bpo-23395: Fix PyErr_SetInterrupt if the SIGINT signal is ignored or not handled (GH-7778)

``_thread.interrupt_main()`` now avoids setting the Python error status if the ``SIGINT`` signal is ignored or not handled by Python.

Doc/c-api/exceptions.rst

@@ -519,13 +519,13 @@ Signal Handling
       single: SIGINT
       single: KeyboardInterrupt (built-in exception)
 
-   This function simulates the effect of a :const:`SIGINT` signal arriving --- the
-   next time :c:func:`PyErr_CheckSignals` is called,  :exc:`KeyboardInterrupt` will
-   be raised.  It may be called without holding the interpreter lock.
-
-   .. % XXX This was described as obsolete, but is used in
-   .. % _thread.interrupt_main() (used from IDLE), so it's still needed.
+   Simulate the effect of a :const:`SIGINT` signal arriving. The next time
+   :c:func:`PyErr_CheckSignals` is called,  the Python signal handler for
+   :const:`SIGINT` will be called.
 
+   If :const:`SIGINT` isn't handled by Python (it was set to
+   :data:`signal.SIG_DFL` or :data:`signal.SIG_IGN`), this function does
+   nothing.
 
 .. c:function:: int PySignal_SetWakeupFd(int fd)
 

Doc/library/_thread.rst

@@ -53,8 +53,12 @@ This module defines the following constants and functions:
 
 .. function:: interrupt_main()
 
-   Raise a :exc:`KeyboardInterrupt` exception in the main thread.  A subthread can
-   use this function to interrupt the main thread.
+   Simulate the effect of a :data:`signal.SIGINT` signal arriving in the main
+   thread. A thread can use this function to interrupt the main thread.
+
+   If :data:`signal.SIGINT` isn't handled by Python (it was set to
+   :data:`signal.SIG_DFL` or :data:`signal.SIG_IGN`), this function does
+   nothing.
 
 
 .. function:: exit()

Lib/test/test_threading.py

@@ -16,6 +16,7 @@ import unittest
 import weakref
 import os
 import subprocess
+import signal
 
 from test import lock_tests
 from test import support
@@ -1168,6 +1169,7 @@ class BoundedSemaphoreTests(lock_tests.BoundedSemaphoreTests):
 class BarrierTests(lock_tests.BarrierTests):
     barriertype = staticmethod(threading.Barrier)
 
+
 class MiscTestCase(unittest.TestCase):
     def test__all__(self):
         extra = {"ThreadError"}
@@ -1175,5 +1177,38 @@ class MiscTestCase(unittest.TestCase):
         support.check__all__(self, threading, ('threading', '_thread'),
                              extra=extra, blacklist=blacklist)
 
+
+class InterruptMainTests(unittest.TestCase):
+    def test_interrupt_main_subthread(self):
+        # Calling start_new_thread with a function that executes interrupt_main
+        # should raise KeyboardInterrupt upon completion.
+        def call_interrupt():
+            _thread.interrupt_main()
+        t = threading.Thread(target=call_interrupt)
+        with self.assertRaises(KeyboardInterrupt):
+            t.start()
+            t.join()
+        t.join()
+
+    def test_interrupt_main_mainthread(self):
+        # Make sure that if interrupt_main is called in main thread that
+        # KeyboardInterrupt is raised instantly.
+        with self.assertRaises(KeyboardInterrupt):
+            _thread.interrupt_main()
+
+    def test_interrupt_main_noerror(self):
+        handler = signal.getsignal(signal.SIGINT)
+        try:
+            # No exception should arise.
+            signal.signal(signal.SIGINT, signal.SIG_IGN)
+            _thread.interrupt_main()
+
+            signal.signal(signal.SIGINT, signal.SIG_DFL)
+            _thread.interrupt_main()
+        finally:
+            # Restore original handler
+            signal.signal(signal.SIGINT, handler)
+
+
 if __name__ == "__main__":
     unittest.main()

Misc/ACKS

@@ -261,7 +261,7 @@ Donn Cave
 Charles Cazabon
 Jesús Cea Avión
 Per Cederqvist
-Matej Cepl
+Matěj Cepl
 Carl Cerecke
 Octavian Cerna
 Michael Cetrulo

Misc/NEWS.d/next/Library/2016-07-27-11-06-43.bpo-23395.MuCEX9.rst

+``_thread.interrupt_main()`` now avoids setting the Python error status
+if the ``SIGINT`` signal is ignored or not handled by Python.

Modules/signalmodule.c

@@ -1683,13 +1683,18 @@ _PyErr_CheckSignals(void)
 }
 
 
-/* Replacements for intrcheck.c functionality
- * Declared in pyerrors.h
- */
+/* Simulate the effect of a signal.SIGINT signal arriving. The next time
+   PyErr_CheckSignals is called,  the Python SIGINT signal handler will be
+   raised.
+
+   Missing signal handler for the SIGINT signal is silently ignored. */
 void
 PyErr_SetInterrupt(void)
 {
-    trip_signal(SIGINT);
+    if ((Handlers[SIGINT].func != IgnoreHandler) &&
+        (Handlers[SIGINT].func != DefaultHandler)) {
+        trip_signal(SIGINT);
+    }
 }
 
 void