It’s possible to trigger the problem on Unix with much smaller sizes, e.g. by interrupting the write() with a signal handler (even if the signal handler doesn’t do anything). The following script starts a subprocess doing a 16MiB write and sends a signal, which is handled but is a no-op, after reading a bit from the pipe:

import signal
import subprocess
import sys

CHILD_PROCESS = '''
import signal, sys
signal.signal(signal.SIGINT, lambda *x: None)
written = sys.stdout.write('x' * 16777216)
print('written:', written, file=sys.stderr, flush=True)
'''

proc = subprocess.Popen(
    [sys.executable, '-u', '-c', CHILD_PROCESS],
    stdin=subprocess.PIPE,
    stdout=subprocess.PIPE,
    stderr=subprocess.PIPE,
)
read = len(proc.stdout.read(1))
proc.send_signal(signal.SIGINT)
read += len(proc.stdout.read())
stdout, stderr = proc.communicate()
assert stdout == b''
print('stderr:', stderr)
assert read == 16777216, "read: {}".format(read)


% python3 test_interrupted_write.py
stderr: b'written: 16777216\n'
Traceback (most recent call last):
  File "test_interrupted_write.py", line 24, in <module>
    assert read == 16777216, "read: {}".format(read)
AssertionError: read: 69632

If I remove the '-u' that gets passed to the subprocess:

% python3 test_interrupted_write.py
stderr: b'written: 16777216\n'