Minimal test case for lit lockup on Mac.

reproducer.py

import _multiprocessing
import multiprocessing

q = multiprocessing.Queue()

COUNT = 2**15 - 1

if COUNT > _multiprocessing.SemLock.SEM_VALUE_MAX:
    print('It is going to lock up')
else:
    print('It should be fine. Wait about 2 seconds.')

def main():
    for i in range(COUNT):
        q.put(i)

    for i in range(COUNT):
        q.get(i)

main()

# This is the sample when it locks up.

#2907 Thread_25011567   DispatchQueue_1: com.apple.main-thread  (serial) ########## <---------------------- main thread, blocked on acquire of semaphore in .put (self._sem of Queue)
#+ 2907 start  (in libdyld.dylib) + 1  [0x7fff895f55c9]
#+   2907 Py_Main  (in Python) + 3051  [0x10b90eb23]
#+     2907 PyRun_SimpleFileExFlags  (in Python) + 769  [0x10b8fd3fd]
#+       2907 PyRun_FileExFlags  (in Python) + 133  [0x10b8fd860]
#+         2907 ???  (in Python)  load address 0x10b85a000 + 0xa37bd  [0x10b8fd7bd]
#+           2907 PyEval_EvalCode  (in Python) + 54  [0x10b8dd7d7]
#+             2907 PyEval_EvalCodeEx  (in Python) + 1413  [0x10b8ddd62]
#+               2907 PyEval_EvalFrameEx  (in Python) + 13389  [0x10b8e13e3]
#+                 2907 ???  (in Python)  load address 0x10b85a000 + 0x8a57d  [0x10b8e457d]
#+                   2907 PyEval_EvalCodeEx  (in Python) + 1413  [0x10b8ddd62]
#+                     2907 PyEval_EvalFrameEx  (in Python) + 14387  [0x10b8e17c9]
#+                       2907 sem_wait  (in libsystem_kernel.dylib) + 10  [0x7fff827c97ba]
#2907 Thread_25011569                                                  ############## <------------------- Queue's internal "feeder thread", blocked on a pipe write
#  2907 thread_start  (in libsystem_pthread.dylib) + 13  [0x7fff871dd41d]
#    2907 _pthread_start  (in libsystem_pthread.dylib) + 176  [0x7fff871df1e5]
#      2907 _pthread_body  (in libsystem_pthread.dylib) + 131  [0x7fff871df268]
#        2907 ???  (in Python)  load address 0x10b85a000 + 0xb6882  [0x10b910882]
#          2907 PyEval_CallObjectWithKeywords  (in Python) + 93  [0x10b8e3d6b]
#            2907 PyObject_Call  (in Python) + 99  [0x10b8642ac]
#              2907 ???  (in Python)  load address 0x10b85a000 + 0x150cf  [0x10b86f0cf]
#                2907 PyObject_Call  (in Python) + 99  [0x10b8642ac]
#                  2907 ???  (in Python)  load address 0x10b85a000 + 0x2830a  [0x10b88230a]
#                    2907 PyEval_EvalCodeEx  (in Python) + 1413  [0x10b8ddd62]
#                      2907 PyEval_EvalFrameEx  (in Python) + 13389  [0x10b8e13e3]
#                        2907 ???  (in Python)  load address 0x10b85a000 + 0x8a60e  [0x10b8e460e]
#                          2907 PyEval_EvalFrameEx  (in Python) + 13389  [0x10b8e13e3]
#                            2907 ???  (in Python)  load address 0x10b85a000 + 0x8a60e  [0x10b8e460e]
#                              2907 PyEval_EvalFrameEx  (in Python) + 10788  [0x10b8e09ba]
#                                2907 PyObject_Call  (in Python) + 99  [0x10b8642ac]
#                                  2907 ???  (in Python)  load address 0x10b85a000 + 0x2830a  [0x10b88230a]
#                                    2907 PyEval_EvalCodeEx  (in Python) + 1413  [0x10b8ddd62]
#                                      2907 PyEval_EvalFrameEx  (in Python) + 15766  [0x10b8e1d2c]
#                                        2907 ???  (in _multiprocessing.so)  load address 0x10bbdc000 + 0x13c0  [0x10bbdd3c0]
#                                          2907 ???  (in _multiprocessing.so)  load address 0x10bbdc000 + 0x1a2d  [0x10bbdda2d]
#                                            2907 write  (in libsystem_kernel.dylib) + 10  [0x7fff827c9c22]

Basically, there is a part of lit that llvm/utils/lit/lit/run.py where it does:

        for i in range(len(tests)):
            self.queue.put(i)