A script spawning a single daemon thread calling print() in a loop (like the attached) will usually hang on shutdown in Python 3.4.2 and hg rev 8d802fb6ae32. Attaching gdb at that point shows the following:

(gdb) thread apply all bt

Thread 1 (Thread 0x7fd927d58700 (LWP 30274)):
#0 sem_wait () at ../sysdeps/unix/sysv/linux/x86_64/sem_wait.S:85
#1 0x00000000005282fe in PyThread_acquire_lock_timed (lock=0x1c5ea30, microseconds=microseconds@entry=-1,
intr_flag=intr_flag@entry=0) at Python/thread_pthread.h:352
#2 0x0000000000528414 in PyThread_acquire_lock (lock=<optimized out>, waitflag=waitflag@entry=1)
at Python/thread_pthread.h:556
#3 0x0000000000567e4c in _enter_buffered_busy (self=0x7fd927bc2b48) at ./Modules/_io/bufferedio.c:327
#4 buffered_flush (self=0x7fd927bc2b48, args=<optimized out>) at ./Modules/_io/bufferedio.c:874
#5 0x000000000042822a in PyObject_Call (func=0x7fd9277b69d8, arg=<optimized out>, kw=<optimized out>)
at Objects/abstract.c:2086
#6 0x00000000004290e4 in call_function_tail (args=0x7fd927b8d048, callable=0x7fd9277b69d8) at Objects/abstract.c:2124
#7 callmethod (is_size_t=1, va=0x7fff5c6cf6b0, format=0x0, func=0x7fd9277b69d8) at Objects/abstract.c:2193
#8 _PyObject_CallMethodId_SizeT (o=<optimized out>, name=<optimized out>, format=0x0) at Objects/abstract.c:2279
#9 0x000000000042822a in PyObject_Call (func=0x7fd9277b6990, arg=<optimized out>, kw=<optimized out>)
at Objects/abstract.c:2086
#10 0x0000000000428cc4 in call_function_tail (args=0x7fd927b8d048, callable=0x7fd9277b6990) at Objects/abstract.c:2124
#11 callmethod (is_size_t=0, va=0x7fff5c6cf7e0, format=0x5b9924 "", func=0x7fd9277b6990) at Objects/abstract.c:2193
#12 _PyObject_CallMethodId (o=o@entry=0x7fd927b5d3a8, name=name@entry=0x862b00 <PyId_flush>,
format=format@entry=0x5b9924 "") at Objects/abstract.c:2238
#13 0x000000000050a521 in flush_std_files () at Python/pylifecycle.c:488
#14 0x000000000050a5aa in Py_Finalize () at Python/pylifecycle.c:550
#15 0x000000000041fc92 in Py_Main (argc=-1, argv=0x1) at Modules/main.c:787
#16 0x000000000041be3c in main (argc=2, argv=<optimized out>) at ./Programs/python.c:69

The daemon thread has exited, and the main thread hangs trying to flush stdout.

I haven't fully tracked down what happens here, but I think it's this:

• daemon thread calls ENTER_BUFFERED on stdout
• daemon thread drops the GIL before writing to stdout
• main thread grabs the GIL and starts exiting
• main thread sets _Py_Finalizing, signaling daemon threads to exit
• main thread calls flush_std_files and drops the GIL
• daemon thread grabs the GIL and immediately exits, without reaching LEAVE_BUFFERED
• main thread deadlocks trying to ENTER_BUFFERED the same file

If that is what happens, I don't really see how to fix it (it's an example of daemon threads not releasing their resources, which the documentation warns about). But it's obviously unfortunate if merely writing to stdout/err is such a resource.

Your analysis sounds right to me.

A possible solution is to tweak the IO lock acquire on shutdown: only wait for a grace period instead of waiting indefinitely. Attached patch demonstrates the solution (and solves the deadlock issue in your script).

(note that trying to print a warning or raise an error only makes things worse if the stream subject to deadlock is stderr)

Attached patch is stricter and dumps a fatal error after the grace period.

I'm afraid I don't have real-world code I can confirm is fixed, since I reported this on behalf of someone on irc in #python.

I think I'd prefer the Py_FatalError version of the patch. It's definitely possible to see writes to stdout/stderr at this time. If I read the first version right, it'll succeed the ENTER_BUFFERED without actually grabbing the lock, which seems problematic. But failing ENTER_BUFFERED is probably also more problematic, as it will probably lead to another exception that Python'll try to write to stderr, which fails the same way... If stdout/stderr are in a broken state during shutdown, Python is probably better off calling Py_FatalError rather than intermittently discarding messages.

Thank you for the feedback. Yes, I now also think the fatal error is the preferrable way. I'll see if I can add a reliable test.

Here is a patch with tests.

New changeset 2c53a5302058 by Antoine Pitrou in branch '3.4':
Issue bpo-23309: Avoid a deadlock at shutdown if a daemon thread is aborted
https://hg.python.org/cpython/rev/2c53a5302058

New changeset 8a3da91737cf by Antoine Pitrou in branch 'default':
Issue bpo-23309: Avoid a deadlock at shutdown if a daemon thread is aborted
https://hg.python.org/cpython/rev/8a3da91737cf

Fix pushed! closing.