Patch #1583 by Adam Olsen.

This adds signal.set_wakeup_fd(fd) which sets a file descriptor to
which a zero byte will be written whenever a C exception handler runs.

I added a simple C API as well, PySignal_SetWakeupFd(fd).

Doc/c-api/exceptions.rst

@@ -375,6 +375,16 @@ is a separate error indicator for each thread.
    .. % thread.interrupt_main() (used from IDLE), so it's still needed.
 
 
+.. cfunction:: int PySignal_SetWakeupFd(int fd)
+
+   This utility function specifies a file descriptor to which a ``'\0'`` byte will
+   be written whenever a signal is received.  It returns the previous such file
+   descriptor.  The value ``-1`` disables the feature; this is the initial state.
+   This is equivalent to :func:`signal.set_wakeup_fd` in Python, but without any
+   error checking.  *fd* should be a valid file descriptor.  The function should
+   only be called from the main thread.
+
+
 .. cfunction:: PyObject* PyErr_NewException(char *name, PyObject *base, PyObject *dict)
 
    This utility function creates and returns a new exception object. The *name*

Doc/library/signal.rst

@@ -110,6 +110,20 @@ The :mod:`signal` module defines the following functions:
    :manpage:`signal(2)`.)
 
 
+.. function:: set_wakeup_fd(fd)
+
+   Set the wakeup fd to *fd*.  When a signal is received, a ``'\0'`` byte is
+   written to the fd.  This can be used by a library to wakeup a poll or select
+   call, allowing the signal to be fully processed.
+
+   The old wakeup fd is returned.  *fd* must be non-blocking.  It is up to the
+   library to remove any bytes before calling poll or select again.
+
+   When threads are enabled, this function can only be called from the main thread;
+   attempting to call it from other threads will cause a :exc:`ValueError`
+   exception to be raised.
+
+
 .. function:: signal(signalnum, handler)
 
    Set the handler for signal *signalnum* to the function *handler*.  *handler* can

Include/pyerrors.h

@@ -238,6 +238,9 @@ PyAPI_FUNC(int) PyErr_WarnExplicit(PyObject *, const char *,
 PyAPI_FUNC(int) PyErr_CheckSignals(void);
 PyAPI_FUNC(void) PyErr_SetInterrupt(void);
 
+/* In signalmodule.c */
+int PySignal_SetWakeupFd(int fd);
+
 /* Support for adding program text to SyntaxErrors */
 PyAPI_FUNC(void) PyErr_SyntaxLocation(const char *, int);
 PyAPI_FUNC(PyObject *) PyErr_ProgramText(const char *, int);

Lib/test/test_signal.py

@@ -165,12 +165,59 @@ class BasicSignalTests(unittest.TestCase):
         self.assertRaises(TypeError, signal.signal,
                           signal.SIGUSR1, None)
 
+class WakeupSignalTests(unittest.TestCase):
+    TIMEOUT_FULL = 10
+    TIMEOUT_HALF = 5
+
+    def test_wakeup_fd_early(self):
+        import select
+
+        signal.alarm(1)
+        before_time = time.time()
+        # We attempt to get a signal during the sleep,
+        # before select is called
+        time.sleep(self.TIMEOUT_FULL)
+        mid_time = time.time()
+        self.assert_(mid_time - before_time < self.TIMEOUT_HALF)
+        select.select([self.read], [], [], self.TIMEOUT_FULL)
+        after_time = time.time()
+        self.assert_(after_time - mid_time < self.TIMEOUT_HALF)
+
+    def test_wakeup_fd_during(self):
+        import select
+
+        signal.alarm(1)
+        before_time = time.time()
+        # We attempt to get a signal during the select call
+        self.assertRaises(select.error, select.select,
+            [self.read], [], [], self.TIMEOUT_FULL)
+        after_time = time.time()
+        self.assert_(after_time - before_time < self.TIMEOUT_HALF)
+
+    def setUp(self):
+        import fcntl
+
+        self.alrm = signal.signal(signal.SIGALRM, lambda x,y:None)
+        self.read, self.write = os.pipe()
+        flags = fcntl.fcntl(self.write, fcntl.F_GETFL, 0)
+        flags = flags | os.O_NONBLOCK
+        fcntl.fcntl(self.write, fcntl.F_SETFL, flags)
+        self.old_wakeup = signal.set_wakeup_fd(self.write)
+
+    def tearDown(self):
+        signal.set_wakeup_fd(self.old_wakeup)
+        os.close(self.read)
+        os.close(self.write)
+        signal.signal(signal.SIGALRM, self.alrm)
+
+
 def test_main():
     if sys.platform[:3] in ('win', 'os2') or sys.platform == 'riscos':
         raise test_support.TestSkipped("Can't test signal on %s" % \
                                        sys.platform)
 
-    test_support.run_unittest(BasicSignalTests, InterProcessSignalTests)
+    test_support.run_unittest(BasicSignalTests, InterProcessSignalTests,
+        WakeupSignalTests)
 
 
 if __name__ == "__main__":

Modules/signalmodule.c

@@ -12,6 +12,8 @@
 
 #include <signal.h>
 
+#include <sys/stat.h>
+
 #ifndef SIG_ERR
 #define SIG_ERR ((PyOS_sighandler_t)(-1))
 #endif
@@ -75,6 +77,8 @@ static struct {
         PyObject *func;
 } Handlers[NSIG];
 
+static sig_atomic_t wakeup_fd = -1;
+
 /* Speed up sigcheck() when none tripped */
 static volatile sig_atomic_t is_tripped = 0;
 
@@ -128,6 +132,8 @@ signal_handler(int sig_num)
                    cleared in PyErr_CheckSignals() before .tripped. */
 		is_tripped = 1;
 		Py_AddPendingCall(checksignals_witharg, NULL);
+		if (wakeup_fd != -1)
+			write(wakeup_fd, "\0", 1);
 #ifdef WITH_THREAD
 	}
 #endif
@@ -267,6 +273,50 @@ None -- if an unknown handler is in effect\n\
 anything else -- the callable Python object used as a handler");
 
 
+static PyObject *
+signal_set_wakeup_fd(PyObject *self, PyObject *args)
+{
+	struct stat buf;
+	int fd, old_fd;
+	if (!PyArg_ParseTuple(args, "i:set_wakeup_fd", &fd))
+		return NULL;
+#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
+	if (fd != -1 && fstat(fd, &buf) != 0) {
+		PyErr_SetString(PyExc_ValueError, "invalid fd");
+		return NULL;
+	}
+	old_fd = wakeup_fd;
+	wakeup_fd = fd;
+	return PyLong_FromLong(old_fd);
+}
+
+PyDoc_STRVAR(set_wakeup_fd_doc,
+"set_wakeup_fd(fd) -> fd\n\
+\n\
+Sets the fd to be written to (with '\\0') when a signal\n\
+comes in.  A library can use this to wakeup select or poll.\n\
+The previous fd is returned.\n\
+\n\
+The fd must be non-blocking.");
+
+/* C API for the same, without all the error checking */
+int
+PySignal_SetWakeupFd(int fd)
+{
+	int old_fd = wakeup_fd;
+	if (fd < 0)
+		fd = -1;
+	wakeup_fd = fd;
+	return old_fd;
+}
+
+
 /* List of functions defined in the module */
 static PyMethodDef signal_methods[] = {
 #ifdef HAVE_ALARM
@@ -274,11 +324,12 @@ static PyMethodDef signal_methods[] = {
 #endif
 	{"signal",	        signal_signal, METH_VARARGS, signal_doc},
 	{"getsignal",	        signal_getsignal, METH_VARARGS, getsignal_doc},
+	{"set_wakeup_fd",	signal_set_wakeup_fd, METH_VARARGS, set_wakeup_fd_doc},
 #ifdef HAVE_PAUSE
 	{"pause",	        (PyCFunction)signal_pause,
 	 METH_NOARGS,pause_doc},
 #endif
-	{"default_int_handler", signal_default_int_handler, 
+	{"default_int_handler", signal_default_int_handler,
 	 METH_VARARGS, default_int_handler_doc},
 	{NULL,			NULL}		/* sentinel */
 };