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gh-159876: Detect the C stack size of non-main threads on musl by serhiy-storchaka · Pull Request #159882 · python/cpython · GitHub

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gh-159876: Detect the C stack size of non-main threads on musl #159882

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gh-159876: Detect the C stack size of non-main threads on musl #159882

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5 changes: 5 additions & 0 deletions Misc/NEWS.d/next/Core_and_Builtins/2026-10-20-16-30-00.gh-issue-159876.Mu5lSt.rst
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Original file line number Diff line number Diff line change
@@ -0,0 +1,5 @@
Query the real C stack size of non-main threads on musl-based Linux again.
The interpreter assumed a fixed ``Py_C_STACK_SIZE`` there, so a thread
created with a smaller stack overflowed it instead of raising
:exc:`RecursionError`. The main thread, whose size musl does not report
correctly, keeps using the estimate.
65 changes: 45 additions & 20 deletions Python/ceval.c
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Original file line number Diff line number Diff line change
Expand Up @@ -123,6 +123,29 @@ int pthread_attr_destroy(pthread_attr_t *a)

#endif

// Any Linux that is neither glibc nor Android (in practice, musl) cannot
// report the main thread's stack through pthread_getattr_np().
#if defined(__linux__) && !defined(__GLIBC__) && !defined(__ANDROID__)
# define _Py_STACK_QUERY_SKIPS_MAIN_THREAD 1
#else
# define _Py_STACK_QUERY_SKIPS_MAIN_THREAD 0
#endif

#if _Py_STACK_QUERY_SKIPS_MAIN_THREAD
static int
stack_on_main_thread(void)
{
# ifdef PY_HAVE_THREAD_NATIVE_ID
return PyThread_get_thread_native_id() == (unsigned long)getpid();
# else
// Without a thread id to compare, assume the worst and keep estimating.
return 1;
# endif
}
#else
# define stack_on_main_thread() 0
#endif

static void
hardware_stack_limits(uintptr_t *base, uintptr_t *top, uintptr_t sp)
{
Expand All @@ -142,27 +165,29 @@ hardware_stack_limits(uintptr_t *base, uintptr_t *top, uintptr_t sp)
*top = (uintptr_t)stack_addr;
*base = ((uintptr_t)stack_addr) - stack_size;
#else
// musl supports HAVE_PTHREAD_GETATTR_NP, but the resulting stack size
// (on alpine at least) is much smaller than expected and imposes undue
// limits compared to the old stack size estimation. musl doesn't give
// us a way to detect it, so we have to assume any non-glibc,
// non-Android Linux is musl and mustn't rely on pthread_getattr_np.
# if defined(HAVE_PTHREAD_GETATTR_NP) && !defined(_AIX) && \
!defined(__NetBSD__) && \
(defined(__GLIBC__) || defined(__ANDROID__) || !defined(__linux__))
size_t stack_size, guard_size;
void *stack_addr;
pthread_attr_t attr;
int err = pthread_getattr_np(pthread_self(), &attr);
if (err == 0) {
err = pthread_attr_getguardsize(&attr, &guard_size);
err |= pthread_attr_getstack(&attr, &stack_addr, &stack_size);
err |= pthread_attr_destroy(&attr);
}
if (err == 0) {
*base = ((uintptr_t)stack_addr) + guard_size;
*top = (uintptr_t)stack_addr + stack_size;
return;
!defined(__NetBSD__)
// On musl, pthread_getattr_np() reports the real stack of a thread that
// the library itself created, but for the process main thread it reports
// musl's default thread stack size rather than the real, RLIMIT_STACK
// sized stack (see gh-131338). Keep estimating the main thread's stack
// there, and query every other thread, whose size the estimate cannot
// know (see gh-159876).
if (!_Py_STACK_QUERY_SKIPS_MAIN_THREAD || !stack_on_main_thread()) {
size_t stack_size, guard_size;
void *stack_addr;
pthread_attr_t attr;
int err = pthread_getattr_np(pthread_self(), &attr);
if (err == 0) {
err = pthread_attr_getguardsize(&attr, &guard_size);
err |= pthread_attr_getstack(&attr, &stack_addr, &stack_size);
err |= pthread_attr_destroy(&attr);
}
if (err == 0) {
*base = ((uintptr_t)stack_addr) + guard_size;
*top = (uintptr_t)stack_addr + stack_size;
return;
}
}
# endif
// Add some space for caller function then round to minimum page size
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