server: stop the server via a threading.Event

It seems that Mac OS does not support calling socket.shutdown(socket.SHUT_RD)
on a listening socket (see https://github.com/romanz/trezor-agent/issues/6).
The following implementation will set the accept() timeout to 0.1s and stop
the server if a threading.Event (named "quit_event") is set by the main thread.

trezor_agent/server.py

@@ -12,6 +12,8 @@ from . import util
 
 log = logging.getLogger(__name__)
 
+UNIX_SOCKET_TIMEOUT = 0.1
+
 
 def remove_file(path, remove=os.remove, exists=os.path.exists):
     try:
@@ -44,19 +46,31 @@ def handle_connection(conn, handler):
             util.send(conn, reply)
     except EOFError:
         log.debug('goodbye agent')
-    except:
-        log.exception('error')
-        raise
 
 
-def server_thread(server, handler):
+def retry(func, exception_type, quit_event):
+    while True:
+        if quit_event.is_set():
+            raise StopIteration
+        try:
+            return func()
+        except exception_type:
+            pass
+
+
+def server_thread(server, handler, quit_event):
     log.debug('server thread started')
+
+    def accept_connection():
+        conn, _ = server.accept()
+        return conn
+
     while True:
         log.debug('waiting for connection on %s', server.getsockname())
         try:
-            conn, _ = server.accept()
-        except socket.error as e:
-            log.debug('server stopped: %s', e)
+            conn = retry(accept_connection, socket.timeout, quit_event)
+        except StopIteration:
+            log.debug('server stopped')
             break
         with contextlib.closing(conn):
             handle_connection(conn, handler)
@@ -64,7 +78,7 @@ def server_thread(server, handler):
 
 
 @contextlib.contextmanager
-def spawn(func, **kwargs):
+def spawn(func, kwargs):
     t = threading.Thread(target=func, kwargs=kwargs)
     t.start()
     yield
@@ -72,20 +86,23 @@ def spawn(func, **kwargs):
 
 
 @contextlib.contextmanager
-def serve(public_keys, signer, sock_path=None):
+def serve(public_keys, signer, sock_path=None, timeout=UNIX_SOCKET_TIMEOUT):
     if sock_path is None:
         sock_path = tempfile.mktemp(prefix='ssh-agent-')
 
     keys = [formats.import_public_key(k) for k in public_keys]
     environ = {'SSH_AUTH_SOCK': sock_path, 'SSH_AGENT_PID': str(os.getpid())}
     with unix_domain_socket_server(sock_path) as server:
+        server.settimeout(timeout)
         handler = protocol.Handler(keys=keys, signer=signer)
-        with spawn(server_thread, server=server, handler=handler):
+        quit_event = threading.Event()
+        kwargs = dict(server=server, handler=handler, quit_event=quit_event)
+        with spawn(server_thread, kwargs):
             try:
                 yield environ
             finally:
                 log.debug('closing server')
-                server.shutdown(socket.SHUT_RD)
+                quit_event.set()
 
 
 def run_process(command, environ, use_shell=False):