Correctly import eventlet to prevent threads blocking each other

Currently, slow running OpenStack API Requests (either stuck connecting
or still waiting for the actual response) from the periodic DataGatherer
task will block HTTPServer connections from being processed. Blocked
HTTPServer connections will also block both other connections and the
DataGatherer task.

Observed Symptoms:
- Slow or failed prometheus requests
- Statistics not being updated as often as you would expect
- HTTP 500 responses and BrokenPipeError tracebacks being logged due to
  later trying to respond to prometheus clients which timed out and
  disconnected the socket
- Hitting the forked process limit

This happens because in the current code, we are intending to use the
eventlet library for asynchronous non-blocking I/O, but we are not using
it correctly. All code within the main application and all imported
dependencies must import the special eventlet "green" versions of many
python libraries (e.g. socket, time, threading, SimpleHTTPServer, etc)
which yield to other green threads when they would have blocked waiting
for I/O or to sleep. Currently this does not always happen.

Fix this by importing eventlet and using eventlet.patcher.monkey_patch()
before importing any other modules. This will automatically intercept
all future imports (including those inside dependencies) and
automatically load the green versions of relevant libraries.

Documentation on correctly import eventlet can be found here:
https://eventlet.readthedocs.io/en/latest/patching.html

A detailed and comprehensive analysis of the issue and multiple previous
attempts to fix it can be found in Issue #130. If you intend to make
further related changes to the use of eventlet, threads or forked
processes please read the detailed history lesson available there.

Fixes: #130, #126, #124, #116, #115, #112

prometheus-openstack-exporter

@@ -22,7 +22,13 @@ You should have received a copy of the GNU General Public License
 along with this program.  If not, see <http://www.gnu.org/licenses/>.
 """
 
-import argparse
+# We must import and monkey patch before importing any other modules:
+# https://eventlet.readthedocs.io/en/latest/patching.html
+import eventlet
+
+eventlet.patcher.monkey_patch()
+
+import argparse  # noqa: I100
 import ast
 import json
 import logging.handlers
@@ -34,15 +40,11 @@ from http.server import BaseHTTPRequestHandler
 from http.server import HTTPServer
 from os import environ as env
 from os import path, rename
-from socketserver import ForkingMixIn
+from threading import Thread
+from time import sleep, time
 
 from cinderclient.v3 import client as cinder_client
 
-from eventlet.green import asyncore  # noqa: F401
-from eventlet.green import socket  # noqa: F401
-from eventlet.green.threading import Thread
-from eventlet.green.time import sleep, time
-
 from netaddr import IPRange
 
 from neutronclient.v2_0 import client as neutron_client
@@ -273,10 +275,8 @@ class DataGatherer(Thread):
 
                 log.debug("DataGatherer: Update keystone")
                 prodstack.update(self._get_keystone_info(keystone))
-                sleep(0)
                 log.debug("DataGatherer: Update neutron")
                 prodstack.update(self._get_neutron_info(neutron))
-                sleep(0)
                 log.debug("DataGatherer: Update nova")
                 prodstack.update(self._get_nova_info(nova, cinder, prodstack))
 
@@ -1143,7 +1143,4 @@ if __name__ == "__main__":
         data_gatherer = DataGatherer()
         data_gatherer.start()
     server = HTTPServer(("", config.get("listen_port")), handler)
-    server.timeout = config.get("cache_refresh_interval", 900)
-    while True:
-        sleep(0)
-        server.handle_request()
+    server.serve_forever()

tox.ini

@@ -37,6 +37,10 @@ deps =
     flake8-colors
 
 [flake8]
+extend-ignore =
+    # E402: E402 module level import not at top of file
+    # eventlet.monkey_patch() is required before importing other modules
+    E402
 exclude =
     .git,
     __pycache__,