Add multiple workers.

This uses the soft-deprecated multiprocessing mode of Tornado. Later
versions will switch away from it.

Ref: tornadoweb/tornado#2801

app.yaml

@@ -3,6 +3,7 @@ entrypoint: >-
     ./pdf_sprinkles_web.py
     --address ''
     --port $PORT
+    --workers 4
     --cloud_debugger
     --cloud_logging
     --golink='go/pdf-sprinkles'

pdf_sprinkles_web.py

@@ -28,6 +28,7 @@
 import google.cloud.logging
 from google.cloud.logging.handlers import setup_logging
 import pdf_sprinkles
+import tornado.httpserver
 import tornado.ioloop
 import tornado.web
 import trace_context
@@ -37,6 +38,7 @@
 flags.DEFINE_integer('port', 8888, 'port to listen to.')
 flags.DEFINE_string('address', '127.0.0.1', 'address to bind to.')
 flags.DEFINE_boolean('debug', False, 'Starts Tornado in debugging mode.')
+flags.DEFINE_integer('workers', 1, 'Number of workers to start.')
 flags.DEFINE_string(
     'golink', None, 'If set, displays a go/ link in the header.')
 flags.DEFINE_boolean('cloud_debugger', False, 'Use cloud debugger.')
@@ -93,6 +95,22 @@ def main(argv: Sequence[str]) -> None:
   if len(argv) > 1:
     raise app.UsageError('Too many command-line arguments.')
 
+  application = tornado.web.Application(
+      [
+          (r'/', MainHandler),
+          (r'/_ah/warmup', WarmupHandler),
+          (r'/recognize', RecognizeHandler, None, 'recognize'),
+      ],
+      static_path=os.path.join(os.path.dirname(__file__), 'static'),
+      template_path=os.path.join(os.path.dirname(__file__), 'templates'),
+      debug=FLAGS.debug,
+      autoreload=FLAGS.debug and FLAGS.workers == 1,
+  )
+  server = tornado.httpserver.HTTPServer(application)
+  server.bind(FLAGS.port, FLAGS.address)
+  server.start(FLAGS.workers)
+
+  # Initialize gRPC clients after fork.
   if FLAGS.cloud_debugger:
     try:
       import googleclouddebugger  # pylint: disable=g-import-not-at-top
@@ -106,18 +124,7 @@ def main(argv: Sequence[str]) -> None:
     setup_logging(handler)
     py_logging.root.removeHandler(logging.get_absl_handler())
 
-  application = tornado.web.Application(
-      [
-          (r'/', MainHandler),
-          (r'/_ah/warmup', WarmupHandler),
-          (r'/recognize', RecognizeHandler, None, 'recognize'),
-      ],
-      static_path=os.path.join(os.path.dirname(__file__), 'static'),
-      template_path=os.path.join(os.path.dirname(__file__), 'templates'),
-      debug=FLAGS.debug,
-  )
-  logging.info('Starting server on %s:%d', FLAGS.address, FLAGS.port)
-  application.listen(FLAGS.port, FLAGS.address)
+  logging.info('Started server on %s:%d', FLAGS.address, FLAGS.port)
   tornado.ioloop.IOLoop.current().start()