gh-59705: Add _thread.set_name() function

[vstinner]

On Linux, threading.Thread now sets the thread name to the operating system.

configure now checks if pthread_setname_np() function is available.

• Issue: Python should support exporting thread names to the OS #59705

[vstinner]

This implementation is very basic on purpose. I plan to add support for more platform in follow-up PRs.

• On Linux, set_name() does nothing if the name is longer than 15 bytes. Should the function truncate silently to 15 bytes instead? I don't think that raising an exception is very convenient here.
• Setting Thread.name after Thread.start() doesn't call again set_name(). set_name() is called only once per thread, at startup.
• I didn't add automated tests since I don't want to add a get_name() function (use Thread.name to get a thread name).

Demo 1 (main thread):

$ ./python
>>> import os
>>> pid=os.getpid()
>>> with open(f"/proc/{pid}/task/{pid}/comm") as fp: print(f"comm = {fp.read()!r}")
... 
comm = 'python\n'

>>> import _thread; _thread.set_name("demo")
>>> with open(f"/proc/{pid}/task/{pid}/comm") as fp: print(f"comm = {fp.read()!r}")
... 
comm = 'demo\n'

Demo 2 (thread):

$ ./python
>>> import threading, os, time
>>> os.getpid()
81921
>>> t=threading.Thread(target=time.sleep, args=(60,), name="sleeper")
>>> t.start()
^Z

$ cat /proc/81921/task/81927/comm 
sleeper

See also a previous attempt to implement the feature: #14578

[vstinner]

I didn't add automated tests since I don't want to add a get_name() function (use Thread.name to get a thread name).

I changed my mind and added a private _thread._get_name() function for tests.

[vstinner]

@pitrou @encukou @serhiy-storchaka: Would you mind to review this change? It's to set the thread name in threading.Thread to the operating system.

[vstinner]

On Linux, set_name() does nothing if the name is longer than 15 bytes. Should the function truncate silently to 15 bytes instead? I don't think that raising an exception is very convenient here.

I modified _thread.set_name(name) to truncate name to 15 bytes on Linux.

Truncating the string in threading.Thread would be more complicated since it requires to encode the string the filesystem encoding, detect the operating system (Linux), and hardcode the 15 bytes limit there. IMO it's more convenient to truncate in _thread.set_name().

[encukou]

This looks great, thank you!

The truncation is not pretty with non-ASCII names. I guess codepoint-preserving truncation is not worth the effort, and Linux tools need to deal with thread names being arbitrary bytes.

But, we can test the edge cases, to ensure this quality-of-life enhancement doesn't start raising exceptions in working code.

[vstinner]

@encukou: I addressed your reviews. Please review the updated PR.

I added tests on long names and non-ASCII names.

[vstinner]

@encukou: Maybe the "replace" error handler can be used, instead of not setting the name if the name cannot be encoded to the filesystem encoding. What do you think?

[serhiy-storchaka]

You can use FS_NONASCII. You can also use TESTFN_UNDECODABLE to test that it works with arbitrary bytes and TESTFN_UNENCODABLE to test for encoding error.

Is it a hard limit for the size? Is it the same on other platforms? I would prefer to use a named constant instead of magic numbers 15, 16, 17.

[vstinner]

@serhiy-storchaka:

Is it a hard limit for the size?

Yes. Using a longer name fails with ERANGE.

Is it the same on other platforms?

It's 16 bytes on Linux and 64 bytes on macOS, so no, it's not the same.

I would prefer to use a named constant instead of magic numbers 15, 16, 17.

I failed to find a public constant for these limits. For example, Darwin MAXTHREADNAMESIZE constant is private (I'm not 100% sure, but I don't have macOS so I cannot check manually, I only read the code).

[vstinner]

You can use FS_NONASCII. You can also use TESTFN_UNDECODABLE to test that it works with arbitrary bytes and TESTFN_UNENCODABLE to test for encoding error.

Ok, I added tests using FS_NONASCII and TESTFN_UNENCODABLE.

[vstinner]

I modified my PR to use the "replace" error handler to not fail if the name cannot be encoded to the filesystem encoding. IMO it's better to replace a few characters in the name rather than not copying any character (all or nothing).