[Telemetry] Terminate any dangling subprocesses

This is designed to prevent deadlocks on the buildbots.

I couldn't figure out a way to kill them without killing the parent as well. I
also haven't looked into a Windows equivalent yet.

BUG=357059

Review URL: https://codereview.chromium.org/301313007

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@274036 0039d316-1c4b-4281-b951-d872f2087c98

tools/telemetry/telemetry/util/global_hooks.py

@@ -4,10 +4,12 @@
 
 """Hooks that apply globally to all scripts that import or use Telemetry."""
 
+import atexit
 import os
 import signal
 import sys
 
+from telemetry.core import platform
 from telemetry.core import util
 from telemetry.util import exception_formatter
 
@@ -18,6 +20,7 @@ def InstallHooks():
   InstallUnhandledExceptionFormatter()
   InstallStackDumpOnSigusr1()
   InstallTerminationHook()
+  InstallAtExitHook()
 
 
 def RemoveAllStalePycFiles(base_dir):
@@ -71,3 +74,24 @@ def InstallTerminationHook():
     exception_formatter.PrintFormattedFrame(stack_frame, exception_string)
     sys.exit(-1)
   signal.signal(signal.SIGTERM, PrintStackAndExit)
+
+
+def InstallAtExitHook():
+  """Ensure all subprocesses are killed.
+
+  Subprocesses should never outlive Telemetry. If they do, they can cause
+  hangs on the builbots.
+  """
+  # TODO(tonyg): Find a way to do something similar on Windows.
+  if platform.GetHostPlatform().GetOSName() == 'win':
+    return
+
+  # Create new process group and become its leader.
+  os.setpgrp()
+
+  # At exit, terminate everything in the group.
+  def KillGroup():
+    # Ignore the TERM we're about to get.
+    signal.signal(signal.SIGTERM, signal.SIG_IGN)
+    os.killpg(0, signal.SIGTERM)
+  atexit.register(KillGroup)