Added ability to abort the command chain by returning a true value from cmd.at_pre_cmd().

src/commands/cmdhandler.py

@@ -401,7 +401,10 @@ def cmdhandler(called_by, raw_string, _testing=False, callertype="session", sess
                 setattr(cmd, key, val)
 
             # pre-command hook
-            yield cmd.at_pre_cmd()
+            abort = yield cmd.at_pre_cmd()
+            if abort:
+                # abort sequence
+                returnValue(abort)
 
             # Parse and execute
             yield cmd.parse()

src/commands/command.py

@@ -255,7 +255,9 @@ class Command(object):
 
     def at_pre_cmd(self):
         """
-        This hook is called before self.parse() on all commands
+        This hook is called before self.parse() on all commands.
+        If this hook returns anything but False/None, the command
+        sequence is aborted.
         """
         pass