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.

Modules/main.c

@@ -9,6 +9,13 @@
 #include "pycore_pystate.h"
 
 #include <locale.h>
+#ifdef HAVE_SIGNAL_H
+#include <signal.h>
+#endif
+#include <stdio.h>
+#if defined(HAVE_GETPID) && defined(HAVE_UNISTD_H)
+#include <unistd.h>
+#endif
 
 #if defined(MS_WINDOWS) || defined(__CYGWIN__)
 #  include <windows.h>
@@ -1830,6 +1837,29 @@ pymain_main(_PyMain *pymain)
 
     pymain_free(pymain);
 
+    if (_Py_UnhandledKeyboardInterrupt) {
+        /* https://bugs.python.org/issue1054041 - We need to exit via the
+         * SIG_DFL handler for SIGINT if KeyboardInterrupt went unhandled.
+         * If we don't, a calling process such as a shell may not know
+         * about the user's ^C.  https://www.cons.org/cracauer/sigint.html */
+#if defined(HAVE_GETPID) && !defined(MS_WINDOWS)
+        if (PyOS_setsig(SIGINT, SIG_DFL) == SIG_ERR) {
+            perror("signal");  /* Impossible in normal environments. */
+        } else {
+            kill(getpid(), SIGINT);
+        }
+        /* If setting SIG_DFL failed, or kill failed to terminate us,
+         * there isn't much else we can do aside from an error code. */
+#endif  /* HAVE_GETPID && !MS_WINDOWS */
+#ifdef MS_WINDOWS
+        /* cmd.exe detects this, prints ^C, and offers to terminate. */
+        /* https://msdn.microsoft.com/en-us/library/cc704588.aspx */
+        pymain->status = STATUS_CONTROL_C_EXIT;
+#else
+        pymain->status = SIGINT + 128;
+#endif  /* !MS_WINDOWS */
+    }
+
     return pymain->status;
 }