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.

docs/wsgi.md

@@ -19,8 +19,9 @@ Then, set up Apache with a configuration like:
 
     <VirtualHost>
         WSGIScriptAlias /nilmdb /home/nilm/nilmdb.wsgi
-        WSGIProcessGroup nilmdb-server
+        WSGIApplicationGroup nilmdb-appgroup
-        WSGIDaemonProcess nilmdb-server threads=32 user=nilm group=nilm
+        WSGIProcessGroup nilmdb-procgroup
+        WSGIDaemonProcess nilmdb-procgroup threads=32 user=nilm group=nilm
 
         # Access control example:
         <Location /nilmdb>

nilmdb/server/server.py

@@ -11,6 +11,7 @@ from nilmdb.utils.time import string_to_timestamp
 import cherrypy
 import sys
 import os
+import socket
 import simplejson as json
 import decorator
 import psutil
@@ -608,6 +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.
@@ -618,29 +624,33 @@ 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:
                 # Build an error message on failure
+                import pprint
                 err = sprintf("Initializing database at path '%s' failed:\n\n",
                               dbpath)
                 err += traceback.format_exc()
                 try:
                     import pwd
                     import grp
-                    err += sprintf("\nRunning as: uid=%d (%s), gid=%d (%s)\n",
+                    err += sprintf("\nRunning as: uid=%d (%s), gid=%d (%s) "
+                                   "on host %s, pid %d\n",
                                    os.getuid(), pwd.getpwuid(os.getuid())[0],
-                                   os.getgid(), grp.getgrgid(os.getgid())[0])
+                                   os.getgid(), grp.getgrgid(os.getgid())[0],
+                                   socket.gethostname(), os.getpid())
                 except ImportError:
                     pass
-        if server[0] is None:
+                err += sprintf("\nEnvironment:\n%s\n", pprint.pformat(environ))
+        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))) ]
@@ -648,5 +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