bpo-1054041: Exit properly after an uncaught ^C. (#11862)

* bpo-1054041: Exit properly by a signal after a ^C.

An uncaught KeyboardInterrupt exception means the user pressed ^C and
our code did not handle it.  Programs that install SIGINT handlers are
supposed to reraise the SIGINT signal to the SIG_DFL handler in order
to exit in a manner that their calling process can detect that they
died due to a Ctrl-C.  https://www.cons.org/cracauer/sigint.html

After this change on POSIX systems

 while true; do python -c 'import time; time.sleep(23)'; done

can be stopped via a simple Ctrl-C instead of the shell infinitely
restarting a new python process.

What to do on Windows, or if anything needs to be done there has not
yet been determined.  That belongs in its own PR.

TODO(gpshead): A unittest for this behavior is still needed.

* Do the unhandled ^C check after pymain_free.

* Return STATUS_CONTROL_C_EXIT on Windows.

* Fix ifdef around unistd.h include.

* 📜🤖 Added by blurb_it.

* Add STATUS_CTRL_C_EXIT to the os module on Windows

* Add unittests.

* Don't send CTRL_C_EVENT in the Windows test.

It was causing CI systems to bail out of the entire test suite.

See https://dev.azure.com/Python/cpython/_build/results?buildId=37980
for example.

* Correct posix test (fail on macOS?) check.

* STATUS_CONTROL_C_EXIT must be unsigned.

* Improve the error message.

* test typo :)

* Skip if the bash version is too old.

...and rename the windows test to reflect what it does.

* min bash version is 4.4, detect no bash.

* restore a blank line i didn't mean to delete.

* PyErr_Occurred() before the Py_DECREF(co);

* Don't add os.STATUS_CONTROL_C_EXIT as a constant.

* Update the Windows test comment.

* Refactor common logic into a run_eval_code_obj fn.

Lib/test/test_signal.py

@@ -78,6 +78,48 @@ class PosixTests(unittest.TestCase):
         self.assertNotIn(signal.NSIG, s)
         self.assertLess(len(s), signal.NSIG)
 
+    @unittest.skipUnless(sys.executable, "sys.executable required.")
+    def test_keyboard_interrupt_exit_code(self):
+        """KeyboardInterrupt triggers exit via SIGINT."""
+        process = subprocess.run(
+                [sys.executable, "-c",
+                 "import os,signal; os.kill(os.getpid(), signal.SIGINT)"],
+                stderr=subprocess.PIPE)
+        self.assertIn(b"KeyboardInterrupt", process.stderr)
+        self.assertEqual(process.returncode, -signal.SIGINT)
+
+    @unittest.skipUnless(sys.executable, "sys.executable required.")
+    def test_keyboard_interrupt_communicated_to_shell(self):
+        """KeyboardInterrupt exits such that shells detect a ^C."""
+        try:
+            bash_proc = subprocess.run(
+                    ["bash", "-c", 'echo "${BASH_VERSION}"'],
+                    stdout=subprocess.PIPE, stderr=subprocess.DEVNULL)
+        except OSError:
+            raise unittest.SkipTest("bash required.")
+        if bash_proc.returncode:
+            raise unittest.SkipTest("could not determine bash version.")
+        bash_ver = bash_proc.stdout.decode("ascii").strip()
+        bash_major_minor = [int(n) for n in bash_ver.split(".", 2)[:2]]
+        if bash_major_minor < [4, 4]:
+            # In older versions of bash, -i does not work as needed
+            # _for this automated test_.  Older shells do behave as
+            # expected in manual interactive use.
+            raise unittest.SkipTest(f"bash version {bash_ver} is too old.")
+        # The motivation for https://bugs.python.org/issue1054041.
+        # An _interactive_ shell (bash -i simulates that here) detects
+        # when a command exits via ^C and stops executing further
+        # commands.
+        process = subprocess.run(
+            ["bash", "-ic",
+             f"{sys.executable} -c 'import os,signal; os.kill(os.getpid(), signal.SIGINT)'; "
+             "echo TESTFAIL using bash \"${BASH_VERSION}\""],
+            stderr=subprocess.PIPE, stdout=subprocess.PIPE)
+        self.assertIn(b"KeyboardInterrupt", process.stderr)
+        # An interactive shell will abort if python exits properly to
+        # indicate that a KeyboardInterrupt occurred.
+        self.assertNotIn(b"TESTFAIL", process.stdout)
+
 
 @unittest.skipUnless(sys.platform == "win32", "Windows specific")
 class WindowsSignalTests(unittest.TestCase):
@@ -112,6 +154,20 @@ class WindowsSignalTests(unittest.TestCase):
         with self.assertRaises(ValueError):
             signal.signal(7, handler)
 
+    @unittest.skipUnless(sys.executable, "sys.executable required.")
+    def test_keyboard_interrupt_exit_code(self):
+        """KeyboardInterrupt triggers an exit using STATUS_CONTROL_C_EXIT."""
+        # We don't test via os.kill(os.getpid(), signal.CTRL_C_EVENT) here
+        # as that requires setting up a console control handler in a child
+        # in its own process group.  Doable, but quite complicated.  (see
+        # @eryksun on https://github.com/python/cpython/pull/11862)
+        process = subprocess.run(
+                [sys.executable, "-c", "raise KeyboardInterrupt"],
+                stderr=subprocess.PIPE)
+        self.assertIn(b"KeyboardInterrupt", process.stderr)
+        STATUS_CONTROL_C_EXIT = 0xC000013A
+        self.assertEqual(process.returncode, STATUS_CONTROL_C_EXIT)
+
 
 class WakeupFDTests(unittest.TestCase):
 
@@ -1217,11 +1273,8 @@ class StressTest(unittest.TestCase):
 class RaiseSignalTest(unittest.TestCase):
 
     def test_sigint(self):
-        try:
+        with self.assertRaises(KeyboardInterrupt):
             signal.raise_signal(signal.SIGINT)
-            self.fail("Expected KeyInterrupt")
-        except KeyboardInterrupt:
-            pass
 
     @unittest.skipIf(sys.platform != "win32", "Windows specific test")
     def test_invalid_argument(self):