Introduce thread job framework.

This is a cleaner generalization of the functionality added
to network_proxy.py a few months ago, whereby jobs can be
added and removed from a thread's main loop.  This allows
us to cut down on unnecessary threads.

A follow-up patch will update the proxy to this framework.

lib/util.py

@@ -3,6 +3,7 @@ import platform
 import shutil
 from datetime import datetime
 from decimal import Decimal
+import traceback
 import urlparse
 import urllib
 import threading
@@ -24,6 +25,20 @@ class MyEncoder(json.JSONEncoder):
             return obj.as_dict()
         return super(MyEncoder, self).default(obj)
 
+class ThreadJob:
+    """A job that is run periodically from a thread's main loop.  run() is
+    called from that thread's context.
+    """
+
+    def print_error(self, *msg):
+        print_error("[%s]" % self.__class__.__name__, *msg)
+
+    def print_msg(self, *msg):
+        print_msg("[%s]" % self.__class__.__name__, *msg)
+
+    def run(self):
+        """Called periodically from the thread"""
+        pass
 
 class DaemonThread(threading.Thread):
     """ daemon thread that terminates cleanly """
@@ -33,6 +48,27 @@ class DaemonThread(threading.Thread):
         self.parent_thread = threading.currentThread()
         self.running = False
         self.running_lock = threading.Lock()
+        self.job_lock = threading.Lock()
+        self.jobs = []
+
+    def add_job(self, job):
+        with self.job_lock:
+            self.jobs.append(job)
+
+    def run_jobs(self):
+        # Don't let a throwing job disrupt the thread, future runs of
+        # itself, or other jobs.  This is useful protection against
+        # malformed or malicious server responses
+        with self.job_lock:
+            for job in self.jobs:
+                try:
+                    job.run()
+                except:
+                    traceback.print_exc(file=sys.stderr)
+
+    def remove_job(self, job):
+        with self.job_lock:
+            self.jobs.remove(job)
 
     def start(self):
         with self.running_lock:
@@ -337,7 +373,6 @@ import socket
 import errno
 import json
 import ssl
-import traceback
 import time
 
 class SocketPipe: