signalmodule: fix thread-safety issue on macOS

The sigaction function is not safe to call concurrently with raise()
on macOS.

Author: colesbury

Modules/signalmodule.c

@@ -122,6 +122,7 @@ typedef struct {
     PyTypeObject *siginfo_type;
 } _signal_module_state;
 
+static _PyMutex signal_mutex;
 
 Py_LOCAL_INLINE(PyObject *)
 get_handler(int i)
@@ -437,9 +438,11 @@ signal_raise_signal_impl(PyObject *module, int signalnum)
 {
     int err;
     Py_BEGIN_ALLOW_THREADS
+    _PyMutex_lock(&signal_mutex);
     _Py_BEGIN_SUPPRESS_IPH
     err = raise(signalnum);
     _Py_END_SUPPRESS_IPH
+    _PyMutex_unlock(&signal_mutex);
     Py_END_ALLOW_THREADS
 
     if (err) {
@@ -479,6 +482,7 @@ signal_signal_impl(PyObject *module, int signalnum, PyObject *handler)
     _signal_module_state *modstate = get_signal_state(module);
     PyObject *old_handler;
     void (*func)(int);
+    PyOS_sighandler_t e;
 #ifdef MS_WINDOWS
     /* Validate that signalnum is one of the allowable signals */
     switch (signalnum) {
@@ -528,7 +532,14 @@ signal_signal_impl(PyObject *module, int signalnum, PyObject *handler)
     if (_PyErr_CheckSignalsTstate(tstate)) {
         return NULL;
     }
-    if (PyOS_setsig(signalnum, func) == SIG_ERR) {
+
+    // On macOS, sigaction isn't thread-safe with concurrent calls to raise(),
+    // so we use a mutex to prevent concurrent calls to raise() and sigaction().
+    _PyMutex_lock(&signal_mutex);
+    e = PyOS_setsig(signalnum, func);
+    _PyMutex_unlock(&signal_mutex);
+
+    if (e == SIG_ERR) {
         PyErr_SetFromErrno(PyExc_OSError);
         return NULL;
     }