Synchronization primitives
**************************

The C-API provides a basic mutual exclusion lock.

type PyMutex

   A mutual exclusion lock.  The "PyMutex" should be initialized to
   zero to represent the unlocked state.  For example:

      PyMutex mutex = {0};

   Instances of "PyMutex" should not be copied or moved.  Both the
   contents and address of a "PyMutex" are meaningful, and it must
   remain at a fixed, writable location in memory.

   Note:

     A "PyMutex" currently occupies one byte, but the size should be
     considered unstable.  The size may change in future Python
     releases without a deprecation period.

   Added in version 3.13.

void PyMutex_Lock(PyMutex *m)
    * Thread safety: Atomic.*

   Lock mutex *m*.  If another thread has already locked it, the
   calling thread will block until the mutex is unlocked.  While
   blocked, the thread will temporarily detach the *thread state* if
   one exists.

   Added in version 3.13.

void PyMutex_Unlock(PyMutex *m)
    * Thread safety: Atomic.*

   Unlock mutex *m*. The mutex must be locked — otherwise, the
   function will issue a fatal error.

   Added in version 3.13.

int PyMutex_IsLocked(PyMutex *m)
    * Thread safety: Atomic.*

   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.

   Added in version 3.14.


Python critical section API
===========================

The critical section API provides a deadlock avoidance layer on top of
per-object locks for *free-threaded* CPython.  They are intended to
replace reliance on the *global interpreter lock*, and are no-ops in
versions of Python with the global interpreter lock.

Critical sections are intended to be used for custom types implemented
in C-API extensions. They should generally not be used with built-in
types like "list" and "dict" because their public C-APIs already use
critical sections internally, with the notable exception of
"PyDict_Next()", which requires critical section to be acquired
externally.

Critical sections avoid deadlocks by implicitly suspending active
critical sections, hence, they do not provide exclusive access such as
provided by traditional locks like "PyMutex".  When a critical section
is started, the per-object lock for the object is acquired. If the
code executed inside the critical section calls C-API functions then
it can suspend the critical section thereby releasing the per-object
lock, so other threads can acquire the per-object lock for the same
object.

Variants that accept "PyMutex" pointers rather than Python objects are
also available. Use these variants to start a critical section in a
situation where there is no "PyObject" – for example, when working
with a C type that does not extend or wrap "PyObject" but still needs
to call into the C API in a manner that might lead to deadlocks.

The functions and structs used by the macros are exposed for cases
where C macros are not available. They should only be used as in the
given macro expansions. Note that the sizes and contents of the
structures may change in future Python versions.

Note:

  Operations that need to lock two objects at once must use
  "Py_BEGIN_CRITICAL_SECTION2".  You *cannot* use nested critical
  sections to lock more than one object at once, because the inner
  critical section may suspend the outer critical sections.  This API
  does not provide a way to lock more than two objects at once.

Example usage:

   static PyObject *
   set_field(MyObject *self, PyObject *value)
   {
      Py_BEGIN_CRITICAL_SECTION(self);
      Py_SETREF(self->field, Py_XNewRef(value));
      Py_END_CRITICAL_SECTION();
      Py_RETURN_NONE;
   }

In the above example, "Py_SETREF" calls "Py_DECREF", which can call
arbitrary code through an object’s deallocation function.  The
critical section API avoids potential deadlocks due to reentrancy and
lock ordering by allowing the runtime to temporarily suspend the
critical section if the code triggered by the finalizer blocks and
calls "PyEval_SaveThread()".

Py_BEGIN_CRITICAL_SECTION(op)

   Acquires the per-object lock for the object *op* and begins a
   critical section.

   In the free-threaded build, this macro expands to:

      {
          PyCriticalSection _py_cs;
          PyCriticalSection_Begin(&_py_cs, (PyObject*)(op))

   In the default build, this macro expands to "{".

   Added in version 3.13.

Py_BEGIN_CRITICAL_SECTION_MUTEX(m)

   Locks the mutex *m* and begins a critical section.

   In the free-threaded build, this macro expands to:

      {
           PyCriticalSection _py_cs;
           PyCriticalSection_BeginMutex(&_py_cs, m)

   Note that unlike "Py_BEGIN_CRITICAL_SECTION", there is no cast for
   the argument of the macro - it must be a "PyMutex" pointer.

   On the default build, this macro expands to "{".

   Added in version 3.14.

Py_END_CRITICAL_SECTION()

   Ends the critical section and releases the per-object lock.

   In the free-threaded build, this macro expands to:

          PyCriticalSection_End(&_py_cs);
      }

   In the default build, this macro expands to "}".

   Added in version 3.13.

Py_BEGIN_CRITICAL_SECTION2(a, b)

   Acquires the per-object locks for the objects *a* and *b* and
   begins a critical section.  The locks are acquired in a consistent
   order (lowest address first) to avoid lock ordering deadlocks.

   In the free-threaded build, this macro expands to:

      {
          PyCriticalSection2 _py_cs2;
          PyCriticalSection2_Begin(&_py_cs2, (PyObject*)(a), (PyObject*)(b))

   In the default build, this macro expands to "{".

   Added in version 3.13.

Py_BEGIN_CRITICAL_SECTION2_MUTEX(m1, m2)

   Locks the mutexes *m1* and *m2* and begins a critical section.

   In the free-threaded build, this macro expands to:

      {
           PyCriticalSection2 _py_cs2;
           PyCriticalSection2_BeginMutex(&_py_cs2, m1, m2)

   Note that unlike "Py_BEGIN_CRITICAL_SECTION2", there is no cast for
   the arguments of the macro - they must be "PyMutex" pointers.

   On the default build, this macro expands to "{".

   Added in version 3.14.

Py_END_CRITICAL_SECTION2()

   Ends the critical section and releases the per-object locks.

   In the free-threaded build, this macro expands to:

          PyCriticalSection2_End(&_py_cs2);
      }

   In the default build, this macro expands to "}".

   Added in version 3.13.


Legacy locking APIs
===================

These APIs are obsolete since Python 3.13 with the introduction of
"PyMutex".

type PyThread_type_lock

   A pointer to a mutual exclusion lock.

type PyLockStatus

   The result of acquiring a lock with a timeout.

   enumerator PY_LOCK_FAILURE

      Failed to acquire the lock.

   enumerator PY_LOCK_ACQUIRED

      The lock was successfully acquired.

   enumerator PY_LOCK_INTR

      The lock was interrupted by a signal.

PyThread_type_lock PyThread_allocate_lock(void)
    * Part of the Stable ABI.*

   Allocate a new lock.

   On success, this function returns a lock; on failure, this function
   returns "0" without an exception set.

   The caller does not need to hold an *attached thread state*.

void PyThread_free_lock(PyThread_type_lock lock)
    * Part of the Stable ABI.*

   Destroy *lock*. The lock should not be held by any thread when
   calling this.

   The caller does not need to hold an *attached thread state*.

PyLockStatus PyThread_acquire_lock_timed(PyThread_type_lock lock, long long microseconds, int intr_flag)
    * Part of the Stable ABI.*

   Acquire *lock* with a timeout.

   This will wait for *microseconds* microseconds to acquire the lock.
   If the timeout expires, this function returns "PY_LOCK_FAILURE". If
   *microseconds* is "-1", this will wait indefinitely until the lock
   has been released.

   If *intr_flag* is "1", acquiring the lock may be interrupted by a
   signal, in which case this function returns "PY_LOCK_INTR". Upon
   interruption, it’s generally expected that the caller makes a call
   to "Py_MakePendingCalls()" to propagate an exception to Python
   code.

   If the lock is successfully acquired, this function returns
   "PY_LOCK_ACQUIRED".

   The caller does not need to hold an *attached thread state*.

int PyThread_acquire_lock(PyThread_type_lock lock, int waitflag)
    * Part of the Stable ABI.*

   Acquire *lock*.

   If *waitflag* is "1" and another thread currently holds the lock,
   this function will wait until the lock can be acquired and will
   always return "1".

   If *waitflag* is "0" and another thread holds the lock, this
   function will not wait and instead return "0". If the lock is not
   held by any other thread, then this function will acquire it and
   return "1".

   Unlike "PyThread_acquire_lock_timed()", acquiring the lock cannot
   be interrupted by a signal.

   The caller does not need to hold an *attached thread state*.

int PyThread_release_lock(PyThread_type_lock lock)
    * Part of the Stable ABI.*

   Release *lock*. If *lock* is not held, then this function issues a
   fatal error.

   The caller does not need to hold an *attached thread state*.
