From f41e9c750e6971c165e055374a1014d6afd2d50e Mon Sep 17 00:00:00 2001
From: Sam Gross <colesbury@gmail.com>
Date: Tue, 1 Jul 2025 13:26:13 -0400
Subject: [PATCH] gh-134009: Expose `PyMutex_IsLocked` in the public C API
 (gh-134365)

The `PyMutex_IsLocked()` function is useful in assertions for verifying
that code maintains certain locking invariants.
---
 Doc/c-api/init.rst                                   | 12 ++++++++++++
 Doc/whatsnew/3.15.rst                                |  7 +++++++
 Include/cpython/lock.h                               | 11 +++++++++++
 Include/internal/pycore_lock.h                       |  7 -------
 .../2025-05-20-17-13-51.gh-issue-134009.CpCmry.rst   |  1 +
 Python/lock.c                                        |  8 ++++++++
 6 files changed, 39 insertions(+), 7 deletions(-)
 create mode 100644 Misc/NEWS.d/next/C_API/2025-05-20-17-13-51.gh-issue-134009.CpCmry.rst

diff --git a/Doc/c-api/init.rst b/Doc/c-api/init.rst
index 41fd4ea14ef..409539dec17 100644
--- a/Doc/c-api/init.rst
+++ b/Doc/c-api/init.rst
@@ -2277,6 +2277,18 @@ The C-API provides a basic mutual exclusion lock.
 
    .. versionadded:: 3.13
 
+.. c:function:: int PyMutex_IsLocked(PyMutex *m)
+
+   Returns non-zero if the mutex *m* is currently locked, zero otherwise.
+
+   .. note::
+
+      This function is intended for use in assertions and debugging only and
+      should not be used to make concurrency control decisions, as the lock
+      state may change immediately after the check.
+
+   .. versionadded:: next
+
 .. _python-critical-section-api:
 
 Python Critical Section API
diff --git a/Doc/whatsnew/3.15.rst b/Doc/whatsnew/3.15.rst
index df636dc9051..f06d4c84ea5 100644
--- a/Doc/whatsnew/3.15.rst
+++ b/Doc/whatsnew/3.15.rst
@@ -346,6 +346,13 @@ Porting to Python 3.15
 
   (Contributed by Serhiy Storchaka in :gh:`133595`.)
 
+* Private functions promoted to public C APIs:
+
+  * ``PyMutex_IsLocked()`` : :c:func:`PyMutex_IsLocked`
+
+  The |pythoncapi_compat_project| can be used to get most of these new
+  functions on Python 3.14 and older.
+
 Deprecated C APIs
 -----------------
 
diff --git a/Include/cpython/lock.h b/Include/cpython/lock.h
index 8ee03e82f74..63886fca28e 100644
--- a/Include/cpython/lock.h
+++ b/Include/cpython/lock.h
@@ -36,6 +36,9 @@ PyAPI_FUNC(void) PyMutex_Lock(PyMutex *m);
 // exported function for unlocking the mutex
 PyAPI_FUNC(void) PyMutex_Unlock(PyMutex *m);
 
+// exported function for checking if the mutex is locked
+PyAPI_FUNC(int) PyMutex_IsLocked(PyMutex *m);
+
 // Locks the mutex.
 //
 // If the mutex is currently locked, the calling thread will be parked until
@@ -61,3 +64,11 @@ _PyMutex_Unlock(PyMutex *m)
     }
 }
 #define PyMutex_Unlock _PyMutex_Unlock
+
+// Checks if the mutex is currently locked.
+static inline int
+_PyMutex_IsLocked(PyMutex *m)
+{
+    return (_Py_atomic_load_uint8(&m->_bits) & _Py_LOCKED) != 0;
+}
+#define PyMutex_IsLocked _PyMutex_IsLocked
diff --git a/Include/internal/pycore_lock.h b/Include/internal/pycore_lock.h
index bd6011b60ac..585120108cf 100644
--- a/Include/internal/pycore_lock.h
+++ b/Include/internal/pycore_lock.h
@@ -25,13 +25,6 @@ PyMutex_LockFast(PyMutex *m)
     return _Py_atomic_compare_exchange_uint8(lock_bits, &expected, _Py_LOCKED);
 }
 
-// Checks if the mutex is currently locked.
-static inline int
-PyMutex_IsLocked(PyMutex *m)
-{
-    return (_Py_atomic_load_uint8(&m->_bits) & _Py_LOCKED) != 0;
-}
-
 // Re-initializes the mutex after a fork to the unlocked state.
 static inline void
 _PyMutex_at_fork_reinit(PyMutex *m)
diff --git a/Misc/NEWS.d/next/C_API/2025-05-20-17-13-51.gh-issue-134009.CpCmry.rst b/Misc/NEWS.d/next/C_API/2025-05-20-17-13-51.gh-issue-134009.CpCmry.rst
new file mode 100644
index 00000000000..f060f09de19
--- /dev/null
+++ b/Misc/NEWS.d/next/C_API/2025-05-20-17-13-51.gh-issue-134009.CpCmry.rst
@@ -0,0 +1 @@
+Expose :c:func:`PyMutex_IsLocked` as part of the public C API.
diff --git a/Python/lock.c b/Python/lock.c
index eb09019e0a2..a49d587a168 100644
--- a/Python/lock.c
+++ b/Python/lock.c
@@ -634,3 +634,11 @@ PyMutex_Unlock(PyMutex *m)
         Py_FatalError("unlocking mutex that is not locked");
     }
 }
+
+
+#undef PyMutex_IsLocked
+int
+PyMutex_IsLocked(PyMutex *m)
+{
+    return _PyMutex_IsLocked(m);
+}