Fix loading homeworks for teachers

[nilsreichardt]

Summary

This PR fixes:

• When signing out, typeOfUser is null for a short moment #608
• Homeworks are not loading for teachers #654
• App freezes when signing out #497 (real fix)

It's required to this all in one pull request because otherwise it would introduce a new bug.

What was the issue?

I started to debug on this problem because of #654. I found out it was because TypeOfUser.student was used as a backup when it's null (see #573). However, just reverting it to TypeOfUser.unknown was not option because this would introduce #497 again.

Therefore, I wondered, why typeOfUser can be null. The reason for this was the last time online reporter. Because .set() has been used for Firestore, Firestore instantly was setting it null (I think null is used for Firestore.currentTimestamp() when the client is offline or the server hasn't responded yet). Using .update() fixed the issue that the user document only contained lastOnline.

But using update() in the last online reporter always caused #497. Even using TypeOfUser.student or TypeOfUser.unknown couldn't be as a workaround. After hours of debugging I found that when signing our the following line was causing the issue:

sharezone-app/app/lib/util/api/homework_api.dart

Line 107 in 47e4ccb

onError: _homeworkSubjectStream.addError);

The issue is that we listened to _homeworkSubjectStream and when an error occurred, we the .addError method of _homeworkSubjectStream used. This ended up into an infinity loop which resulted in the freeze (#497),

This was by far one of my hardest bugs... I needed ~5,5h to find the solution and it was extremely hard to debug.

Fixes #608
Fixes #654
Fixes #497

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[Jonas-Sander]

See comment above

[Jonas-Sander]

Hard bug? Do you mean hard to find?

[nilsreichardt]

Yes, hard bugs means bugs that are hard to debug, hard to find, hard to locate, etc

[nilsreichardt]

I have added this definition to the comment

[Jonas-Sander]

This was by far one of my hardest bugs... I needed ~5,5h to find the solution and it was extremely hard to debug.

I think this bug was on me... Sorry!! 🙈😁

[Jonas-Sander]

Although in my opinion there should be:

• A lint that warns of piping a stream into itself in this way (at least the error I guess?)
• A debug feature that warns when a stream goes into this kind of infinity loop

If I have time maybe I'm gonna raise an issue on the Dart side, but no promises yet. I guess that there might be reasons why it might be hard to actually implement it as well.

[nilsreichardt]

Although in my opinion there should be:

• A lint that warns of piping a stream into itself in this way (at least the error I guess?)
• A debug feature that warns when a stream goes into this kind of infinity loop

If I have time maybe I'm gonna raise an issue on the Dart side, but no promises yet. I guess that there might be reasons why it might be hard to actually implement it as well.

Definitely, agree. Unfortunately, I couldn't also use performance tools, like DevTools because they're not really working for web and say that I should use Chrome DevTools, however even Chrome DevTools couldn't record it properly.

[nilsreichardt]

@Jonas-Sander Any ideas why the android integration test are not passing anymore because of the changes?

[Jonas-Sander]

@Jonas-Sander Any ideas why the android integration test are not passing anymore because of the changes?

No, no idea, sorry

[Jonas-Sander]

@nilsreichardt Should we release a new version after this is merged?

[nilsreichardt]

Yes, makes sense also because of #671