Add Facebook Limited Login backend

[Tenzer]

Proposed changes

This adds a new backend for Facebook Limited Login

This is a new login method for Facebook which uses OpenID Connect and doesn't have the ability to associate a user logging into one app with the IDFA token on iOS devices: https://developers.facebook.com/docs/facebook-login/limited-login.

Types of changes

Please check the type of change your PR introduces:

• [ ] Release (new release request)
• [ ] Bugfix (non-breaking change which fixes an issue)
• [x] New feature (non-breaking change which adds functionality)
• [ ] Code style update (PEP8, lint, formatting, renaming, etc)
• [ ] Refactoring (no functional changes, no api changes)
• [ ] Breaking change (fix or feature that would cause existing functionality to not work as expected)
• [ ] Build related changes (build process, tests runner, etc)
• [ ] Other (please describe):

Checklist

Put an x in the boxes that apply. You can also fill these out after creating the PR. If you're unsure about any of them, don't hesitate to ask. We're here to help! This is simply a reminder of what we are going to look for before merging your code.

• [x] Lint and unit tests pass locally with my changes
• [x] I have added tests that prove my fix is effective or that my feature works

Other information

Any other information that is important to this PR such as screenshots of how the component looks before and after the change.

[codecov[bot]]

Codecov Report

Merging #590 (bd12218) into master (4b57a1d) will decrease coverage by 0.03%. The diff coverage is 70.96%.

@@            Coverage Diff             @@
##           master     #590      +/-   ##
==========================================
- Coverage   77.29%   77.27%   -0.03%     
==========================================
  Files         325      325              
  Lines        9845     9875      +30     
  Branches     1176     1180       +4     
==========================================
+ Hits         7610     7631      +21     
- Misses       2080     2088       +8     
- Partials      155      156       +1     

Flag	Coverage Δ
unittests	77.27% <70.96%> (-0.03%)	:arrow_down:

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
social_core/backends/facebook.py	55.12% <60.86%> (+0.99%)	:arrow_up:
social_core/tests/backends/test_facebook.py	100.00% <100.00%> (ø)
social_core/tests/backends/test_open_id_connect.py	99.08% <100.00%> (ø)

Help us with your feedback. Take ten seconds to tell us how you rate us. Have a feature suggestion? Share it here.

[Tenzer]

I have converted this to a draft while I test it out more thoroughly. It turned out the "simple" approach I was hoping for doesn't quite work since this backend doesn't have any APIs available for validating the user, nonce, etc.

[Tenzer]

I have rebased the branch as I noticed there were some conflicts with the master branch. The sole test failure is the same as what is currently in the master branch.

[nijel]

Looks good, but can't merge until SAML errors in the tests are sorted out...

[Tenzer]

Those SAML tests look pretty broken to me. I just tried to run them locally to have a look at how they are failing, and I managed to run it three times in a row which produced different failures each time. The first run was successful but the two following ones failed with different errors: https://gist.github.com/Tenzer/12720fc757ec4d595e4b775825ff8323

[nijel]

Strange is that those worked fine for years, so this is most likely result of upgrade of some underlying library. I haven't found time to dig into that so far...

[Tenzer]

@nijel Any chance you could take a look at this PR again? I have rebased it to get the latest fixes in from the master branch, and that seems to have fixed the unrelated failing tests.

[nijel]

Merged, thanks for your contribution!

[Tenzer]

Thanks a lot!

[nijel]

This has introduced non-declared hard dependency on jose, see https://github.com/python-social-auth/social-app-django/issues/427#issuecomment-1471630584

[Tenzer]

Sorry about that! I didn't really think that importing an internal module would cause issues like this.

I guess another solution which wasn't mentioned in your linked comment, would be to move all the imports of jose inside social_core/backends/open_id_connect.py into the methods where they are used. Then they won't create errors at import time.

Switching everything to use PyJWT would probably be the nicest solution to this, but will likely take more time and be more complicated than other workarounds, such as either always depending on jose or splitting Facebook Limited Login into a separate file.

Is there anything you would like me to do to help out with this?

[nijel]

Moving the imports will make the backend fail at authentication time instead of at import time, what sort of hides the configuration error.

[nijel]

Proposed fix: https://github.com/python-social-auth/social-core/pull/778