[devil] Add signal handler context manager.

devil/devil/utils/signal_handler.py

Index: devil/devil/utils/signal_handler.py
diff --git a/devil/devil/utils/signal_handler.py b/devil/devil/utils/signal_handler.py
new file mode 100644
index 0000000000000000000000000000000000000000..566bef94e93cea7db9f0b1ad2b5360e9aa35659d
--- /dev/null
+++ b/devil/devil/utils/signal_handler.py
@@ -0,0 +1,30 @@
+# Copyright 2016 The Chromium Authors. All rights reserved.
+# Use of this source code is governed by a BSD-style license that can be
+# found in the LICENSE file.
+
+import contextlib
+import signal
+
+
+@contextlib.contextmanager
+def AddSignalHandler(signalnum, additional_handler):
+  """Adds a signal handler for the given signal in the wrapped context.
+
+  This runs the new handler after any existing handler rather than
+  replacing the existing handler.
+
+  Args:
+    signum: The signal for which a handler should be added.
+    additional_handler: The handler to add.
+  """
+  existing_handler = signal.getsignal(signalnum)
+
+  def handler(signum, frame):
+    if callable(existing_handler):
+      existing_handler(signum, frame)
+    additional_handler(signum, frame)
+
+  signal.signal(signalnum, handler)
+  yield
+  signal.signal(signalnum, existing_handler)
+