I figured out why this is happening in this try push: https://treeherder.mozilla.org/#/jobs?repo=try&revision=91de3338f4c0. If you look at one of the failure logs such as <http://mozilla-releng-blobs.s3.amazonaws.com/blobs/try/sha512/c18d72576781d7e01d2ba65d2ec9c79a24d3b65530acbcec22c350c1d0b1eb3f2e1ba0f71391ed1a954111f11898eab9b1cbe495ef89dc173c099471ab655493> you'll see: 10-08 06:18:38.679 628 639 I GeckoDump: ⰲ겿{"action":"test_status","time":1444310318680,"thread":null,"pid":null,"source":"mochitest","test":"/tests/dom/workers/test/serviceworkers/test_app_protocol.html","subtest":"Messaging from app complete","status":"PASS","js_source":"TestRunner.js"}ⰲ겿 10-08 06:18:39.119 628 639 D GeckoWebappManager: uninstall: http://mochi.test:8888/tests/dom/workers/test/serviceworkers/app-protocol/update.webapp 10-08 06:18:39.149 628 639 I Gecko : -*- AppsUtils.jsm: getAppByManifestURL http://mochi.test:8888/tests/dom/workers/test/serviceworkers/app-protocol/update.webapp 10-08 06:18:39.159 628 639 I Gecko : -*- AppsUtils.jsm: getCoreAppsBasePath() 10-08 06:18:39.409 628 639 D GeckoWebapps: cancelDownload http://mochi.test:8888/tests/dom/workers/test/serviceworkers/app-protocol/update.webapp 10-08 06:18:39.419 628 639 I Gecko : -*-*- AppDownloadManager.jsm : Getting http://mochi.test:8888/tests/dom/workers/test/serviceworkers/app-protocol/update.webapp 10-08 06:18:39.419 628 639 D GeckoWebapps: Could not find a download for http://mochi.test:8888/tests/dom/workers/test/serviceworkers/app-protocol/update.webapp 10-08 06:18:39.439 628 639 I Gecko : -*- AppsService.js: getAppByLocalId( 1007 ) 10-08 06:18:39.439 628 639 I Gecko : -*- AppsUtils.jsm: getAppByLocalId 1007 10-08 06:18:39.709 628 639 I Gecko : -*-*- UserCustomizations Starting customization unregistration for http://mochi.test:8888/tests/dom/workers/test/serviceworkers/app-protocol/update.webapp 10-08 06:18:39.729 628 639 D GeckoWebapps: Saving /mnt/sdcard/tests/webapps/webapps.json 10-08 06:18:41.229 628 639 D GeckoWebapps: Success saving /mnt/sdcard/tests/webapps/webapps.json 10-08 06:18:41.279 628 639 D GeckoWebapps: XXXXehsan not sending the async message The "not sending the async message" piece comes form <https://hg.mozilla.org/try/rev/5977728cb495#l2.21> which means that the underlying message manager has been destroyed by the time that we want to signal back to the caller that the uninstall has finished successfully. This can impact any test that tries to uninstall an app, including this one. I'm running a try build to see what happens when I take this code out.

The try server is happy <https://treeherder.mozilla.org/#/jobs?repo=try&revision=1eed2704fbf8>

The comment in the removed code is incorrect in that the yields do eventually finish, and without the message manager we have no way to finish the uninstallation process.

Comment on attachment 8671678 [details] [diff] [review] Keep the message manager alive in doUninstall() to be able to signal the result back There's some risk either way, so it's a tradeoff, but I see how this makes sense, especially given the known intermittent test failures, which should also show up as runtime failures in the field.