Filed by: apavel [at] mozilla.com
Parsed log: https://treeherder.mozilla.org/logviewer.html#?job_id=258278374&repo=autoland
Full log: https://queue.taskcluster.net/v1/task/ZRapiksUT0iyHya3vzQqow/runs/0/artifacts/public/logs/live_backing.log

[task 2019-07-25T11:45:59.787Z] 11:45:59 INFO - TEST-START | dom/canvas/test/webgl-conf/generated/test_conformance__extensions__webgl-compressed-texture-s3tc.html
[task 2019-07-25T11:45:59.930Z] 11:45:59 INFO - GECKO(1671) | JavaScript error: http://mochi.test:8888/tests/dom/canvas/test/webgl-conf/checkout/conformance/extensions/webgl-compressed-texture-s3tc.html, line 96: ReferenceError: WebGLTestUtils is not defined
[task 2019-07-25T11:45:59.930Z] 11:45:59 INFO - TEST-INFO | started process screencapture
[task 2019-07-25T11:46:00.168Z] 11:46:00 INFO - TEST-INFO | screencapture: exit 0
[task 2019-07-25T11:46:00.168Z] 11:46:00 INFO - Buffered messages logged at 11:45:59
[task 2019-07-25T11:46:00.168Z] 11:46:00 INFO - TEST-PASS | dom/canvas/test/webgl-conf/generated/test_conformance__extensions__webgl-compressed-texture-s3tc.html | A valid string reason is expected
[task 2019-07-25T11:46:00.168Z] 11:46:00 INFO - TEST-PASS | dom/canvas/test/webgl-conf/generated/test_conformance__extensions__webgl-compressed-texture-s3tc.html | Reason cannot be empty
[task 2019-07-25T11:46:00.169Z] 11:46:00 INFO - Buffered messages finished
[task 2019-07-25T11:46:00.169Z] 11:46:00 INFO - TEST-UNEXPECTED-FAIL | dom/canvas/test/webgl-conf/generated/test_conformance__extensions__webgl-compressed-texture-s3tc.html | successfullyParsed should be true (of type boolean). Was undefined (of type undefined).
[task 2019-07-25T11:46:00.169Z] 11:46:00 INFO - SimpleTest.ok@SimpleTest/SimpleTest.js:275:18
[task 2019-07-25T11:46:00.169Z] 11:46:00 INFO - reportResults@dom/canvas/test/webgl-conf/mochi-single.html?checkout/conformance/extensions/webgl-compressed-texture-s3tc.html:22:14
[task 2019-07-25T11:46:00.169Z] 11:46:00 INFO - reportTestResultsToHarness@dom/canvas/test/webgl-conf/checkout/js/js-test-pre.js:111:36
[task 2019-07-25T11:46:00.169Z] 11:46:00 INFO - testFailed@dom/canvas/test/webgl-conf/checkout/js/js-test-pre.js:241:5
[task 2019-07-25T11:46:00.169Z] 11:46:00 INFO - shouldBe@dom/canvas/test/webgl-conf/checkout/js/js-test-pre.js:408:9
[task 2019-07-25T11:46:00.169Z] 11:46:00 INFO - shouldBeTrue@dom/canvas/test/webgl-conf/checkout/js/js-test-pre.js:434:29
[task 2019-07-25T11:46:00.169Z] 11:46:00 INFO - @dom/canvas/test/webgl-conf/checkout/js/js-test-post.js:24:1
[task 2019-07-25T11:46:00.169Z] 11:46:00 INFO - GECKO(1671) | MEMORY STAT | vsize 7611MB | residentFast 241MB | heapAllocated 85MB
[task 2019-07-25T11:46:00.170Z] 11:46:00 INFO - TEST-OK | dom/canvas/test/webgl-conf/generated/test_conformance__extensions__webgl-compressed-texture-s3tc.html | took 133ms

I think this is a new bug, though it can happen on any webgl test.

(In reply to Gian-Carlo Pascutto [:gcp] from bug 1471001#c104)

Jeff, bug 1546442 is a security improvement that causes this intermittent to get slightly more frequent, and it got backed out because of that. That patch changes how our memory is laid out, and there's no connection to WebGL at all as it affects all allocations in the browser. I don't see anything I can do to that patch that would affect or help this. Can you advise how to proceed? Is disabling this test an option?

"This test" is the whole webgl conformance suite, so disabling it is not an option.
"WebGLTestUtils is not defined" is new to me, I think, and not something I would expect to see as the point of (or first indication of) failure. This instead sounds like a script loading behavior change, either race condition or occasional load failure.

This was introduced by bug 1546442 until it was backed out.

https://wiki.mozilla.org/Bug_Triage#Intermittent_Test_Failure_Cleanup