[3.13] gh-117511: Make PyMutex public in the non-limited API (GH-117731) (#120800)

(cherry picked from commit 3af7263037)

Include/cpython/lock.h

@@ -0,0 +1,63 @@
+#ifndef Py_CPYTHON_LOCK_H
+#  error "this header file must not be included directly"
+#endif
+
+#define _Py_UNLOCKED    0
+#define _Py_LOCKED      1
+
+// A mutex that occupies one byte. The lock can be zero initialized to
+// represent the unlocked state.
+//
+// Typical initialization:
+//   PyMutex m = (PyMutex){0};
+//
+// Or initialize as global variables:
+//   static PyMutex m;
+//
+// Typical usage:
+//   PyMutex_Lock(&m);
+//   ...
+//   PyMutex_Unlock(&m);
+//
+// The contents of the PyMutex are not part of the public API, but are
+// described to aid in understanding the implementation and debugging. Only
+// the two least significant bits are used. The remaining bits are always zero:
+// 0b00: unlocked
+// 0b01: locked
+// 0b10: unlocked and has parked threads
+// 0b11: locked and has parked threads
+typedef struct PyMutex {
+    uint8_t _bits;  // (private)
+} PyMutex;
+
+// exported function for locking the mutex
+PyAPI_FUNC(void) PyMutex_Lock(PyMutex *m);
+
+// exported function for unlocking the mutex
+PyAPI_FUNC(void) PyMutex_Unlock(PyMutex *m);
+
+// Locks the mutex.
+//
+// If the mutex is currently locked, the calling thread will be parked until
+// the mutex is unlocked. If the current thread holds the GIL, then the GIL
+// will be released while the thread is parked.
+static inline void
+_PyMutex_Lock(PyMutex *m)
+{
+    uint8_t expected = _Py_UNLOCKED;
+    if (!_Py_atomic_compare_exchange_uint8(&m->_bits, &expected, _Py_LOCKED)) {
+        PyMutex_Lock(m);
+    }
+}
+#define PyMutex_Lock _PyMutex_Lock
+
+// Unlocks the mutex.
+static inline void
+_PyMutex_Unlock(PyMutex *m)
+{
+    uint8_t expected = _Py_LOCKED;
+    if (!_Py_atomic_compare_exchange_uint8(&m->_bits, &expected, _Py_UNLOCKED)) {
+        PyMutex_Unlock(m);
+    }
+}
+#define PyMutex_Unlock _PyMutex_Unlock

Include/cpython/weakrefobject.h

@@ -36,7 +36,7 @@ struct _PyWeakReference {
      * Normally this can be derived from wr_object, but in some cases we need
      * to lock after wr_object has been set to Py_None.
      */
-    struct _PyMutex *weakrefs_lock;
+    PyMutex *weakrefs_lock;
 #endif
 };