Logging in signal handlers may be unsafe

[vanschelven]

In the arbiter's signal-handling code, there are many uses of logging. E.g.

https://github.com/benoitc/gunicorn/blob/master/gunicorn/arbiter.py#L252

however, the docs say this:

If you are implementing asynchronous signal handlers using the signal module, you may not be able to use logging from within such handlers. This is because lock implementations in the threading module are not always re-entrant, and so cannot be invoked from such signal handlers.

https://docs.python.org/3/library/logging.html#thread-safety

elsewhere Gregory Smith says the following:

I'd still rank "doing IO" even in a python signal handler (which is effectively possible to be executed between any given bytecode and between some operations using the C API that check the internal signal context) as a bad idea.

What the consequences of the above remarks are... may depend on how many signals you send.

[pajod]

Related: #2816

We do have suggested changes, notably #3148 attempting to address this.