[Python][S3] Segfault when using S3FileSystem in uwsgi

[lucasmo]

Describe the bug, including details regarding any error messages, version, and platform.

Simply instantiating S3FileSystem within a uwsgi container causes a segfault on process exit.

Backtrace

(gdb) bt
#0 __pthread_kill_implementation (no_tid=0, signo=6, threadid=<optimized out>) at ./nptl/pthread_kill.c:44
#1 __pthread_kill_internal (signo=6, threadid=<optimized out>) at ./nptl/pthread_kill.c:78
#2 __GI___pthread_kill (threadid=<optimized out>, signo=signo@entry=6) at ./nptl/pthread_kill.c:89
#3 0x00007f5a4669826e in __GI_raise (sig=sig@entry=6) at ../sysdeps/posix/raise.c:26
#4 0x00007f5a4667b8ff in __GI_abort () at ./stdlib/abort.c:79
#5 0x00007f5a4667c7b6 in __libc_message_impl (fmt=fmt@entry=0x7f5a468218d7 "%s\n") at ../sysdeps/posix/libc_fatal.c:132
#6 0x00007f5a466fbfe5 in malloc_printerr (str=str@entry=0x7f5a468248d8 "malloc_consolidate(): invalid chunk size") at ./malloc/malloc.c:5772
#7 0x00007f5a466fcd58 in malloc_consolidate (av=av@entry=0x7f5a46856ac0 <main_arena>) at ./malloc/malloc.c:4851
#8 0x00007f5a466fea80 in _int_malloc (av=av@entry=0x7f5a46856ac0 <main_arena>, bytes=bytes@entry=1808) at ./malloc/malloc.c:4041
#9 0x00007f5a467006e4 in __GI___libc_malloc (bytes=bytes@entry=1808) at ./malloc/malloc.c:3336
#10 0x00007f5a4678906b in __backtrace_symbols (array=0x55f7e1600c20, size=<optimized out>) at ./debug/backtracesyms.c:68
#11 0x000055f7dfd72b61 in uwsgi_backtrace ()
#12 0x000055f7dfd72f87 in uwsgi_segfault ()
#13 <signal handler called>
#14 0x00007f5a40a59d27 in std::_Sp_counted_base<(__gnu_cxx::_Lock_policy)2>::_M_release_last_use_cold() () from /home/ubuntu/.cache/pypoetry/virtualenvs/uwsgiboom-qBOj13LC-py3.11/lib/python3.11/site-packages/pyarrow/libarrow.so.1400
#15 0x00007f5a418b1d15 in std::unique_ptr<arrow::fs::(anonymous namespace)::AwsInstance, std::default_delete<arrow::fs::(anonymous namespace)::AwsInstance> >::~unique_ptr() ()
 from /home/ubuntu/.cache/pypoetry/virtualenvs/uwsgiboom-qBOj13LC-py3.11/lib/python3.11/site-packages/pyarrow/libarrow.so.1400
#16 0x00007f5a4669aa66 in __run_exit_handlers (status=30, listp=<optimized out>, run_list_atexit=run_list_atexit@entry=true, run_dtors=run_dtors@entry=true) at ./stdlib/exit.c:108
#17 0x00007f5a4669abae in __GI_exit (status=<optimized out>) at ./stdlib/exit.c:138
#18 0x000055f7dfd252c5 in uwsgi_exit ()
#19 0x000055f7dfd6f82b in end_me ()
#20 <signal handler called>
#21 0x00007f5a4677d042 in epoll_wait (epfd=7, events=0x7ffc1469aa0c, maxevents=1, timeout=-1) at ../sysdeps/unix/sysv/linux/epoll_wait.c:30
#22 0x000055f7dfd65428 in event_queue_wait ()
#23 0x000055f7dfd22a0e in wsgi_req_accept ()
#24 0x000055f7dfd6e6ae in simple_loop_run ()
#25 0x000055f7dfd6e79b in simple_loop ()
#26 0x000055f7dfd732d9 in uwsgi_ignition ()
#27 0x000055f7dfd779ab in uwsgi_worker_run ()
#28 0x000055f7dfd77f2a in uwsgi_run ()
#29 0x000055f7dfd21e64 in main ()

Reproducing

I tried this with pyarrow 17.0.0 and 16.1.0. I tried with the minimum versions of each package that didn't have critical CVEs and worked with python 3.11 or 3.12.

In a virtualenv, install uwsgi==2.0.23 and pyarrow==17.0.0.

Create the following program called app.py:

from pyarrow.fs import S3FileSystem

def application(env, start_response):
 start_response('200 OK', [('Content-Type','text/html')])
 fs = S3FileSystem()
 return [b"Hello World"]

Run uwsgi --http :9090 --wsgi-file app.py
In another shell, run curl http://localhost:9090/
Press ^C to terminate the uwsgi container (this also happens after the container serves several requests, or if the container sits idle for 15 minutes).

See:

/arrow/cpp/src/arrow/filesystem/s3fs.cc:2913: arrow::fs::FinalizeS3 was not called even though S3 was initialized. This could lead to a segmentation fault at exit
!!! uWSGI process 36031 got Segmentation Fault !!!
malloc_consolidate(): invalid chunk size
Aborted (core dumped)

Component(s)

Python

[pitrou]

We have an atexit handler that finalizes S3 at process shutdown:

arrow/python/pyarrow/fs.py

Lines 65 to 70 in 046e7cb

	# GH-38364: we don't initialize S3 eagerly as that could lead
	# to crashes at shutdown even when S3 isn't used.
	# Instead, S3 is initialized lazily using `ensure_s3_initialized`
	# in assorted places.
	import atexit
	atexit.register(ensure_s3_finalized)

Does uwsgi call atexit handlers at shutdown? @methane

[lucasmo]

It does look like that is called, from the core dump:

__run_exit_handlers (...) at ./stdlib/exit.c:108

This is the line before the pointer free happens. Is it possible the finalize is called twice somehow, resulting in a double free?

[pitrou]

It does look like that is called, from the core dump:

No, this is the libc exit, and it is often too late at this point to finalize S3. Python atexit handlers are called before, in Py_Finalize.

[pitrou]

Ok, I think I understand what happens. uwsgi finalizes its plugins thanks to a C-level atexit hook:
https://github.com/unbit/uwsgi/blob/b9618b619a73f2fc9680094eba7c202f5422fa42/core/uwsgi.c#L3202

The atexit handler for the Python handler then does call Py_Finalize at the end:
https://github.com/unbit/uwsgi/blob/b9618b619a73f2fc9680094eba7c202f5422fa42/plugins/python/python_plugin.c#L388-L446

So we have this:

1. uwsgi receives a Ctrl-C (or decides to shutdown a worker for another reason)
2. for this, it calls the libc's exit function
3. the libc's exit function shuts down the process
4. at some point the process shutdown triggers C-level atexit handlers
5. this calls back into uwsgi, especially uwsgi_plugins_atexit
6. uwsgi_plugins_atexit calls the Python plugin's uwsgi_python_atexit
7. uwsgi_python_atexit calls Py_Finalize which calls the Python atexit handlers
8. our atexit handler is then executed to finalize S3:

   arrow/python/pyarrow/fs.py

   Lines 69 to 70 in 6a38205

   	import atexit
   	atexit.register(ensure_s3_finalized)

However, at this point, it's already too late, because calling the libc's exit function has already deallocated critical resources in the AWS SDK. S3 finalization fails and crashes the process.

[pitrou]

So, to solve this, we'd like to find a way for uwsgi to call Py_Finalize before calling the C exit function.
I'm not sure that is possible @xrmx @methane @niol ?