Add a debug thread that dumps tracebacks.

Start and extra thread that blocks on a threading event until SIGUSR1 is
received. Then dump the tracebacks for all threads except itself. This
is only really useful as a debug tool for deadlocks, so we might want to
hide it behind a flag?

mopidy/__main__.py

@@ -36,7 +36,7 @@ from mopidy.utils.deps import list_deps_optparse_callback
 from mopidy.utils.log import setup_logging
 from mopidy.utils.path import get_or_create_folder, get_or_create_file
 from mopidy.utils.process import (exit_handler, stop_remaining_actors,
-    stop_actors_by_class)
+    stop_actors_by_class, DebugThread)
 from mopidy.utils.settings import list_settings_optparse_callback
 
 
@@ -44,7 +44,12 @@ logger = logging.getLogger('mopidy.main')
 
 
 def main():
+    debug_thread = DebugThread()
+    debug_thread.start()
+
+    signal.signal(signal.SIGUSR1, debug_thread.handler)
     signal.signal(signal.SIGTERM, exit_handler)
+
     loop = gobject.MainLoop()
     try:
         options = parse_options()

mopidy/utils/process.py

@@ -1,7 +1,9 @@
 import logging
 import signal
+import sys
 import thread
 import threading
+import traceback
 
 from pykka import ActorDeadError
 from pykka.registry import ActorRegistry
@@ -10,6 +12,9 @@ from mopidy import SettingsError
 
 logger = logging.getLogger('mopidy.utils.process')
 
+signals = dict((k, v) for v, k in signal.__dict__.iteritems()
+    if v.startswith('SIG') and not v.startswith('SIG_'))
+
 def exit_process():
     logger.debug(u'Interrupting main...')
     thread.interrupt_main()
@@ -17,8 +22,6 @@ def exit_process():
 
 def exit_handler(signum, frame):
     """A :mod:`signal` handler which will exit the program on signal."""
-    signals = dict((k, v) for v, k in signal.__dict__.iteritems()
-        if v.startswith('SIG') and not v.startswith('SIG_'))
     logger.info(u'Got %s signal', signals[signum])
     exit_process()
 
@@ -65,3 +68,27 @@ class BaseThread(threading.Thread):
 
     def run_inside_try(self):
         raise NotImplementedError
+
+class DebugThread(threading.Thread):
+    daemon = True
+    name = 'DebugThread'
+
+    event = threading.Event()
+
+    def handler(self, signum, frame):
+        logger.info(u'Got %s signal', signals[signum])
+        self.event.set()
+
+    def run(self):
+        while True:
+            self.event.wait()
+            threads = dict((t.ident, t.name) for t in threading.enumerate())
+
+            for ident, frame in sys._current_frames().items():
+                if self.ident == ident:
+                    continue
+
+                print "## Thread: %s (%s) ##" % (threads[ident], ident)
+                print ''.join(traceback.format_stack(frame))
+
+            self.event.clear()