application: factor out middleware installation (#2732)

Summary:
Currently, we apply the `_handle_errors` middleware as a decorator
directly on `__call__`. We plan to add middleware that is parameterized
on instance variables, so this pattern will no longer suffice. This
commit simply refactors the existing code to apply middleware at
initialization time.

Test Plan:
It suffices to run `bazel test //tensorboard/backend:application_test`.

wchargin-branch: app-middleware

tensorboard/backend/application.py

@@ -323,6 +323,13 @@ def __init__(self, plugins, path_prefix=''):
             key=lambda x: len(x[0]),
             reverse=True))
 
+    self._app = self._create_wsgi_app()
+
+  def _create_wsgi_app(self):
+    """Apply middleware to create the final WSGI app."""
+    app = self._route_request
+    app = _handling_errors(app)
+    return app
 
   @wrappers.Request.application
   def _serve_plugins_listing(self, request):
@@ -392,23 +399,31 @@ def _serve_plugins_listing(self, request):
       response[plugin.plugin_name] = output_metadata
     return http_util.Respond(request, response, 'application/json')
 
-  @_handling_errors
-  def __call__(self, environ, start_response):  # pylint: disable=invalid-name
+  def __call__(self, environ, start_response):
     """Central entry point for the TensorBoard application.
 
-    This method handles routing to sub-applications. It does simple routing
-    using strict string matching.  Regular expressions are not supported.
-    Wildcard routes such as `/foo/*` are supported as a special case.
-
     This __call__ method conforms to the WSGI spec, so that instances of this
     class are WSGI applications.
 
     Args:
-      environ: See WSGI spec.
-      start_response: See WSGI spec.
+      environ: See WSGI spec (PEP 3333).
+      start_response: See WSGI spec (PEP 3333).
+    """
+    return self._app(environ, start_response)
 
-    Returns:
-      A werkzeug Response.
+  def _route_request(self, environ, start_response):
+    """Delegate an incoming request to sub-applications.
+
+    This method supports strict string matching and wildcard routes of a
+    single path component, such as `/foo/*`. Other routing patterns,
+    like regular expressions, are not supported.
+
+    This is the main TensorBoard entry point before middleware is
+    applied. (See `_create_wsgi_app`.)
+
+    Args:
+      environ: See WSGI spec (PEP 3333).
+      start_response: See WSGI spec (PEP 3333).
     """
     request = wrappers.Request(environ)
     parsed_url = urlparse.urlparse(request.path)