scratch_link.py: Avoid eternal loop by hostname resolve failure

When the address of device-manager.scratch.mit.edu can not be resolved,
scratch_link.py catches the exception for the resolve failure and
restarts itself. This results in eternal loop.

To avoid the eternal loop, catch the resolve failure, print error
message and break the loop. Also improve the error message for the other
exceptions caught in the loop.

Signed-off-by: Shin'ichiro Kawasaki <kawasaki@juno.dti.ne.jp>

pyscrlink/scratch_link.py

@@ -8,6 +8,7 @@ import asyncio
 import pathlib
 import ssl
 import websockets
+import socket
 import json
 import base64
 import logging
@@ -42,6 +43,8 @@ logger.setLevel(logLevel)
 logger.addHandler(handler)
 logger.propagate = False
 
+HOSTNAME="device-manager.scratch.mit.edu"
+
 class Session():
     """Base class for BTSession and BLESession"""
     def __init__(self, websocket, loop):
@@ -738,7 +741,7 @@ def main():
     ssl_context.load_cert_chain(localhost_cer, localhost_key)
 
     start_server = websockets.serve(
-         ws_handler, "device-manager.scratch.mit.edu", 20110, ssl=ssl_context
+         ws_handler, HOSTNAME, 20110, ssl=ssl_context
     )
 
     while True:
@@ -749,7 +752,13 @@ def main():
         except KeyboardInterrupt as e:
             stack_trace()
             break
+        except socket.gaierror as e:
+            logger.error(f"{type(e).__name__}: {e}")
+            logger.info(f"Check internet connection to {HOSTNAME}. If not "
+                        f"available, add '127.0.0.1 {HOSTNAME}' to /etc/hosts.")
+            break
         except Exception as e:
+            logger.error(f"{type(e).__name__}: {e}")
             logger.info("Restarting scratch-link...")
 
 if __name__ == "__main__":