This is a ticket for experimentation. We'd like to support something like firefox --backgroundtask update that would pump the UpdateService loop from the command line. Here's a place to hang work in progress.

Obvious TODOs:

• Use the static BackgroundTasks class rather than hardcoding the
  changes to the manifest parser and to the static component
  registration, so that regular Firefox still works.

• Make the static component registration futzing data driven.

Depends on D91372

Depends on D91373

Depends on D94209

Depends on D94210

Adds a couple of useful APIs for background tasks. The main one is for safely
locking the profile so that data can be extracted and then unlocking it. A
second API allows for parsing the prefs file from the profile.

Depends on D95060

Depends on D95198

Depends on D95199

Depends on D95200

Depends on D91373

Depends on D95200

Depends on D97509

Depends on D97510

Depends on D97511

This establishes a high water mark for code loaded (even after a short
delay) by a background task that does nothing.

Code loaded here means:

1. Chrome JSMs imported using ChromeUtils.import;

2. XPCOM services, generally long-lived, loaded using do_getService
   or Services.* or an equivalent;

3. XPCOM components defined in JavaScript and loaded via
   chrome.manifest entries.

At this time background tasks do not load any of category 3. The
distinction is made because they are reported separately by Gecko.

Depends on D97512

Comment on attachment 9184605 [details]
Bug 1667276: Disable persistent cookie storage when running background tasks.

Revision D95200 was moved to bug 1675829. Setting attachment 9184605 [details] to obsolete.

Backed out for causing browser_all_files_referenced.js failures.

backout: https://hg.mozilla.org/integration/autoland/rev/06ee1ff0214b90a8b2689141d33e9d756b2351ae

push: https://treeherder.mozilla.org/jobs?repo=autoland&group_state=expanded&test_paths=browser%2Fbase%2Fcontent%2Ftest%2Fstatic%2Fbrowser.ini&revision=f1a65c9b3ca29e352a6fa427ec811b816bf328c2&selectedTaskRun=aVIAbSqGSQ2qYD8IKfWdUQ.1

failure log: https://treeherder.mozilla.org/logviewer?job_id=327785851&repo=autoland&lineNumber=2582

[task 2021-01-26T03:44:56.643Z] 03:44:56 INFO - indirectly whitelisted file: chrome://fxr/content/prefs.js used from chrome://fxr/content/prefs.html
[task 2021-01-26T03:44:56.644Z] 03:44:56 INFO - Buffered messages finished
[task 2021-01-26T03:44:56.648Z] 03:44:56 INFO - TEST-UNEXPECTED-FAIL | browser/base/content/test/static/browser_all_files_referenced.js | there should be no unreferenced files - Got 4, expected +0
[task 2021-01-26T03:44:56.648Z] 03:44:56 INFO - Stack trace:
[task 2021-01-26T03:44:56.648Z] 03:44:56 INFO - chrome://mochikit/content/browser-test.js:test_is:1351
[task 2021-01-26T03:44:56.649Z] 03:44:56 INFO - chrome://mochitests/content/browser/browser/base/content/test/static/browser_all_files_referenced.js:checkAllTheFiles:948
[task 2021-01-26T03:44:56.649Z] 03:44:56 INFO - Not taking screenshot here: see the one that was previously logged
[task 2021-01-26T03:44:56.650Z] 03:44:56 INFO - TEST-UNEXPECTED-FAIL | browser/base/content/test/static/browser_all_files_referenced.js | unreferenced file: resource://gre/defaults/backgroundtasks/backgroundtasks.js -
[task 2021-01-26T03:44:56.650Z] 03:44:56 INFO - Stack trace:
[task 2021-01-26T03:44:56.651Z] 03:44:56 INFO - chrome://mochikit/content/browser-test.js:test_ok:1323
[task 2021-01-26T03:44:56.651Z] 03:44:56 INFO - chrome://mochitests/content/browser/browser/base/content/test/static/browser_all_files_referenced.js:checkAllTheFiles:952
[task 2021-01-26T03:44:56.652Z] 03:44:56 INFO - Not taking screenshot here: see the one that was previously logged

Also failing: https://treeherder.mozilla.org/logviewer?job_id=327785366&repo=autoland&lineNumber=5074

Backed out for causing failure on test_crash_backgroundtask_moz_crash.js.

Backout link: https://hg.mozilla.org/integration/autoland/rev/e379907d0452a34155518062e838b465f5e4a37c

Push with failures: https://treeherder.mozilla.org/jobs?repo=autoland&searchStr=linux%2C18.04%2Cx64%2Copt%2Cxpcshell%2Ctests%2Ctest-linux1804-64%2Fopt-xpcshell-e10s%2Cx2&revision=f06504e3219f5a31ce538829376eeb53be317715&selectedTaskRun=SF47dsaJR4uEMyYymhI9nQ.0

Failure log: https://treeherder.mozilla.org/logviewer?job_id=328010578&repo=autoland&lineNumber=3050

https://hg.mozilla.org/mozilla-central/rev/68c50add73aa
https://hg.mozilla.org/mozilla-central/rev/09db6a973d97
https://hg.mozilla.org/mozilla-central/rev/b238efac91b4
https://hg.mozilla.org/mozilla-central/rev/14441979c730
https://hg.mozilla.org/mozilla-central/rev/73f56e24b72c
https://hg.mozilla.org/mozilla-central/rev/0e0e6bbf6f79

Updating the ticket title to be more representative of what we've actually landed.