bglib: convert uncaught exceptions to events

This does not solve the underlying problem of an unhandled exception in
the connection handler thread, but means that the outer app layer
actually receives an event, giving them a chance of taking action.

We only have a single string field, but the traceback is still
important, so include it anyway, as a string, it's better than nothing.

Signed-off-by: Karl Palsson <karlp@etactica.com>

bgapi/bglib.py

@@ -22,6 +22,7 @@
 
 from array import array
 import threading
+import traceback
 try:
     import queue
 except ImportError:
@@ -76,6 +77,8 @@ def __init__(self, connection, event_handler, *apis):
         threading.Thread.__init__(self)
         self.daemon = True # Daemon thread
         self.stop_flag = threading.Event()
+        threading.excepthook = self._handle_thread_exceptions
+        self._apis, = apis
 
         self.conn = connection
         self.event_handler = event_handler
@@ -86,6 +89,14 @@ def __init__(self, connection, event_handler, *apis):
         self.response_queue = queue.Queue(maxsize=1)
         self.waiting_response = threading.Event()
 
+    def _handle_thread_exceptions(self, ee):
+        # This is called on the thread that excepted
+        # It's just nicer than wrapping everything in a try: block
+        # Create a synthetic event for the outer world to discover that the thread has gone.
+        # We can't keep the traceback, as system.error only has definitions for code+string
+        val = f"{ee.exc_type}:{ee.exc_value} {traceback.format_tb(ee.exc_traceback)}"
+        self.event_handler(BGEvent(self._apis["system"].events["error"], (99, val)))
+
     def send_command(self, bgcmd, response_timeout):
         try:
             # Send command