New submission from Deomid Ryabkov :
I know, I know - forking a multi-threaded process is bad. But it happens.
This is related to https://bugs.python.org/issue40399 and
https://bugs.python.org/issue6721 but is distinct because the problem is
entirely self-inflicted.
What happens:
1) A multithreaded program forks using one of the functions, such as
os.forkpty()
2) In the child process the Python interpreter, in its PyOS_AfterFork_Child
function ([1]) tries to kill all the threads other than the one doing the
forking.
3) Among the objects being destroyed may include file or socket objects that
are now being destroyed too, without having been previosuly closed, which
triggers a ResourceWarning in the finalizer [2], [3].
4) Default action for warnings is to write to sys.stderr
5) A mutex used in BufferedIO is held by some other (now deceased thread).
6) Deadlock in _enter_buffered_busy [4].
This is bad because there is absolutely no way to avoid it without disabling
warnings.
Even if the program is super careful to not do anything after forking other
than exec, it doesn't help because the resource warning and the resulting
deadlock is triggered by activity of the interpreter: it is the interpreter
that orphans and is forcibly destroying the files and sockets, not the program
that lost track of them.
[1]
https://github.com/python/cpython/blob/beba1a808000d5fc445cb28eab96bdb4cdb7c959/Modules/posixmodule.c#L451
[2]
https://github.com/python/cpython/blob/beba1a808000d5fc445cb28eab96bdb4cdb7c959/Modules/_io/fileio.c#L95
[3]
https://github.com/python/cpython/blob/beba1a808000d5fc445cb28eab96bdb4cdb7c959/Modules/socketmodule.c#L4800
[4]
https://github.com/python/cpython/blob/beba1a808000d5fc445cb28eab96bdb4cdb7c959/Modules/_io/bufferedio.c#L282
--
components: Library (Lib)
messages: 367701
nosy: rojer
priority: normal
severity: normal
status: open
title: Spurious warning emitted during fork() can deadlock a multi-threaded
process
versions: Python 3.7
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