While running all the Marionette unit tests with actors enabled I noticed that the given tests currently perma fail:

0:03.25 INFO test_expected.py test_expected.TestExpected.test_element_displayed_when_stale_element
0:03.25 INFO test_expected.py test_expected.TestExpected.test_element_not_displayed_when_stale_element
0:03.25 INFO test_expected.py test_expected.TestExpected.test_element_stale

It actually regressed with my first patch from bug 1662460.

Here the error details:

ERROR testing/marionette/harness/marionette_harness/tests/unit/test_expected.py TestExpected.test_element_displayed_when_stale_element - UnknownException: Error: The element reference of <p> is stale; either the element is no longer attached to the DOM, it is not in the current frame context, or the document has been refreshed
stacktrace:
	WebDriverError@chrome://marionette/content/error.js:181:5
	StaleElementReferenceError@chrome://marionette/content/error.js:442:5
	element.resolveElement@chrome://marionette/content/element.js:808:11
	evaluate.fromJSON@chrome://marionette/content/evaluate.js:233:26
	evaluate.fromJSON@chrome://marionette/content/evaluate.js:253:29
	receiveMessage@chrome://marionette/content/actors/MarionetteFrameChild.jsm:56:27
Traceback (most recent call last):

It should hopefully be simple to get fixed.

The problem here is that we now no longer raise a stale element reference but unknown error error:

before:

1601642508323 Marionette DEBUG 2 <- [1,8,{"error":"stale element reference","message":"The element reference of <p> is stale; either the element is no longer att ... /MarionetteFrameChild.jsm:378:21\nreceiveMessage@chrome://marionette/content/actors/MarionetteFrameChild.jsm:108:31\n"},null]

now:

1601642318325 Marionette DEBUG 2 <- [1,8,{"error":"unknown error","message":"Error: The element reference of <p> is stale; either the element is no longer attach ... onette/content/evaluate.js:258:29\nreceiveMessage@chrome://marionette/content/actors/MarionetteFrameChild.jsm:56:27\n"},null]

And it's easy to fix. So formerly we resolved the element id to a DOM node within the appropriate method as called by receiveMessage, but now it's done directly in evaluate.fromJSON(serializedData):

https://searchfox.org/mozilla-central/rev/222e4f64b769413ac1a1991d2397b13a0acb5d9d/testing/marionette/actors/MarionetteFrameChild.jsm#57-61

Note that if an error is thrown we no longer send back an actual error type, but a result.

Without catching the error from evaluate.fromJSON() the default error handling
will kick-in and an UnknownError will be returned.

https://hg.mozilla.org/mozilla-central/rev/e74872af6889