gh-149162: Use _Py_LOCK_DONT_DETACH in intern_common

Avoids potential deadlocks with C++ static local initialization in
extensions that call PyUnicode_InternFromString during one-time
initialization.

Author: colesbury

Include/internal/pycore_pyatomic_ft_wrappers.h

@@ -138,6 +138,7 @@ extern "C" {
 #define FT_ATOMIC_ADD_SSIZE(value, new_value) \
     (void)_Py_atomic_add_ssize(&value, new_value)
 #define FT_MUTEX_LOCK(lock) PyMutex_Lock(lock)
+#define FT_MUTEX_LOCK_FLAGS(lock, flags) PyMutex_LockFlags(lock, flags)
 #define FT_MUTEX_UNLOCK(lock) PyMutex_Unlock(lock)
 
 #else
@@ -201,6 +202,7 @@ extern "C" {
 #define FT_ATOMIC_STORE_ULLONG_RELAXED(value, new_value) value = new_value
 #define FT_ATOMIC_ADD_SSIZE(value, new_value) (void)(value += new_value)
 #define FT_MUTEX_LOCK(lock) do {} while (0)
+#define FT_MUTEX_LOCK_FLAGS(lock, flags) do {} while (0)
 #define FT_MUTEX_UNLOCK(lock) do {} while (0)
 
 #endif

Misc/NEWS.d/next/Core_and_Builtins/2026-04-29-15-10-59.gh-issue-149162.BPPyrq.rst

@@ -0,0 +1,3 @@
+Fix a potential deadlock in :c:func:`PyUnicode_InternFromString` and other
+interning functions in the :term:`free-threaded build` when called from C++
+static local initializers.

Objects/unicodeobject.c

@@ -14347,7 +14347,7 @@ intern_common(PyInterpreterState *interp, PyObject *s /* stolen */,
     }
 #endif
 
-    FT_MUTEX_LOCK(INTERN_MUTEX);
+    FT_MUTEX_LOCK_FLAGS(INTERN_MUTEX, _Py_LOCK_DONT_DETACH);
     PyObject *t;
     {
         int res = PyDict_SetDefaultRef(interned, s, s, &t);