Add support for making Tornado dump its stack via SIGUSR1/2.

With this change,

pkill -SIGUSR1 -f runtornado

will dump the stack and SIGUSR2 will enable an interactive debugging
session.

This fixes #613 for Tornado which was the original motive for that
ticket; I'm not sure whether we want to do this for our Django
processes as well, but it would be easy to do so if we did.

(imported from commit a7de7c6070f4bf0404bed6f434e6a6b291d66a26)

zephyr/lib/debug.py

@@ -0,0 +1,26 @@
+import code
+import traceback
+import signal
+
+# Interactive debugging code from
+# http://stackoverflow.com/questions/132058/showing-the-stack-trace-from-a-running-python-application
+# (that link also points to code for an interactive remote debugger
+# setup, which we might want if we move Tornado to run in a daemon
+# rather than via screen).
+def interactive_debug(sig, frame):
+    """Interrupt running process, and provide a python prompt for
+    interactive debugging."""
+    d={'_frame':frame}         # Allow access to frame object.
+    d.update(frame.f_globals)  # Unless shadowed by global
+    d.update(frame.f_locals)
+
+    message  = "Signal recieved : entering python shell.\nTraceback:\n"
+    message += ''.join(traceback.format_stack(frame))
+    i = code.InteractiveConsole(d)
+    i.interact(message)
+
+# SIGUSR1 => Just print the stack
+# SIGUSR2 => Print stack + open interactive debugging shell
+def interactive_debug_listen():
+    signal.signal(signal.SIGUSR1, lambda sig, stack: traceback.print_stack(stack))
+    signal.signal(signal.SIGUSR2, interactive_debug)

zephyr/management/commands/runtornado.py

@@ -8,6 +8,7 @@ import tornado.web
 import logging
 import time
 from tornado import ioloop
+from zephyr.lib.debug import interactive_debug_listen
 
 # A hack to keep track of how much time we spend working, versus sleeping in
 # the event loop.
@@ -78,6 +79,7 @@ class Command(BaseCommand):
         # setup unbuffered I/O
         sys.stdout = os.fdopen(sys.stdout.fileno(), 'w', 0)
         sys.stderr = os.fdopen(sys.stderr.fileno(), 'w', 0)
+        interactive_debug_listen()
 
         import django
         from django.core.handlers.wsgi import WSGIHandler