Fix SFTP code to properly handle SFTPServer's exit() being async

This commit adds code to properly call into the exit() method of the
SFTPServer class when it is declared async. Thanks go to C. R.  Oldham
for reporting this issue!

Author: ronf

asyncssh/sftp.py

@@ -5931,7 +5931,10 @@ async def _cleanup(self, exc: Optional[Exception]) -> None:
                 if inspect.isawaitable(result):
                     await result
 
-            self._server.exit()
+            result = self._server.exit()
+
+            if inspect.isawaitable(result):
+                await result
 
             self._file_handles = {}
             self._dir_handles = {}

tests/test_sftp.py

@@ -617,6 +617,11 @@ async def fsync(self, file_obj):
 
         super().fsync(file_obj)
 
+    async def exit(self):
+        """Shut down this SFTP server"""
+
+        super().exit()
+
 
 class _CheckSFTP(ServerTestCase):
     """Utility functions for AsyncSSH SFTP unit tests"""