Merge pull request #587 from python-cmd2/signal_threads

Ruggedized registration of SIGINT signal handler

Author: tleonhardt

CHANGELOG.md

@@ -1,4 +1,6 @@
 ## 0.9.7 (TBD, 2018)
+* Enhancements
+    * **cmdloop** now only attempts to register a custom signal handler for SIGINT if running in the main thread
 * Deletions (potentially breaking changes)
     * Deleted ``Cmd.colorize()`` and ``Cmd._colorcodes`` which were deprecated in 0.9.5
 

cmd2/cmd2.py

@@ -3588,10 +3588,12 @@ def cmdloop(self, intro: Optional[str]=None) -> None:
             if callargs:
                 self.cmdqueue.extend(callargs)
 
-        # Register a SIGINT signal handler for Ctrl+C
-        import signal
-        original_sigint_handler = signal.getsignal(signal.SIGINT)
-        signal.signal(signal.SIGINT, self.sigint_handler)
+        # Only the main thread is allowed to set a new signal handler in Python, so only attempt if that is the case
+        if threading.current_thread() is threading.main_thread():
+            # Register a SIGINT signal handler for Ctrl+C
+            import signal
+            original_sigint_handler = signal.getsignal(signal.SIGINT)
+            signal.signal(signal.SIGINT, self.sigint_handler)
 
         # Grab terminal lock before the prompt has been drawn by readline
         self.terminal_lock.acquire()
@@ -3625,8 +3627,9 @@ def cmdloop(self, intro: Optional[str]=None) -> None:
         # This will also zero the lock count in case cmdloop() is called again
         self.terminal_lock.release()
 
-        # Restore the original signal handler
-        signal.signal(signal.SIGINT, original_sigint_handler)
+        if threading.current_thread() is threading.main_thread():
+            # Restore the original signal handler
+            signal.signal(signal.SIGINT, original_sigint_handler)
 
         if self.exit_code is not None:
             sys.exit(self.exit_code)

docs/settingchanges.rst

@@ -140,6 +140,10 @@ set to ``False``, then the current line will simply be cancelled.
   (Cmd) typing a comma^C
   (Cmd)
 
+.. warning::
+    The default SIGINT behavior will only function properly if **cmdloop** is running
+    in the main thread.
+
 
 Timing
 ======