terminate the server tailing process on exit

Summary:
We were running into an issue where the pyre incremental process would terminate,
but leave behind a process tailing server.stdout - meant to give progress messages. Register a function
to terminate said tailer at the exit of the program.

Reviewed By: dark

Differential Revision: D7979191

fbshipit-source-id: 8ab0167cb9e7b07c547fcb56c1544dfe17e04c6b

scripts/commands/incremental.py

@@ -3,6 +3,7 @@
 # This source code is licensed under the MIT license found in the
 # LICENSE file in the root directory of this source tree.
 
+import atexit
 import logging
 import subprocess
 import os
@@ -36,6 +37,7 @@ def _read_stderr(self, _stream, source_directory):
                 ['tail', '-f', stderr_file],
                 stdout=subprocess.PIPE,
                 stderr=subprocess.DEVNULL) as stderr_tail:
+            atexit.register(stderr_tail.terminate)
             super(Incremental, self)._read_stderr(
                 stderr_tail.stdout,
                 source_directory)