Fix for race in ZooKeeper when joining send_thread/receive_thread

[davenger]

Changelog category (leave one):

• Bug Fix (user-visible misbehavior in an official stable release)

Changelog entry (a user-readable short description of the changes that goes to CHANGELOG.md):

Fixes #45431

Documentation entry for user-facing changes

• Documentation is written (mandatory for new features)

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[antonio2368]

It works as a quick fix, but I think the core problem of calling join from many different places is still confusing.
Still, I'm okay with this because it doesn't seem straightforward to me how to change the code to only have joins in the destructor.

[davenger]

It works as a quick fix, but I think the core problem of calling join from many different places is still confusing. Still, I'm okay with this because it doesn't seem straightforward to me how to change the code to only have joins in the destructor.

Yep, that was exactly my thinking! It would be much cleaner to only have join()-s in dtor, but it's not obvious how to fix it.