sync: implement watch::Receiver::mark_unseen()

[victor-timofei]

Fixes: #5871

Solution

I've added an extra flag to sync::watch::Receiver.

Mutex is used for thread safety and Cell for interior mutability.

Could try to use an AtomicBool instead of Mutex<Cell<bool>> since a Receiver instance is not shared across threads?

[Darksonn]

I'm pretty sure you can just decrement the version instead. Since whether it's seen is computed based on what the version is.

[victor-timofei]

I was thinking to change it to isize to be able to mark as unread even a newly created Receiver.

I was a little worried about the fact that there is also the CLOSED bit, but after a little experimentation I should be able to make it work.

[Darksonn]

You can just make the starting version be 1. Then you can always subtract one from the version.

[Darksonn]

I would like a test that actually sends a value somewhere.

[Darksonn]

Thanks.

[uklotzde]

I didn't notice this PR and left some comments at the merge commit: 61042b4

[uklotzde]

Most crucial issue is the choice of naming which introduces "unseen" while we already have changed() and has_changed(). IMHO mark_unchanged() mark_changed() would be the more consistent choice here.

[victor-timofei]

Hi @uklotzde, sounds reasonable. Since the change hasn't been released yet I could update the name if we want to.

[uklotzde]

I have created a PR to prevent that this change is released accidentally: #6014

[uklotzde]

Got it wrong 🙈 Of course the method must be named mark_changed(), not mark_unchanged()!