Make ConcurrentMultiSpanProcessor fork safe

[gregoiredx]

Description

ConcurrentMultiSpanProcessor is not "fork safe". This MR tries to solve this problem.

Forking a threaded process is not a good idea in the first place, citing the Python doc, see https://docs.python.org/3/library/multiprocessing.html#contexts-and-start-methods

Note that safely forking a multithreaded process is problematic.

Also, the OTel Python SDK suggest to only initialize the SDK post fork, see https://opentelemetry-python.readthedocs.io/en/latest/examples/fork-process-model/README.html

Nevertheless, on an existing code base, it's not always easy to avoid all forks. So it looks like a good idea to try and make the SDK as fork safe as possible.

Note: BatchProcessor is already relying on register_at_fork used here, see

opentelemetry-python/opentelemetry-sdk/src/opentelemetry/sdk/_shared_internal/__init__.py

Line 125 in 102fec2

os.register_at_fork(after_in_child=lambda: weak_reinit()()) # pyright: ignore[reportOptionalCall] pylint: disable=unnecessary-lambda

Fixes #2349

Type of change

Please delete options that are not relevant.

• Bug fix (non-breaking change which fixes an issue)

How Has This Been Tested?

It's quite hard to unit test.

An equivalent patch is currently running in our stack.

Does This PR Require a Contrib Repo Change?

?

Checklist:

• Followed the style guidelines of this project
• Changelogs have been updated
• Unit tests have been added
• Documentation has been updated

[linux-foundation-easycla]

The committers listed above are authorized under a signed CLA.

• ✅ login: gregoiredx / name: Grégoire Deveaux (f98d672)