Eventlet green threads not released back to pool

Presently, the wsgi server allows persist connections hence even after
the response is sent to the client, it doesn't close the client socket
connection.
Because of this problem, the green thread is not released back to the pool.

In order to close the client socket connection explicitly after the
response is sent and read successfully by the client, you simply have to
set keepalive to False when you create a wsgi server.

Add a parameter to take advantage of the new(ish) eventlet socket timeout
behaviour. Allows closing idle client connections after a period of
time, eg:

$ time nc localhost 8776
real    1m0.063s
Setting 'client_socket_timeout = 0' means do not timeout.

DocImpact:
Added wsgi_keep_alive option (default=True).
Added client_socket_timeout option (default=900).
SecurityImpact

Change-Id: I303d87addeed8b103eeb26dbcc48e3acce06ee6a
Closes-Bug: #1361360

heat/common/wsgi.py

@@ -159,10 +159,25 @@ cfg.CONF.register_group(api_cw_group)
 cfg.CONF.register_opts(api_cw_opts,
                        group=api_cw_group)
 
+wsgi_elt_opts = [
+    cfg.BoolOpt('wsgi_keep_alive',
+                default=True,
+                help=_("If False, closes the client socket connection "
+                       "explicitly.")),
+    cfg.IntOpt('client_socket_timeout', default=900,
+               help=_("Timeout for client connections' socket operations. "
+                      "If an incoming connection is idle for this number of "
+                      "seconds it will be closed. A value of '0' means "
+                      "wait forever.")),
+]
+wsgi_elt_group = cfg.OptGroup('eventlet_opts')
+cfg.CONF.register_group(wsgi_elt_group)
+cfg.CONF.register_opts(wsgi_elt_opts,
+                       group=wsgi_elt_group)
 json_size_opt = cfg.IntOpt('max_json_body_size',
                            default=1048576,
-                           help='Maximum raw byte size of JSON request body.'
-                                ' Should be larger than max_template_size.')
+                           help=_('Maximum raw byte size of JSON request body.'
+                                  ' Should be larger than max_template_size.'))
 cfg.CONF.register_opt(json_size_opt)
 
 
@@ -171,6 +186,7 @@ def list_opts():
     yield 'heat_api', api_opts
     yield 'heat_api_cfn', api_cfn_opts
     yield 'heat_api_cloudwatch', api_cw_opts
+    yield 'eventlet_opts', wsgi_elt_opts
 
 
 def get_bind_addr(conf, default_port=None):
@@ -334,13 +350,17 @@ class Server(object):
         eventlet.hubs.use_hub('poll')
         eventlet.patcher.monkey_patch(all=False, socket=True)
         self.pool = eventlet.GreenPool(size=self.threads)
+        socket_timeout = cfg.CONF.eventlet_opts.client_socket_timeout or None
         try:
-            eventlet.wsgi.server(self.sock,
-                                 self.application,
-                                 custom_pool=self.pool,
-                                 url_length_limit=URL_LENGTH_LIMIT,
-                                 log=self._wsgi_logger,
-                                 debug=cfg.CONF.debug)
+            eventlet.wsgi.server(
+                self.sock,
+                self.application,
+                custom_pool=self.pool,
+                url_length_limit=URL_LENGTH_LIMIT,
+                log=self._wsgi_logger,
+                debug=cfg.CONF.debug,
+                keepalive=cfg.CONF.eventlet_opts.wsgi_keep_alive,
+                socket_timeout=socket_timeout)
         except socket.error as err:
             if err[0] != errno.EINVAL:
                 raise