Improve client hook subscriber management #17276
Description
Problem Statement
Today, we simply store client hook callbacks in an array per hook. This generally works fine, but as we learned in #17272, causes incorrect behaviour when removing a callback (i.e. unsubscribing) synchronously while subscribers are still being invoked.
Solution Brainstorm
Ideally, we can find a solution that
- enables synchronous unsibscribing without compromising other subscribers
- requires no breaking API changes