Can you add a unit test that fails without the fix and passes with it?

Can you add a unit test that fails without the fix and passes with it?

Run this code:

import zlib
import threading

with open('txt.tar', 'rb') as f:
    b = f.read()
    b = b[:100*1024*1024]
print('read', len(b))

cobj = zlib.compressobj()

def f(name, cobj, b):
    print(name)
    dat = cobj.compress(b)
    print(name, len(dat))

thread1 = threading.Thread(target=f, args=('T1', cobj, b))
thread2 = threading.Thread(target=f, args=('T2', cobj, b))
thread1.start()
thread2.start()

print('ok')

Before the patch, output:

read 104857600
T1
T2
ok
T1 57142315

It seems thread2 did not finish the compressing.

After the patch, output:

read 104857600
T1
T2
ok
T1 57165411
T2 57166659

Can you add a unit test that fails without the fix and passes with it?

Seems a little troublesome. If the data is large, it may be time-consuming. If the data is small, it may not trigger the bug.

Above code can reliably reproduce the bug.

@ambv: Please replace # with GH- in the commit message next time. Thanks!