WIP (don't merge!): bpo-21895: Experiment signal handlers in any thread

Doc/library/signal.rst

@@ -46,18 +46,17 @@ This has consequences:
   signal handlers will be called when the calculation finishes.
 
 
-.. _signals-and-threads:
-
-
 Signals and threads
 ^^^^^^^^^^^^^^^^^^^
 
-Python signal handlers are always executed in the main Python thread,
-even if the signal was received in another thread.  This means that signals
-can't be used as a means of inter-thread communication.  You can use
-the synchronization primitives from the :mod:`threading` module instead.
+Only the main thread is allowed to set a new signal handler.
+
+You can use the synchronization primitives from the :mod:`threading` module
+instead.
 
-Besides, only the main thread is allowed to set a new signal handler.
+.. versionchanged:: 3.7
+   Python signal handlers can now be executed in threads other than the main
+   Python thread.
 
 
 Module contents
@@ -221,11 +220,7 @@ The :mod:`signal` module defines the following functions:
 
    Send the signal *signalnum* to the thread *thread_id*, another thread in the
    same process as the caller.  The target thread can be executing any code
-   (Python or not).  However, if the target thread is executing the Python
-   interpreter, the Python signal handlers will be :ref:`executed by the main
-   thread <signals-and-threads>`.  Therefore, the only point of sending a
-   signal to a particular Python thread would be to force a running system call
-   to fail with :exc:`InterruptedError`.
+   (Python or not).
 
    Use :func:`threading.get_ident()` or the :attr:`~threading.Thread.ident`
    attribute of :class:`threading.Thread` objects to get a suitable value

Modules/_winapi.c

@@ -1350,9 +1350,9 @@ _winapi_WaitForMultipleObjects_impl(PyObject *module, PyObject *handle_seq,
         handles[i] = h;
         Py_DECREF(v);
     }
-    /* If this is the main thread then make the wait interruptible
-       by Ctrl-C unless we are waiting for *all* handles */
-    if (!wait_flag && _PyOS_IsMainThread()) {
+    /* Make the wait interruptible by Ctrl-C unless
+       we are waiting for *all* handles */
+    if (!wait_flag) {
         sigint_event = _PyOS_SigintEvent();
         assert(sigint_event != NULL);
         handles[nhandles++] = sigint_event;

Modules/signalmodule.c

@@ -417,13 +417,6 @@ signal_signal_impl(PyObject *module, int signalnum, PyObject *handler)
             PyErr_SetString(PyExc_ValueError, "invalid signal value");
             return NULL;
     }
-#endif
-#ifdef WITH_THREAD
-    if (PyThread_get_thread_ident() != main_thread) {
-        PyErr_SetString(PyExc_ValueError,
-                        "signal only works in main thread");
-        return NULL;
-    }
 #endif
     if (signalnum < 1 || signalnum >= NSIG) {
         PyErr_SetString(PyExc_ValueError,
@@ -436,7 +429,8 @@ signal_signal_impl(PyObject *module, int signalnum, PyObject *handler)
         func = SIG_DFL;
     else if (!PyCallable_Check(handler)) {
         PyErr_SetString(PyExc_TypeError,
-"signal handler must be signal.SIG_IGN, signal.SIG_DFL, or a callable object");
+                        "signal handler must be signal.SIG_IGN, "
+                        "signal.SIG_DFL, or a callable object");
                 return NULL;
     }
     else
@@ -550,14 +544,6 @@ signal_set_wakeup_fd(PyObject *self, PyObject *args)
         return NULL;
 #endif
 
-#ifdef WITH_THREAD
-    if (PyThread_get_thread_ident() != main_thread) {
-        PyErr_SetString(PyExc_ValueError,
-                        "set_wakeup_fd only works in main thread");
-        return NULL;
-    }
-#endif
-
 #ifdef MS_WINDOWS
     is_socket = 0;
     if (sockfd != INVALID_FD) {
@@ -1501,11 +1487,6 @@ PyErr_CheckSignals(void)
     if (!is_tripped)
         return 0;
 
-#ifdef WITH_THREAD
-    if (PyThread_get_thread_ident() != main_thread)
-        return 0;
-#endif
-
     /*
      * The is_tripped variable is meant to speed up the calls to
      * PyErr_CheckSignals (both directly or via pending calls) when no
@@ -1553,7 +1534,16 @@ PyErr_CheckSignals(void)
 void
 PyErr_SetInterrupt(void)
 {
+#if defined(HAVE_PTHREAD_KILL) && defined(WITH_THREAD)
+    if (PyThread_get_thread_ident() == main_thread) {
+        trip_signal(SIGINT);
+    }
+    else {
+        pthread_kill(main_thread, SIGINT);
+    }
+#else
     trip_signal(SIGINT);
+#endif
 }
 
 void

Modules/timemodule.c

@@ -1457,11 +1457,9 @@ pysleep(_PyTime_t secs)
             return -1;
         }
 
-        /* Allow sleep(0) to maintain win32 semantics, and as decreed
-         * by Guido, only the main thread can be interrupted.
-         */
+        /* Allow sleep(0) to maintain win32 semantics. */
         ul_millis = (unsigned long)millisecs;
-        if (ul_millis == 0 || !_PyOS_IsMainThread()) {
+        if (ul_millis == 0) {
             Py_BEGIN_ALLOW_THREADS
             Sleep(ul_millis);
             Py_END_ALLOW_THREADS

Python/ceval.c

@@ -355,6 +355,8 @@ PyEval_RestoreThread(PyThreadState *tstate)
    callback.
  */
 
+/* FIXME: move this array into PyThreadState to avoid race conditions between
+   threads when Py_AddPendingCall() is called by signal handlers. */
 #define NPENDINGCALLS 32
 static struct {
     int (*func)(void *);