Assertion failure: `!_PyFrame_IsIncomplete(frame->f_frame)`

Crash report

What happened?

It's possible to cause an abort on a free-threaded build even with PYTHON_GIL=1 by running the code below (only reproduces with free threading enabled builds).

I attempted to replicate the trigger without pdb (using sys.monitoring directly, frame trace setters, sys.settrace, Bdb directly) without success. pdb's specific call chain seems necessary to reliably trigger it.

MRE:

from threading import Thread
import sys, pdb
sys.stdin = open("/dev/null")

# ONE shared Pdb instance so threads call set_trace on the SAME one
shared_pdb = pdb.Pdb(mode='inline', backend='monitoring', colorize=False)

def thread_target():
    shared_pdb.set_trace(sys._getframe().f_back)

def set_trace():
    return sys._getframe().f_back

for x in range(5):
    threads = []
    for x in range(50):
        threads.append(Thread(target=thread_target))
        threads.append(Thread(target=set_trace))
        threads.append(Thread(target=sys._clear_internal_caches))

    for t in threads:
        t.start()
        sys._clear_internal_caches()
    for t in threads:
        t.join()

print("NO ABORT")

Backtrace:

python: Objects/frameobject.c:2400: PyFrameObject *PyFrame_GetBack(PyFrameObject *): Assertion `!_PyFrame_IsIncomplete(frame->f_frame)' failed.

Thread 23 "Thread-22 (thre" received signal SIGABRT, Aborted.

#0  __pthread_kill_implementation (threadid=<optimized out>, signo=6, no_tid=0) at ./nptl/pthread_kill.c:44
#1  __pthread_kill_internal (threadid=<optimized out>, signo=6) at ./nptl/pthread_kill.c:89
#2  __GI___pthread_kill (threadid=<optimized out>, signo=signo@entry=6) at ./nptl/pthread_kill.c:100
#3  0x00007ffff7c45e2e in __GI_raise (sig=sig@entry=6) at ../sysdeps/posix/raise.c:26
#4  0x00007ffff7c28888 in __GI_abort () at ./stdlib/abort.c:77
#5  0x00007ffff7c287f0 in __assert_fail_base (fmt=<optimized out>, assertion=<optimized out>, file=<optimized out>, line=<optimized out>, function=<optimized out>) at ./assert/assert.c:118
#6  0x00007ffff7c3c19f in __assert_fail (assertion=<optimized out>, file=<optimized out>, line=<optimized out>, function=<optimized out>) at ./assert/assert.c:127
#7  0x0000555555b50d29 in PyFrame_GetBack (frame=frame@entry=0x7bffcc160410) at Objects/frameobject.c:2400
#8  0x0000555555b58c76 in frame_back_get_impl (self=0x7bffcc160410) at Objects/frameobject.c:1119
#9  frame_back_get (self=<optimized out>, _unused_context=<optimized out>) at Objects/clinic/frameobject.c.h:212
#10 0x0000555555c2cfb4 in _PyObject_GenericGetAttrWithDict (obj=0x7bffcc160410, name=0x7bffb60c0610, dict=0x0, suppress=0) at Objects/object.c:1924
#11 0x0000555555c2cab3 in _PyObject_GetAttrStackRef (v=0x7bffcc160410, name=0x7bffb60c0610) at Objects/object.c:1367
#12 0x0000555555edc322 in _PyEval_EvalFrameDefault (tstate=<optimized out>, frame=<optimized out>, throwflag=<optimized out>) at Python/generated_cases.c.h:8041
#13 0x0000555555ea8d04 in _PyEval_EvalFrame (tstate=0x7bffb4249010, frame=0x7e8ff709e328, throwflag=0) at ./Include/internal/pycore_ceval.h:118
#14 _PyEval_Vector (tstate=0x7bffb4249010, func=0x7bffb64cb1b0, locals=0x0, args=0x7bff9d4b9720, argcount=1, kwnames=0x0) at Python/ceval.c:2179
#15 0x0000555555ac6030 in _PyObject_VectorcallTstate (tstate=tstate@entry=0x7bffb4249010, callable=callable@entry=0x7bffb64cb1b0, args=0x7bff9d4b9720, nargsf=nargsf@entry=1,
    kwnames=kwnames@entry=0x0) at ./Include/internal/pycore_call.h:144
#16 0x0000555555acbf7f in _PyObject_VectorcallPrepend (tstate=<optimized out>, callable=<optimized out>, arg=<error reading variable: Cannot access memory at address 0x6>,
    args=<optimized out>, nargsf=0, kwnames=<optimized out>) at Objects/call.c:855
#17 0x0000555555f6b113 in _PyObject_VectorcallTstate (tstate=0x7bffb4249010, callable=0x7bffc82200d0, args=0x7bff9a2ab6c0, nargsf=22, kwnames=0x0) at ./Include/internal/pycore_call.h:144
#18 context_run (self=0x7bffb757cc10, args=args@entry=0x7bff9a2a96b0, nargs=nargs@entry=1, kwnames=kwnames@entry=0x0) at Python/context.c:727
#19 0x0000555555af812d in method_vectorcall_FASTCALL_KEYWORDS (func=func@entry=0x7bffb60a8820, args=args@entry=0x7bff9a2a96a8, nargsf=nargsf@entry=9223372036854775810,
    kwnames=kwnames@entry=0x0) at Objects/descrobject.c:421
#20 0x0000555555ac6030 in _PyObject_VectorcallTstate (tstate=0x7bffb4249010, callable=0x7bffb60a8820, args=0x7bff9a2a96a8, nargsf=9223372036854775810, kwnames=0x0)
    at ./Include/internal/pycore_call.h:144
#21 0x0000555555ea9e3b in _Py_VectorCallInstrumentation_StackRefSteal (callable=..., arguments=arguments@entry=0x7e8ff709e2f8, total_args=total_args@entry=2, kwnames=kwnames@entry=...,
    call_instrumentation=false, frame=frame@entry=0x7e8ff709e298, this_instr=0x7bffc81901fe, tstate=0x7bffb4249010) at Python/ceval.c:775
#22 0x0000555555ee754d in _PyEval_EvalFrameDefault (tstate=<optimized out>, frame=<optimized out>, throwflag=<optimized out>) at Python/generated_cases.c.h:1841
#23 0x0000555555ea8d04 in _PyEval_EvalFrame (tstate=0x7bffb4249010, frame=0x7e8ff709e220, throwflag=0) at ./Include/internal/pycore_ceval.h:118
#24 _PyEval_Vector (tstate=0x7bffb4249010, func=0x7bffb64cb290, locals=0x0, args=0x7bff9d4b8220, argcount=1, kwnames=0x0) at Python/ceval.c:2179
#25 0x0000555555ac6030 in _PyObject_VectorcallTstate (tstate=tstate@entry=0x7bffb4249010, callable=callable@entry=0x7bffb64cb290, args=0x7bff9d4b8220, nargsf=nargsf@entry=1,
    kwnames=kwnames@entry=0x0) at ./Include/internal/pycore_call.h:144
#26 0x0000555555acbf7f in _PyObject_VectorcallPrepend (tstate=<optimized out>, callable=<optimized out>, arg=<error reading variable: Cannot access memory at address 0x6>,
    args=<optimized out>, nargsf=0, kwnames=<optimized out>) at Objects/call.c:855
#27 0x0000555555ac8802 in _PyVectorcall_Call (tstate=<optimized out>, func=<optimized out>, callable=<optimized out>, tuple=0x55555695f428 <_PyRuntime+173928>, kwargs=<optimized out>)
    at Objects/call.c:273
#28 0x00005555562c9019 in thread_run (boot_raw=boot_raw@entry=0x7bffb41f08d0) at ./Modules/_threadmodule.c:387
#29 0x000055555610e226 in pythread_wrapper (arg=0x7bffb4010b50) at Python/thread_pthread.h:234
#30 0x00005555558a034b in asan_thread_start(void*) ()

Found using fusil by @vstinner.

CPython versions tested on:

CPython main branch, 3.15

Operating systems tested on:

Linux

Output from running 'python -VV' on the command line:

Python 3.15.0a7+ free-threading build (heads/main:4d0e8ee649c, Mar 29 2026, 23:20:29) [Clang 21.1.2 (2ubuntu6)]

Linked PRs

• gh-156001

Start with the reproducer and backtrace, then inspect Objects/frameobject.c around PyFrame_GetBack and the pdb/sys.monitoring call chain described in the issue. Run the MRE on a free-threaded CPython 3.15 build; done means the abort no longer occurs under the reported threaded workload and appropriate regression coverage is added.