Segfault in contextvars HAMT during isolation_scope() / new_scope() with uvicorn multiprocessing workers

[sysradium]

How do you use Sentry?

Self-hosted/on-premise

Version

2.45.0

Steps to Reproduce

Environment

• sentry-sdk version: 2.45.0
• Python version: 3.10.x
• OS: Linux (x86_64)
• Framework: FastAPI + uvicorn with --workers 10 (multiprocessing spawn)
• Event loop: uvloop 0.21.0

Problem

Workers crash with SIGSEGV when Sentry SDK calls _current_scope.set() inside isolation_scope() or new_scope(). The crash occurs in Python's HAMT (Hash Array Mapped Trie) implementation used by contextvars.

GDB analysis shows that the HAMT node pointer has been corrupted - it points to a PyFunctionObject (_safe_repr_wrapper from sentry_sdk/serializer.py) instead of a HAMT node. When Python tries to clone this "node", it reads the function's vectorcall field as if it were b_array, then crashes trying to Py_INCREF a C function pointer.

Stack trace:

#0  _Py_INCREF (op=0x7b4d9a872260 <_PyFunction_Vectorcall>) at ./Include/object.h:472
#1  _Py_XINCREF (op=0x7b4d9a872260 <_PyFunction_Vectorcall>) at ./Include/object.h:558
#2  hamt_node_bitmap_clone (node=0x7b4c301c7e20) at Python/hamt.c:583
#3  hamt_node_bitmap_assoc (...) at Python/hamt.c:771
#4  _PyHamt_Assoc (...) at Python/hamt.c:2308
#5  contextvar_set (var=0x7b4d9916a5c0, val=<Scope at remote 0x7b4c30d62b60>) at Python/context.c:738
#6  PyContextVar_Set (ovar=<_contextvars.ContextVar at remote 0x7b4d9916a5c0>, ...) at Python/context.c:285
...
#12 _PyEval_EvalFrame (...) for file sentry_sdk/scope.py, line 1785, in isolation_scope

Python stack trace

File "sentry_sdk/scope.py", line 1785, in isolation_scope
    current_token = _current_scope.set(forked_current_scope)
File "contextlib.py", line 135, in __enter__
    return next(self.gen)
File "sentry_sdk/integrations/asgi.py", line 198, in _run_app
    with sentry_sdk.isolation_scope() as sentry_scope:

Corrupted HAMT node:

(gdb) p *node
$2 = {
  ob_base = {ob_type = 0x7b4d9aa8a7a0 <PyFunction_Type>},
  b_bitmap = 2587471808,
  b_array = {'_safe_repr_wrapper'}  # Does not look like HAMT data
}

(gdb) p node->b_array[0]
$5 = '_safe_repr_wrapper'

(gdb) x/20x 0x7b4d9a872260
0x7b4d9a872260 <_PyFunction_Vectorcall>: ....

Sentry configuration

sentry_sdk.init(
    dsn=settings.SENTRY_DSN,
    traces_sample_rate=1.0,
    profiles_sample_rate=0.0,
    enable_tracing=True,
    send_default_pii=True,
    integrations=[
        FastApiIntegration(),
        HttpxIntegration(),
        AioHttpIntegration(),
        RedisIntegration(),
    ],
)

How to reproduce

1. FastAPI app with sentry-sdk 2.45.0
2. Run with uvicorn app:app --workers 10 (multiprocessing spawn)
3. Send concurrent HTTP requests
4. Workers eventually crash with SIGSEGV

Observations

• Crash happens in spawned worker processes, not the main process
• The corrupted pointer always points to Sentry SDK's _safe_repr_wrapper function or _PyFunction_Vectorcall
• Workers that don't use Sentry SDK (different project, same infrastructure) don't crash
• Same codebase works fine in single-process mode

We tried setting OPENBLAS_NUM_THREADS=1 and it did not help.

It appears that a function object created during Sentry serialization (_safe_repr_wrapper) is somehow being written to a location that should contain a HAMT node pointer.

This could be:

• A use-after-free where HAMT memory is reused for function objects
• A buffer overflow in serialization code
• Race condition in scope/context handling with multiprocessing

Expected Result

No segfault

Actual Result

Eventual segfault

[linear]

PY-2002