Drop ShieldCancellation.

[T-256]

I didn't see any discussion about this, I think that's proper place to handle it in unasync.py.

[lovelydinosaur]

As with #805, can you describe and demonstrate a behaviour that you want to see changed?

Driving changes in the project should be through describing changes in terms of behaviour & motivation.

[T-256]

Driving changes in the project should be through describing changes in terms of behaviour & motivation.

Sorry, ShieldCancellation is not member of public API, I thought I shouldn't consider it as behavior change.

The main motivation is when I was reading few files in httpcore/_sync/, I confused on the error handlings (task cancellation shield on non-async codes!?). Then by going to httpcore/_synchronization.py, I noticed that is only for prevent unasync.py errors.

[lovelydinosaur]

ShieldCancellation is a nop in the sync codebase.

There isn't any obvious gracefully way to approach "shielded cancellation blocks in our async code" that doesn't involve also mirroring that into the sync part of the codebase, so we make do with having a no-op context block there. If you have a look through the history you'd probably be able to find some discussion about that.