Use a global cached server object for WSGI app

This is instead of caching it inside nilmdb.server.wsgi_application.
Might make things work a bit better in case the web server decides
to call wsgi_application multiple times.

nilmdb/server/server.py

@@ -609,6 +609,11 @@ class Server(object):
     def stop(self):
         cherrypy.engine.exit()
 
+# Use a single global nilmdb.server.NilmDB and nilmdb.server.Server
+# instance since the database can only be opened once.  For this to
+# work, the web server must use only a single process and single
+# Python interpreter.  Multiple threads are OK.
+_wsgi_server = None
 def wsgi_application(dbpath, basepath): # pragma: no cover
     """Return a WSGI application object with a database at the
     specified path.
@@ -619,13 +624,13 @@ def wsgi_application(dbpath, basepath): # pragma: no cover
     is the same as the first argument to Apache's WSGIScriptAlias
     directive.
     """
-    server = [None]
     def application(environ, start_response):
-        if server[0] is None:
+        global _wsgi_server
+        if _wsgi_server is None:
             # Try to start the server
             try:
                 db = nilmdb.utils.serializer_proxy(nilmdb.server.NilmDB)(dbpath)
-                server[0] = nilmdb.server.Server(
+                _wsgi_server = nilmdb.server.Server(
                     db, embedded = True,
                     basepath = basepath.rstrip('/'))
             except Exception:
@@ -645,7 +650,7 @@ def wsgi_application(dbpath, basepath): # pragma: no cover
                 except ImportError:
                     pass
                 err += sprintf("\nEnvironment:\n%s\n", pprint.pformat(environ))
-        if server[0] is None:
+        if _wsgi_server is None:
             # Serve up the error with our own mini WSGI app.
             headers = [ ('Content-type', 'text/plain'),
                         ('Content-length', str(len(err))) ]
@@ -653,5 +658,5 @@ def wsgi_application(dbpath, basepath): # pragma: no cover
             return [err]
 
         # Call the normal application
-        return server[0].wsgi_application(environ, start_response)
+        return _wsgi_server.wsgi_application(environ, start_response)
     return application