Add on_thread_exit hook

[biinari]

Closes #2100

Description

For apps that need to tidy up thread-local resources when a thread exits, add a new on_thread_exit hook.
This will be called when a thread is being trimmed, just before the thread exits.

An example config entry involving rabbitmq channels being held in thread local variables (one channel per thread):

on_thread_exit do
  unless Thread.current[MY_KEY].nil?
    Thread.current[MY_KEY].disconnect
    Thread.current[MY_KEY] = nil
  end
end

The hook should be configured with a quick non-blocking operation as it will block new requests from being started in other threads. I wondered about moving the call to the hook outside of the mutex section, but that becomes quite a bit less readable and I can't see what anyone would need to do that would be a blocking operation.

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[dentarg]

LGTM 👌 Needs a rebase though.

One thought, should we call it before_thread_exit_hooks? As you can have multiple hooks. I guess you followed out_of_band_hook which also could use an s on the end IMHO.

[MSP-Greg]

Should there be a hook on the front end, immediately after the thread block starts?

Also, current code in this PR rescues outside of before_thread_exit_hook.each(&:call). With other hooks it's done inside the iteration, so if one hook errors, the others are still called.

[biinari]

Thanks for the suggestions. I've rebased and followed the naming from the out_of_band which now has no _hook suffix in the instance variable.

@MSP-Greg, it sounds like you're asking for an additional feature with a hook at the start of running a thread. If you have a use case for needing this, perhaps create a separate issue? (Or correct me if I've misunderstood).

Changed the rescue to be per hook call as suggested, that's a good idea.

[MSP-Greg]

@biinari

to tidy up thread-local resources

Not an additional feature, but possibly a more complete implementation of the feature. In this case, a logical place to create the 'thread-local resources' might be a companion on_thread_start (or on_thread_enter) hook.

Edit: I'll defer to others on this one, so maybe wait for their input. I just want to avoid someone asking for it at a later time.

[biinari]

@MSP-Greg, actually that does sound sensible to add in with this as it would be a place to create thread-local resources or to monitor when threads are starting and stopping. I'll take a look at it soon assuming nobody disagrees.

[nateberkopec]

@biinari Works for me too, please do investigate 👍

[nateberkopec]

It looks like OP needed to move on to other things.

I don't think adding on_thread_start should actually block this though? We can merge as-is and make adding the complementary hook a separate issue.

[biinari]

Thanks for merging this in. Sorry you're right, I've had my focus other projects for a while.

[nateberkopec]

No worries homie, no one's on a timeline here 👍 Thanks for your contribution 🙇