T04:53:11.519Z] 04:53:11     INFO - _updateURLBarPlaceholderFromDefaultEngine@chrome://browser/content/browser.js:4112:10
[task 2020-12-04T04:53:11.519Z] 04:53:11     INFO - Rejection date: Fri Dec 04 2020 04:53:09 GMT+0000 (Coordinated Universal Time) - false == true - JS frame :: resource://testing-common/PromiseTestUtils.jsm :: assertNoUncaughtRejections :: line 282
[task 2020-12-04T04:53:11.520Z] 04:53:11     INFO - Stack trace:
[task 2020-12-04T04:53:11.520Z] 04:53:11     INFO - resource://testing-common/PromiseTestUtils.jsm:assertNoUncaughtRejections:282
[task 2020-12-04T04:53:11.520Z] 04:53:11     INFO - chrome://mochikit/content/browser-test.js:Tester_execTest/<:1100
[task 2020-12-04T04:53:11.520Z] 04:53:11     INFO - Leaving test bound test_window_swap
[task 2020-12-04T04:53:11.521Z] 04:53:11     INFO - Console message: BrowserTestUtils.withNewTab: Tab was already closed before removeTab would have been called
[task 2020-12-04T04:53:11.522Z] 04:53:11     INFO - GECKO(12439) | ###!!! [Parent][RunMessage] Error: Channel closing: too late to send/recv, messages will be lost
[task 2020-12-04T04:53:11.522Z] 04:53:11     INFO - GECKO(12439) | MEMORY STAT | vsize 3504MB | residentFast 431MB | heapAllocated 151MB
[task 2020-12-04T04:53:11.525Z] 04:53:11     INFO - TEST-OK | browser/modules/test/browser/browser_PermissionUI.js | took 7593ms
[task 2020-12-04T04:53:11.526Z] 04:53:11     INFO - GECKO(12439) | [Child 12497: Main Thread]: I/DocShellAndDOMWindowLeak ++DOCSHELL 0x7f646556f000 == 2 [pid = 12497] [id = 12]
[task 2020-12-04T04:53:11.526Z] 04:53:11     INFO - GECKO(12439) | [Child 12497: Main Thread]: I/DocShellAndDOMWindowLeak ++DOMWINDOW == 3 (0x7f64679e7040) [pid = 12497] [serial = 35] [outer = (nil)]
[task 2020-12-04T04:53:11.526Z] 04:53:11     INFO - GECKO(12439) | [Child 12497: Main Thread]: I/DocShellAndDOMWindowLeak ++DOMWINDOW == 4 (0x7f6465570400) [pid = 12497] [serial = 36] [outer = 0x7f64679e7040]
[task 2020-12-04T04:53:11.527Z] 04:53:11     INFO - checking window state
[task 2020-12-04T04:53:11.527Z] 04:53:11     INFO - GECKO(12439) | must wait for focus
[task 2020-12-04T04:53:11.527Z] 04:53:11     INFO - GECKO(12439) | [Child 13092: Main Thread]: I/DocShellAndDOMWindowLeak --DOCSHELL 0x7f168bb46400 == 1 [pid = 13092] [id = 0] [url = http://example.com/]
[task 2020-12-04T04:53:11.528Z] 04:53:11     INFO - GECKO(12439) | [Child 13092: Main Thread]: I/DocShellAndDOMWindowLeak --DOCSHELL 0x7f168bb9e000 == 0 [pid = 13092] [id = 1] [url = about:blank]```

Aligns with the landing of bug 1675624. Dale, please investigate this frequent failure.

Looking into it now

There have been 46 total failures in the last 7 days: https://treeherder.mozilla.org/intermittent-failures/bugdetails?startday=2020-12-14&endday=2020-12-21&tree=trunk&bug=1680672

Recent failure log: https://treeherder.mozilla.org/logviewer?job_id=325073766&repo=autoland&lineNumber=51475

Dale, are there any updates?

https://hg.mozilla.org/mozilla-central/rev/33335bf6c31f

We should fix the root cause of these failures and enable the test again. On first glance I don't see any issues with the test itself.
Mark, is the problem you mention here: https://bugzilla.mozilla.org/show_bug.cgi?id=1685124#c3 related to this failure? Can we dupe the bug?

Yes, that's causing this. We can mark as dupe, or re-closed as fixed, I'm fine with either.