gh-151546: Fix stack limits on musl

If the thread stack size is set by linker flags, pass the stack size
to Python/ceval.c via the new _Py_LINKER_THREAD_STACK_SIZE variable
to set Py_C_STACK_SIZE macro.

Author: vstinner

Misc/NEWS.d/next/Core_and_Builtins/2026-06-16-17-23-37.gh-issue-151546.LhiaZz.rst

@@ -0,0 +1,3 @@
+Fix the stack limit check if Python is linked to musl (ex: Alpine Linux).
+Use the stack size set by the linker to compute the stack limits. Patch by
+Victor Stinner.

Python/ceval.c

@@ -49,7 +49,9 @@ _Py_ReachedRecursionLimitWithMargin(PyThreadState *tstate, int margin_count)
 #endif
 }
 
-#if defined(__s390x__)
+#if defined(_Py_LINKER_THREAD_STACK_SIZE)
+#  define Py_C_STACK_SIZE _Py_LINKER_THREAD_STACK_SIZE
+#elif defined(__s390x__)
 #  define Py_C_STACK_SIZE 320000
 #elif defined(_WIN32)
    // Don't define Py_C_STACK_SIZE, ask the O/S

configure

@@ -2507,6 +2507,9 @@ EOF
 
     if test "$ac_cv_thread_stack_size" != "default" -a "$ac_cv_thread_stack_size" != "unknown"; then
         LDFLAGS="$LDFLAGS -Wl,-z,stack-size=$ac_cv_thread_stack_size"
+        # Stack size used by Python/ceval.c to set Py_C_STACK_SIZE
+        AC_DEFINE_UNQUOTED([_Py_LINKER_THREAD_STACK_SIZE], [$ac_cv_thread_stack_size],
+                  [Thread stack size set by the linker (in bytes).])
     fi
 fi
 

configure.ac

@@ -2088,6 +2088,9 @@
 /* Define if you have the 'PR_SET_VMA_ANON_NAME' constant. */
 #undef _Py_HAVE_PR_SET_VMA_ANON_NAME
 
+/* Thread stack size set by the linker (in bytes). */
+#undef _Py_LINKER_THREAD_STACK_SIZE
+
 /* Define to 1 if the machine stack grows down (default); 0 if it grows up. */
 #undef _Py_STACK_GROWS_DOWN