Bring back the signal handlers (and the "killed" event).

Now done more responsibly, saving any existing signal handlers
and calling them when an interrupt occurs in addition to the
one Sleek installs.

NOTE: You may need to explicitly use "kill <process id>" in
order to trigger the proper signal handler execution, and
to raise the "killed" event.

sleekxmpp/xmlstream/xmlstream.py

@@ -10,6 +10,7 @@ from __future__ import with_statement, unicode_literals
 
 import copy
 import logging
+import signal
 import socket as Socket
 import ssl
 import sys
@@ -195,6 +196,53 @@ class XMLStream(object):
         self.auto_reconnect = True
         self.is_client = False
 
+    def use_signals(self, signals=None):
+        """
+        Register signal handlers for SIGHUP and SIGTERM, if possible,
+        which will raise a "killed" event when the application is
+        terminated.
+
+        If a signal handler already existed, it will be executed first,
+        before the "killed" event is raised.
+
+        Arguments:
+            signals -- A list of signal names to be monitored.
+                       Defaults to ['SIGHUP', 'SIGTERM'].
+        """
+        if signals is None:
+            signals = ['SIGHUP', 'SIGTERM']
+
+        existing_handlers = {}
+        for sig_name in signals:
+            if hasattr(signal, sig_name):
+                sig = getattr(signal, sig_name)
+                handler = signal.getsignal(sig)
+                if handler:
+                    existing_handlers[sig] = handler
+
+        def handle_kill(signum, frame):
+            """
+            Capture kill event and disconnect cleanly after first
+            spawning the "killed" event.
+            """
+
+            if signum in existing_handlers and \
+                   existing_handlers[signum] != handle_kill:
+                existing_handlers[signum](signum, frame)
+
+            self.event("killed", direct=True)
+            self.disconnect()
+
+        try:
+            for sig_name in signals:
+                if hasattr(signal, sig_name):
+                    sig = getattr(signal, sig_name)
+                    signal.signal(sig, handle_kill)
+            self.__signals_installed = True
+        except:
+            log.debug("Can not set interrupt signal handlers. " + \
+                      "SleekXMPP is not running from a main thread.")
+
     def new_id(self):
         """
         Generate and return a new stream ID in hexadecimal form.
@@ -731,6 +779,7 @@ class XMLStream(object):
             if not self.stop.isSet() and self.auto_reconnect:
                 self.reconnect()
             else:
+                self.event('killed', direct=True)
                 self.disconnect()
                 self.event_queue.put(('quit', None, None))
         self.scheduler.run = False
@@ -909,6 +958,7 @@ class XMLStream(object):
                     return False
         except KeyboardInterrupt:
             log.debug("Keyboard Escape Detected in _event_runner")
+            self.event('killed', direct=True)
             self.disconnect()
             return
         except SystemExit:
@@ -934,6 +984,7 @@ class XMLStream(object):
                     self.disconnect(self.auto_reconnect)
         except KeyboardInterrupt:
             log.debug("Keyboard Escape Detected in _send_thread")
+            self.event('killed', direct=True)
             self.disconnect()
             return
         except SystemExit: