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gh-118138: Adds test for multithreaded writes to files. #119107

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gh-118138: Adds test for multithreaded writes to files. #119107

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dca6eb6
Adds test for multithreaded writes to files.
elistevens May 17, 2024
997dbc2
Calls executor.shutdown before closing file.
elistevens May 17, 2024
3f05fd9
Fixes race conditions from multithreaded writes.
elistevens May 17, 2024
d156d3c
Update test_io_threading.py to not suffer from interleaved prints
elistevens May 30, 2024
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Adds test for multithreaded writes to files. 
Multithreaded writes to files (including standard calls to print() from
threads) can result in assertion failures and potentially missed output due to
race conditions in _io_TextIOWrapper_write_impl and _textiowrapper_writeflush.

See: #118138
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@elistevens
elistevens committed May 17, 2024
commit dca6eb690f19ce49af1d570623ae26f402d7d285
36 changes: 36 additions & 0 deletions Lib/test/test_io_threading.py
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Original file line number Diff line number Diff line change
@@ -0,0 +1,36 @@
import unittest
from concurrent.futures import ThreadPoolExecutor


NUM_THREADS = 100
NUM_LOOPS = 10

def looping_print_to_file(f, i):
for j in range(NUM_LOOPS):
# p = 'x' * 90 + '123456789'
p = f"{i:2}i,{j}j\n" + '0123456789' * 10
print(p, file=f)

class IoThreadingTestCase(unittest.TestCase):
def test_io_threading(self):
with ThreadPoolExecutor(max_workers=NUM_THREADS) as executor:
with open('test_io_threading.out', 'w') as f:
for i in range(NUM_THREADS):
executor.submit(looping_print_to_file, f, i)

executor.shutdown(wait=True)

with open('test_io_threading.out', 'r') as f:
lines = set(x.rstrip() for x in f.readlines() if ',' in x)

assert len(lines) == NUM_THREADS * NUM_LOOPS, repr(len(lines))

for i in range(NUM_THREADS):
for j in range(NUM_LOOPS):
p = f"{i:2}i,{j}j"

assert p in lines, repr(p)


if __name__ == "__main__":
unittest.main()
4 changes: 4 additions & 0 deletions Modules/_io/textio.c
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Expand Up @@ -1729,6 +1729,10 @@ _io_TextIOWrapper_write_impl(textio *self, PyObject *text)
Py_DECREF(b);
return NULL;
}
// The assumption that self won't have been modified from another
// thread between the flush above and the assignment below is
// incorrect.
assert(self->pending_bytes == NULL);
self->pending_bytes = b;
}
else if (!PyList_CheckExact(self->pending_bytes)) {
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