bytearray.resize is not thread-safe

Description

Currently, bytearray.resize is not thread-safe (as opposed to, from what I can see, other bytearray operations).

The problem seems to come from the fact bytearray.resize (which wraps bytearray_resize_impl) calls into PyByteArray_Resize (which locks), then does its own thing (setting the new bytes to 0) without the lock held. This means if another thread is calling into resize at the same time, one of the threads may end up writing to memory that doesn't "exist" anymore.
To be thread safe, resize would need to lock the bytearray for the duration of the whole resize operation.

Example of a problematic sequence:

Main thread initialises a bytearray with 10 elements
T1 calls resize(1000), completes the PyByteArray_Resize call
T2 calls resize(10), completes the PyByteArray_Resize call
T1 proceeds to filling (or continues to fill) the "new buffer space" with 0's, but most of it (in our case 1000 - 10) is now invalid memory as the underlying buffer has been replaced with a smaller one.

(Note: there are several paths to PyByteArray_Resize, some of which keep the same existing buffer and just change the "apparent size" of the bytearray, but going from 1000 to 10 crosses the n / 2 threshold at which a new, smaller buffer is allocated and replaces the existing one through PyBytes_FromStringAndSize.)

Proposed fix

Add a @critical_section like other methods in bytearray do it.

I have a ~~branch in my fork~~ PR #145714 that implements this fix and that I can open once the bug is confirmed.

After making this change, my reproducer does not crash anymore.

Reproducer

Running the following on the latest main, free-threaded build with TSan...

from threading import Thread

ba = bytearray(100)

def f():
    for _ in range(100_000):
        try:
            ba.resize(10_000)
            ba.resize(1)
        except (BufferError, ValueError):
            pass

threads = [Thread(target=f) for _ in range(4)]
for t in threads: t.start()
for t in threads: t.join()

... gives me ...

==================
WARNING: ThreadSanitizer: data race (pid=67656)
  Write of size 8 at 0x00012491b848 by thread T2:
    #0 bytearray_resize_lock_held bytearrayobject.c:285 (python.exe:arm64+0x10006875c)
    #1 bytearray_resize bytearrayobject.c.h:628 (python.exe:arm64+0x100072d6c)
    #2 _PyEval_EvalFrameDefault generated_cases.c.h:4041 (python.exe:arm64+0x10028fa58)
    #3 _PyEval_Vector ceval.c:2132 (python.exe:arm64+0x1002856a8)
    #4 _PyFunction_Vectorcall call.c (python.exe:arm64+0x10008e0f0)
    #5 method_vectorcall classobject.c:74 (python.exe:arm64+0x1000923ac)
    #6 context_run context.c:727 (python.exe:arm64+0x1002d0d64)
    #7 method_vectorcall_FASTCALL_KEYWORDS descrobject.c:421 (python.exe:arm64+0x1000a5258)
    #8 PyObject_Vectorcall call.c:327 (python.exe:arm64+0x10008daac)
    #9 _Py_VectorCallInstrumentation_StackRefSteal ceval.c:769 (python.exe:arm64+0x100286174)
    #10 _PyEval_EvalFrameDefault generated_cases.c.h:1817 (python.exe:arm64+0x1002929f0)
    #11 _PyEval_Vector ceval.c:2132 (python.exe:arm64+0x1002856a8)
    #12 _PyFunction_Vectorcall call.c (python.exe:arm64+0x10008e0f0)
    #13 method_vectorcall classobject.c:74 (python.exe:arm64+0x1000923ac)
    #14 _PyObject_Call call.c:348 (python.exe:arm64+0x10008dd68)
    #15 PyObject_Call call.c:373 (python.exe:arm64+0x10008dddc)
    #16 thread_run _threadmodule.c:387 (python.exe:arm64+0x10043afe8)
    #17 pythread_wrapper thread_pthread.h:234 (python.exe:arm64+0x10037b2a0)

  Previous read of size 8 at 0x00012491b848 by thread T1:
    #0 bytearray_resize bytearrayobject.c.h:628 (python.exe:arm64+0x100072e08)
    #1 _PyEval_EvalFrameDefault generated_cases.c.h:4041 (python.exe:arm64+0x10028fa58)
    #2 _PyEval_Vector ceval.c:2132 (python.exe:arm64+0x1002856a8)
    #3 _PyFunction_Vectorcall call.c (python.exe:arm64+0x10008e0f0)
    #4 method_vectorcall classobject.c:74 (python.exe:arm64+0x1000923ac)
    #5 context_run context.c:727 (python.exe:arm64+0x1002d0d64)
    #6 method_vectorcall_FASTCALL_KEYWORDS descrobject.c:421 (python.exe:arm64+0x1000a5258)
    #7 PyObject_Vectorcall call.c:327 (python.exe:arm64+0x10008daac)
    #8 _Py_VectorCallInstrumentation_StackRefSteal ceval.c:769 (python.exe:arm64+0x100286174)
    #9 _PyEval_EvalFrameDefault generated_cases.c.h:1817 (python.exe:arm64+0x1002929f0)
    #10 _PyEval_Vector ceval.c:2132 (python.exe:arm64+0x1002856a8)
    #11 _PyFunction_Vectorcall call.c (python.exe:arm64+0x10008e0f0)
    #12 method_vectorcall classobject.c:74 (python.exe:arm64+0x1000923ac)
    #13 _PyObject_Call call.c:348 (python.exe:arm64+0x10008dd68)
    #14 PyObject_Call call.c:373 (python.exe:arm64+0x10008dddc)
    #15 thread_run _threadmodule.c:387 (python.exe:arm64+0x10043afe8)
    #16 pythread_wrapper thread_pthread.h:234 (python.exe:arm64+0x10037b2a0)

  Thread T2 (tid=26268866, running) created by main thread at:
    ...

  Thread T1 (tid=26268865, running) created by main thread at:
    ...

SUMMARY: ThreadSanitizer: data race bytearrayobject.c:285 in bytearray_resize_lock_held
==================
==================
WARNING: ThreadSanitizer: data race (pid=67656)
  Write of size 8 at 0x00012491b848 by thread T2:
    #0 bytearray_resize_lock_held bytearrayobject.c:285 (python.exe:arm64+0x10006875c)
    #1 bytearray_resize bytearrayobject.c.h:628 (python.exe:arm64+0x100072d98)
    #2 _PyEval_EvalFrameDefault generated_cases.c.h:4041 (python.exe:arm64+0x10028fa58)
    #3 _PyEval_Vector ceval.c:2132 (python.exe:arm64+0x1002856a8)
    #4 _PyFunction_Vectorcall call.c (python.exe:arm64+0x10008e0f0)
    #5 method_vectorcall classobject.c:74 (python.exe:arm64+0x1000923ac)
    #6 context_run context.c:727 (python.exe:arm64+0x1002d0d64)
    #7 method_vectorcall_FASTCALL_KEYWORDS descrobject.c:421 (python.exe:arm64+0x1000a5258)
    #8 PyObject_Vectorcall call.c:327 (python.exe:arm64+0x10008daac)
    #9 _Py_VectorCallInstrumentation_StackRefSteal ceval.c:769 (python.exe:arm64+0x100286174)
    #10 _PyEval_EvalFrameDefault generated_cases.c.h:1817 (python.exe:arm64+0x1002929f0)
    #11 _PyEval_Vector ceval.c:2132 (python.exe:arm64+0x1002856a8)
    #12 _PyFunction_Vectorcall call.c (python.exe:arm64+0x10008e0f0)
    #13 method_vectorcall classobject.c:74 (python.exe:arm64+0x1000923ac)
    #14 _PyObject_Call call.c:348 (python.exe:arm64+0x10008dd68)
    #15 PyObject_Call call.c:373 (python.exe:arm64+0x10008dddc)
    #16 thread_run _threadmodule.c:387 (python.exe:arm64+0x10043afe8)
    #17 pythread_wrapper thread_pthread.h:234 (python.exe:arm64+0x10037b2a0)

  Previous read of size 8 at 0x00012491b848 by thread T3:
    #0 bytearray_resize bytearrayobject.c.h:628 (python.exe:arm64+0x100072e08)
    #1 _PyEval_EvalFrameDefault generated_cases.c.h:4041 (python.exe:arm64+0x10028fa58)
    #2 _PyEval_Vector ceval.c:2132 (python.exe:arm64+0x1002856a8)
    #3 _PyFunction_Vectorcall call.c (python.exe:arm64+0x10008e0f0)
    #4 method_vectorcall classobject.c:74 (python.exe:arm64+0x1000923ac)
    #5 context_run context.c:727 (python.exe:arm64+0x1002d0d64)
    #6 method_vectorcall_FASTCALL_KEYWORDS descrobject.c:421 (python.exe:arm64+0x1000a5258)
    #7 PyObject_Vectorcall call.c:327 (python.exe:arm64+0x10008daac)
    #8 _Py_VectorCallInstrumentation_StackRefSteal ceval.c:769 (python.exe:arm64+0x100286174)
    #9 _PyEval_EvalFrameDefault generated_cases.c.h:1817 (python.exe:arm64+0x1002929f0)
    #10 _PyEval_Vector ceval.c:2132 (python.exe:arm64+0x1002856a8)
    #11 _PyFunction_Vectorcall call.c (python.exe:arm64+0x10008e0f0)
    #12 method_vectorcall classobject.c:74 (python.exe:arm64+0x1000923ac)
    #13 _PyObject_Call call.c:348 (python.exe:arm64+0x10008dd68)
    #14 PyObject_Call call.c:373 (python.exe:arm64+0x10008dddc)
    #15 thread_run _threadmodule.c:387 (python.exe:arm64+0x10043afe8)
    #16 pythread_wrapper thread_pthread.h:234 (python.exe:arm64+0x10037b2a0)

  Thread T2 (tid=26268866, running) created by main thread at:
    ...

  Thread T3 (tid=26268867, running) created by main thread at:
    ...

SUMMARY: ThreadSanitizer: data race bytearrayobject.c:285 in bytearray_resize_lock_held
==================
ThreadSanitizer:DEADLYSIGNAL
==67656==ERROR: ThreadSanitizer: SEGV on unknown address 0x0000000000f0 (pc 0x00010276beb4 bp 0x0001719ee340 sp 0x0001719ee310 T26268868)
==67656==The signal is caused by a READ memory access.
==67656==Hint: address points to the zero page.
    #0 _PyForIter_VirtualIteratorNext ceval.c:3715 (python.exe:arm64+0x1002a3eb4)
    #1 _PyEval_EvalFrameDefault generated_cases.c.h:5842 (python.exe:arm64+0x10029944c)
    #2 _PyEval_Vector ceval.c:2132 (python.exe:arm64+0x1002856a8)
    #3 _PyFunction_Vectorcall call.c (python.exe:arm64+0x10008e0f0)
    #4 method_vectorcall classobject.c:74 (python.exe:arm64+0x1000923ac)
    #5 context_run context.c:727 (python.exe:arm64+0x1002d0d64)
    #6 method_vectorcall_FASTCALL_KEYWORDS descrobject.c:421 (python.exe:arm64+0x1000a5258)
    #7 PyObject_Vectorcall call.c:327 (python.exe:arm64+0x10008daac)
    #8 _Py_VectorCallInstrumentation_StackRefSteal ceval.c:769 (python.exe:arm64+0x100286174)
    #9 _PyEval_EvalFrameDefault generated_cases.c.h:1817 (python.exe:arm64+0x1002929f0)
    #10 _PyEval_Vector ceval.c:2132 (python.exe:arm64+0x1002856a8)
    #11 _PyFunction_Vectorcall call.c (python.exe:arm64+0x10008e0f0)
    #12 method_vectorcall classobject.c:74 (python.exe:arm64+0x1000923ac)
    #13 _PyObject_Call call.c:348 (python.exe:arm64+0x10008dd68)
    #14 PyObject_Call call.c:373 (python.exe:arm64+0x10008dddc)
    #15 thread_run _threadmodule.c:387 (python.exe:arm64+0x10043afe8)
    #16 pythread_wrapper thread_pthread.h:234 (python.exe:arm64+0x10037b2a0)
    #17 __tsan_thread_start_func <null> (libclang_rt.tsan_osx_dynamic.dylib:arm64e+0x2f678)
    #18 _pthread_start <null> (libsystem_pthread.dylib:arm64e+0x6c04)
    #19 thread_start <null> (libsystem_pthread.dylib:arm64e+0x1ba4)

==67656==Register values:
 x[0] = 0x00000001054f8000   x[1] = 0x00001000000001e0   x[2] = 0x000000004df51dff   x[3] = 0x00000001719ee490  
 x[4] = 0x0000000000000003   x[5] = 0x0000000000000000   x[6] = 0x0000000000000005   x[7] = 0x0000000000000000  
 x[8] = 0x000000000f080d9b   x[9] = 0x000000000df51dff  x[10] = 0x000000000df51d00  x[11] = 0x000000010f9607c0  
x[12] = 0x00001000000001e0  x[13] = 0x0000000000000000  x[14] = 0x0000000000000000  x[15] = 0x000000000000000e  
x[16] = 0x0000000199a1ac18  x[17] = 0x000000010345c9f8  x[18] = 0x0000000000000000  x[19] = 0x0000000132060140  
x[20] = 0x0000000122100000  x[21] = 0x00000001054fc1a8  x[22] = 0x0000000000000000  x[23] = 0x0000000000000001  
x[24] = 0x00000001054fc210  x[25] = 0x0000000122100000  x[26] = 0x000000013207019c  x[27] = 0x0000000102acbde0  
x[28] = 0x00000001054fc1a8     fp = 0x00000001719ee340     lr = 0x000000010276beb4     sp = 0x00000001719ee310  
ThreadSanitizer can not provide additional info.
SUMMARY: ThreadSanitizer: SEGV ceval.c:3715 in _PyForIter_VirtualIteratorNext
==67656==ABORTING
zsh: abort      ./python.exe

Linked PRs

Indeed this looks like a legitimate bug. Maybe I was too explicit about "waiting" for triaging but usually:

Hard crashes that you can yourself reproduce without messing up with code objects themselves (that is, using code.__replace__) should usually be assumed to be bugs themselves. When the patch is really simple and the regression test as well, it's fine to open a PR. If the issue is much more complex or touches sensitive parts of the interpreter, it's usually better to wait for other core devs to decide how to tackle the issue (unless there's an obvious way to do it).
Bugs in pure Python code may or may not bugs so it's better to wait for the issue to be triaged or be really sure about it. However, trivial fixes such as typos in the docs can be done without an issue (only typos in user-facing docs are usually accepted as otherwise they introduce code churn). Most of the time, bugs are either issues in the documentation where it's imprecise but the behavior is expected, or real bugs where the documentation is correct but the actual code does something else. And ... sometimes it's just something that we can't change because it'd be highly backward incompatible and the ship has already sailed.
Features usually require to wait for a discussion. Opening a PR without a discussion is a no go in general and I tend to close them because otherwise they remain open for too long.

Thanks for taking the time to write that comment, and thanks for all the additional explanations, I appreciate it. I opened the PR and will follow those guidelines in the future.

gh-145713: make bytearray.resize thread-safe on free-threading (#145714)

pythongh-145713: make bytearray.resize thread-safe on free-threading (p…

I just opened the backport PR (it was a draft so far).

Merged, thanks!

[3.14] gh-145713: make bytearray.resize thread-safe on free-threading (…

pythongh-145713: make bytearray.resize thread-safe on free-threading (p…

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`bytearray.resize` is not thread-safe #145713

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bytearray.resize is not thread-safe #145713

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picnixz commented on Mar 9, 2026

KowalskiThomas commented on Mar 9, 2026

akanil18 commented on Mar 10, 2026

picnixz commented on Mar 10, 2026

KowalskiThomas commented on Mar 13, 2026

kumaraditya303 commented on Mar 13, 2026

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`bytearray.resize` is not thread-safe #145713