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@vstinner: please review the changes made to this pull request.

@vstinner please advise if there are any additional suggestions you have or changes that should be implemented before the merge.

This is the second version of PR #11993 (original reverted due to test failures on unsupported operating systems).

cc @vstinner @pitrou

From the original PR:

This functionality adds a native Thread ID to threading.Thread objects. This ID (TID), similar to the PID of a process, is assigned by the OS (kernel) and is generally used for externally monitoring resources consumed by the running thread (or process).
This does not replace the ident attribute within Thread objects, which is assigned by the Python interpreter and is guaranteed as unique for the lifetime of the Python instance.
This new functionality fully supports Linux, Apple, and Windows platforms.

I expect the reason it broke AIX - is because AIX supports - for nearly 30 years is my guess (not known if it was already in AIX 3.X, but always in AIX 4.X).
From the AIX 6.1 documentation:

pthread_self Subroutine
Purpose
Returns the calling thread's ID.
Library
Threads Library (libpthreads.a)
Syntax
#include <pthread.h>
pthread_t pthread_self (void);
Description
The pthread_self subroutine returns the calling thread's ID.
Note: The pthread.h header file must be the first included file of each source file using the threads
library. Otherwise, the -D_THREAD_SAFE compilation flag should be used, or the cc_r compiler
used. In this case, the flag is automatically set.
Return Values
The calling thread's ID is returned.
Errors
No errors are defined.
The pthread_self function will not return an error code of EINTR.

The 'key' issue being that <pthread.h> needs to be first (or -D_THREAD_SAFE, which may be 'better' to ensure that all C-code is compiled "thread-safe" when using gcc. I normally use xlc_r, which sets _THREAD_SAFE - but I cannot assume that gcc (which is what the AIX bots currently use) are compiling everything "thread-safe".
Iirc, among other things, "thread-safe" means the linker links with the "thread_safe" libraries (provided by IBM. No guarantee that third-party libraries are thread-safe. etc..

Anyway, moving forward - how can I best contribute some code specific to AIX. Never tried working "together" before.

To be clear, in all the POSIX-based platforms that I’ve worked with to build this feature (notably excluding AIX, which I have zero experience with), *_self()-type functions are NOT the same value/ID as the one used by this feature.

Can you confirm whether or not AIX has the capability to return the integral ID, or whether the pthread_self() function simply returns the same struct reference as other POSIX systems? Given the return type pthread_t, at first glance I’m inclined to believe it’s not the same functionality.

Looking at docs, I think this might be what we’re looking for: https://www.ibm.com/support/knowledgecenter/en/ssw_aix_72/com.ibm.aix.basetrf2/thread_self.htm

I will look into this and see if it’d be easy to add for this PR.

I'd recommend tackling AIX as a separate PR.

Either way :)

In that case, I think this is mostly good to go (pending review of course).

I could continue to nitpick to death, but well, you addressed my main concern: not add the function/attribute on unsupported platforms, so I approve your change. Thanks for fixing it!