I can see this exception when I was investigating a test failure in our Firefox ui tests logged as bug 1215499. The following output is visible in the Gecko log: MARIONETTE LOG: INFO: TEST-START: c:\jenkins\workspace\release-mozilla-release_functional\venv\Lib\site-packages\firefox_ui_tests\functional\private_browsing\test_about_private_browsing.py:testCheckAboutPrivateBrowsing 1444997539656 Marionette: Element does not have an accessible object************************* A coding exception was thrown in a Promise rejection callback. See https://developer.mozilla.org/Mozilla/JavaScript_code_modules/Promise.jsm/Promise Full message: TypeError: 'toString' called on an object that does not implement interface Exception. Full stack: error.isError@chrome://marionette/content/error.js:104:12 error.isWebDriverError@chrome://marionette/content/error.js:112:10 error.toJSON@chrome://marionette/content/error.js:72:8 Response.prototype.sendError@chrome://marionette/content/command.js:100:16 Handler.prototype.process@resource://gre/modules/Promise.jsm -> resource://gre/modules/Promise-backend.js:925:21 this.PromiseWalker.walkerLoop@resource://gre/modules/Promise.jsm -> resource://gre/modules/Promise-backend.js:801:7 this.PromiseWalker.scheduleWalkerLoop/<@resource://gre/modules/Promise.jsm -> resource://gre/modules/Promise-backend.js:740:39 ************************* Marionette threw an error: TypeError: 'toString' called on an object that does not implement interface Exception. error.isError@chrome://marionette/content/error.js:104:12 error.isWebDriverError@chrome://marionette/content/error.js:112:10 error.toJSON@chrome://marionette/content/error.js:72:8 Response.prototype.sendError@chrome://marionette/content/command.js:100:16 Handler.prototype.process@resource://gre/modules/Promise.jsm -> resource://gre/modules/Promise-backend.js:925:21 this.PromiseWalker.walkerLoop@resource://gre/modules/Promise.jsm -> resource://gre/modules/Promise-backend.js:801:7 this.PromiseWalker.scheduleWalkerLoop/<@resource://gre/modules/Promise.jsm -> resource://gre/modules/Promise-backend.js:740:39

The error.toJSON function has since been removed. Does this still reproduce?

Yes, still a problem with a Nightly build from Tuesday: Full message: TypeError: 'toString' called on an object that does not implement interface Exception. Full stack: error.isError@chrome://marionette/content/error.js:78:12 error.isWebDriverError@chrome://marionette/content/error.js:86:10 Response.prototype.sendError@chrome://marionette/content/command.js:89:12

This is caused by an incomplete test for whether val in error.isError is an XPCOM exception. The correct check would be to do val instanceof Ci.nsIException

The current test of the `result' property does not always pass for all types of XPCOM exceptions. A safer test is to do an instance check against Components.interfaces.nsIException. This fixes hangs such as the one described in bug 1202576. r=dburns Review commit: https://reviewboard.mozilla.org/r/29881/diff/#index_header See other reviews: https://reviewboard.mozilla.org/r/29881/

This fixes serialisation of XPCOM exceptions to the client. Response.sendError was incorrectly serialising the err input variable instead of the wrapped WebDriverError, we. r=dburns Review commit: https://reviewboard.mozilla.org/r/29883/diff/#index_header See other reviews: https://reviewboard.mozilla.org/r/29883/

Comment on attachment 8705146 [details] MozReview Request: Bug 1215502: Perform nsIException instance test for XPCOM exceptions https://reviewboard.mozilla.org/r/29881/#review26687

Comment on attachment 8705147 [details] MozReview Request: Bug 1215502: Send wrapped WebDriverError to client https://reviewboard.mozilla.org/r/29883/#review26691

https://hg.mozilla.org/integration/mozilla-inbound/rev/26a9634f6ec2 https://hg.mozilla.org/integration/mozilla-inbound/rev/a9da976060a8

Thanks for the quick fix Andreas! I wonder if a unit test would also be good to have here.

https://hg.mozilla.org/mozilla-central/rev/26a9634f6ec2 https://hg.mozilla.org/mozilla-central/rev/a9da976060a8

This looks great! The hang (bug 1202576) is fixed which was caused by this bug. David, can we please get this backported to the aurora branch to have it included in the next ESR? Thanks.

https://hg.mozilla.org/releases/mozilla-aurora/rev/dcd136c18c7a https://hg.mozilla.org/releases/mozilla-aurora/rev/9e810b77b412