Bug 1636859 - Shut down the `fix-stacks` child process when `fix_stacks.py` exits. r=glandium

Hopefully this will avoid some intermittent failures that occur when
`fix-stacks` is deleted at the end of a test run.

Differential Revision: https://phabricator.services.mozilla.com/D76980

tools/rb/fix_stacks.py

@@ -9,6 +9,7 @@
 
 from __future__ import absolute_import, print_function
 from subprocess import Popen, PIPE
+import atexit
 import os
 import platform
 import re
@@ -64,6 +65,14 @@ def fixSymbols(line, jsonMode=False, slowWarning=False, breakpadSymsDir=None, hi
 
         fix_stacks = Popen(args, stdin=PIPE, stdout=PIPE, stderr=stderr)
 
+        # Shut down the fix_stacks process on exit. We use `terminate()`
+        # because it is more forceful than `wait()`, and the Python docs warn
+        # about possible deadlocks with `wait()`.
+        def cleanup(fix_stacks):
+            fix_stacks.stdin.close()
+            fix_stacks.terminate()
+        atexit.register(cleanup, fix_stacks)
+
         if slowWarning:
             print("Initializing stack-fixing for the first stack frame, this may take a while...")