[test] Ensure restoring of signal handler when tests terminate

Otherwise, the last tests that ran in a worker keep sitting on their
sigterm handlers without any running processes. This creates
exceptions when workers terminate.

Bug: v8:8292
Change-Id: Iefb9a4a353399c1e3168eae2916e3cedca4e09b3
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2011831Reviewed-by: Tamer Tas <tmrts@chromium.org>
Commit-Queue: Michael Achenbach <machenbach@chromium.org>
Cr-Commit-Position: refs/heads/master@{#65907}

tools/testrunner/local/command.py

@@ -5,6 +5,7 @@
 # for py2/py3 compatibility
 from __future__ import print_function
 
+from contextlib import contextmanager
 import os
 import re
 import signal
@@ -39,6 +40,28 @@ class AbortException(Exception):
   pass
 
 
+@contextmanager
+def handle_sigterm(process, handle_sigterm):
+  """Handle sigterm for `process` and restore previous handler to prevent
+  erroneous termination of an already terminated process.
+  """
+  # Variable to communicate with the signal handler.
+  abort_occured = [False]
+  def handler(signum, frame):
+    self._abort(process, abort_occured)
+
+  if handle_sigterm:
+    previous = signal.signal(signal.SIGTERM, handler)
+  try:
+    yield
+  finally:
+    if handle_sigterm:
+      signal.signal(signal.SIGTERM, previous)
+
+  if abort_occured[0]:
+    raise AbortException()
+
+
 class BaseCommand(object):
   def __init__(self, shell, args=None, cmd_prefix=None, timeout=60, env=None,
                verbose=False, resources_func=None, handle_sigterm=False):
@@ -72,28 +95,18 @@ class BaseCommand(object):
 
     process = self._start_process()
 
-    # Variable to communicate with the signal handler.
-    abort_occured = [False]
-    def handler(signum, frame):
-      self._abort(process, abort_occured)
-
-    if self.handle_sigterm:
-      signal.signal(signal.SIGTERM, handler)
-
-    # Variable to communicate with the timer.
-    timeout_occured = [False]
-    timer = threading.Timer(
-        self.timeout, self._abort, [process, timeout_occured])
-    timer.start()
-
-    start_time = time.time()
-    stdout, stderr = process.communicate()
-    duration = time.time() - start_time
+    with handle_sigterm(process, self.handle_sigterm):
+      # Variable to communicate with the timer.
+      timeout_occured = [False]
+      timer = threading.Timer(
+          self.timeout, self._abort, [process, timeout_occured])
+      timer.start()
 
-    timer.cancel()
+      start_time = time.time()
+      stdout, stderr = process.communicate()
+      duration = time.time() - start_time
 
-    if abort_occured[0]:
-      raise AbortException()
+      timer.cancel()
 
     return output.Output(
       process.returncode,