application: extract path prefix logic to middleware

Summary:
This way, we can reason about and test it in isolation as we add more
middleware to the application.

Note that when using `--path_prefix`, loading the main TensorBoard page
without a trailing slash on the URL has long been broken (#1117). This
commit does not fix that, but changes the exact failure mode: rather
than 404ing, we now load a broken TensorBoard (relative URLs don’t
resolve). A follow-up commit will fix this properly.

Test Plan:
Verify that TensorBoard still works fully, with `--path_prefix` and
without it.

wchargin-branch: path-prefix-middleware
wchargin-source: 3e07d2b681f12c4bab310bef2afc066590ae5cb7

tensorboard/backend/BUILD

@@ -63,6 +63,7 @@ py_library(
     visibility = ["//visibility:public"],
     deps = [
         ":http_util",
+        ":path_prefix",
         "//tensorboard:errors",
         "//tensorboard:expect_sqlite3_installed",
         "//tensorboard/backend/event_processing:data_provider",
@@ -97,6 +98,29 @@ py_test(
     ],
 )
 
+py_library(
+    name = "path_prefix",
+    srcs = ["path_prefix.py"],
+    srcs_version = "PY2AND3",
+    deps = [
+        "//tensorboard:errors",
+    ],
+)
+
+py_test(
+    name = "path_prefix_test",
+    size = "small",
+    srcs = ["path_prefix_test.py"],
+    srcs_version = "PY2AND3",
+    tags = ["support_notf"],
+    deps = [
+        ":path_prefix",
+        "//tensorboard:errors",
+        "//tensorboard:test",
+        "@org_pocoo_werkzeug",
+    ],
+)
+
 py_library(
     name = "process_graph",
     srcs = ["process_graph.py"],

tensorboard/backend/application.py

@@ -40,6 +40,7 @@
 
 from tensorboard import errors
 from tensorboard.backend import http_util
+from tensorboard.backend import path_prefix
 from tensorboard.backend.event_processing import db_import_multiplexer
 from tensorboard.backend.event_processing import data_provider as event_data_provider  # pylint: disable=line-too-long
 from tensorboard.backend.event_processing import plugin_event_accumulator as event_accumulator  # pylint: disable=line-too-long
@@ -258,8 +259,7 @@ def __init__(self, plugins, path_prefix=''):
         # TODO(@chihuahua): Delete this RPC once we have skylark rules that
         # obviate the need for the frontend to determine which plugins are
         # active.
-        self._path_prefix + DATA_PREFIX + PLUGINS_LISTING_ROUTE:
-            self._serve_plugins_listing,
+        DATA_PREFIX + PLUGINS_LISTING_ROUTE: self._serve_plugins_listing,
     }
     unordered_prefix_routes = {}
 
@@ -291,10 +291,11 @@ def __init__(self, plugins, path_prefix=''):
                            'route does not start with a slash' %
                            (plugin.plugin_name, route))
         if type(plugin) is core_plugin.CorePlugin:  # pylint: disable=unidiomatic-typecheck
-          path = self._path_prefix + route
+          path = route
         else:
-          path = (self._path_prefix + DATA_PREFIX + PLUGIN_PREFIX + '/' +
-                  plugin.plugin_name + route)
+          path = (
+              DATA_PREFIX + PLUGIN_PREFIX + '/' + plugin.plugin_name + route
+          )
 
         if path.endswith('/*'):
           # Note we remove the '*' but leave the slash in place.
@@ -328,6 +329,7 @@ def __init__(self, plugins, path_prefix=''):
   def _create_wsgi_app(self):
     """Apply middleware to create the final WSGI app."""
     app = self._route_request
+    app = path_prefix.PathPrefixMiddleware(app, self._path_prefix)
     app = _handling_errors(app)
     return app
 
@@ -384,7 +386,7 @@ def _serve_plugins_listing(self, request):
         loading_mechanism = {
             'type': 'IFRAME',
             'module_path': ''.join([
-                self._path_prefix, DATA_PREFIX, PLUGIN_PREFIX, '/',
+                request.script_root, DATA_PREFIX, PLUGIN_PREFIX, '/',
                 plugin.plugin_name, es_module_handler,
             ]),
         }
@@ -427,7 +429,7 @@ def _route_request(self, environ, start_response):
     """
     request = wrappers.Request(environ)
     parsed_url = urlparse.urlparse(request.path)
-    clean_path = _clean_path(parsed_url.path, self._path_prefix)
+    clean_path = _clean_path(parsed_url.path)
 
     # pylint: disable=too-many-function-args
     if clean_path in self.exact_routes:
@@ -579,22 +581,16 @@ def _get_connect_params(query):
   return {k: json.loads(v[0]) for k, v in params.items()}
 
 
-def _clean_path(path, path_prefix=""):
-  """Cleans the path of the request.
-
-  Removes the ending '/' if the request begins with the path prefix and pings a
-  non-empty route.
+def _clean_path(path):
+  """Removes a trailing slash from a non-root path.
 
   Arguments:
     path: The path of a request.
-    path_prefix: The prefix string that every route of this TensorBoard instance
-    starts with.
 
   Returns:
-    The route to use to serve the request (with the path prefix stripped if
-    applicable).
+    The route to use to serve the request.
   """
-  if path != path_prefix + '/' and path.endswith('/'):
+  if path != '/' and path.endswith('/'):
     return path[:-1]
   return path
 

tensorboard/backend/path_prefix.py

@@ -0,0 +1,65 @@
+# Copyright 2019 The TensorFlow Authors. All Rights Reserved.
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+# ==============================================================================
+"""Internal path prefix support for TensorBoard.
+
+Using a path prefix of `/foo/bar` enables TensorBoard to serve from
+`http://localhost:6006/foo/bar/` rather than `http://localhost:6006/`.
+See the `--path_prefix` flag docs for more details.
+"""
+
+from __future__ import absolute_import
+from __future__ import division
+from __future__ import print_function
+
+from tensorboard import errors
+
+
+class PathPrefixMiddleware(object):
+  """WSGI middleware for path prefixes.
+
+  All requests to this middleware must begin with the specified path
+  prefix (otherwise, a 404 will be returned immediately). Requests will
+  be forwarded to the underlying application with the path prefix
+  stripped and appended to `SCRIPT_NAME` (see the WSGI spec, PEP 3333,
+  for details).
+  """
+
+  def __init__(self, application, path_prefix):
+    """Initializes this middleware.
+
+    Args:
+      application: The WSGI application to wrap (see PEP 3333).
+      path_prefix: A string path prefix to be stripped from incoming
+        requests. If empty, this middleware is a no-op. If non-empty,
+        the path prefix must start with a slash and not end with one
+        (e.g., "/tensorboard").
+    """
+    if path_prefix.endswith("/"):
+      raise ValueError("Path prefix must not end with slash: %r" % path_prefix)
+    if path_prefix and not path_prefix.startswith("/"):
+      raise ValueError(
+          "Non-empty path prefix must start with slash: %r" % path_prefix
+      )
+    self._application = application
+    self._path_prefix = path_prefix
+    self._strict_prefix = self._path_prefix + "/"
+
+  def __call__(self, environ, start_response):
+    path = environ.get("PATH_INFO", "")
+    if path != self._path_prefix and not path.startswith(self._strict_prefix):
+      raise errors.NotFoundError()
+    environ["PATH_INFO"] = path[len(self._path_prefix):]
+    environ["SCRIPT_NAME"] = environ.get("SCRIPT_NAME", "") + self._path_prefix
+    return self._application(environ, start_response)

tensorboard/backend/path_prefix_test.py

@@ -0,0 +1,112 @@
+# Copyright 2019 The TensorFlow Authors. All Rights Reserved.
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+# ==============================================================================
+"""Tests for `tensorboard.backend.path_prefix`."""
+
+from __future__ import absolute_import
+from __future__ import division
+from __future__ import print_function
+
+import json
+
+import werkzeug
+
+from tensorboard import errors
+from tensorboard import test as tb_test
+from tensorboard.backend import path_prefix
+
+
+class PathPrefixMiddlewareTest(tb_test.TestCase):
+  """Tests for `PathPrefixMiddleware`."""
+
+  def _echo_app(self, environ, start_response):
+    # https://www.python.org/dev/peps/pep-0333/#environ-variables
+    data = {
+        "path": environ.get("PATH_INFO", ""),
+        "script": environ.get("SCRIPT_NAME", ""),
+    }
+    body = json.dumps(data, sort_keys=True)
+    start_response("200 OK", [("Content-Type", "application/json")])
+    return [body]
+
+  def _assert_ok(self, response, path, script):
+    self.assertEqual(response.status_code, 200)
+    actual = json.loads(response.get_data())
+    expected = dict(path=path, script=script)
+    self.assertEqual(actual, expected)
+
+  def test_bad_path_prefix_without_leading_slash(self):
+    with self.assertRaises(ValueError) as cm:
+      path_prefix.PathPrefixMiddleware(self._echo_app, "hmm")
+    msg = str(cm.exception)
+    self.assertIn("must start with slash", msg)
+    self.assertIn(repr("hmm"), msg)
+
+  def test_bad_path_prefix_with_trailing_slash(self):
+    with self.assertRaises(ValueError) as cm:
+      path_prefix.PathPrefixMiddleware(self._echo_app, "/hmm/")
+    msg = str(cm.exception)
+    self.assertIn("must not end with slash", msg)
+    self.assertIn(repr("/hmm/"), msg)
+
+  def test_empty_path_prefix(self):
+    app = path_prefix.PathPrefixMiddleware(self._echo_app, "")
+    server = werkzeug.test.Client(app, werkzeug.BaseResponse)
+
+    with self.subTest("at empty"):
+      self._assert_ok(server.get(""), path="", script="")
+
+    with self.subTest("at root"):
+      self._assert_ok(server.get("/"), path="/", script="")
+
+    with self.subTest("at subpath"):
+      response = server.get("/foo/bar")
+      self._assert_ok(server.get("/foo/bar"), path="/foo/bar", script="")
+
+  def test_nonempty_path_prefix(self):
+    app = path_prefix.PathPrefixMiddleware(self._echo_app, "/pfx")
+    server = werkzeug.test.Client(app, werkzeug.BaseResponse)
+
+    with self.subTest("at root"):
+      response = server.get("/pfx")
+      self._assert_ok(response, path="", script="/pfx")
+
+    with self.subTest("at root with slash"):
+      response = server.get("/pfx/")
+      self._assert_ok(response, path="/", script="/pfx")
+
+    with self.subTest("at subpath"):
+      response = server.get("/pfx/foo/bar")
+      self._assert_ok(response, path="/foo/bar", script="/pfx")
+
+    with self.subTest("at non-path-component extension"):
+      with self.assertRaises(errors.NotFoundError):
+        server.get("/pfxz")
+
+    with self.subTest("above path prefix"):
+      with self.assertRaises(errors.NotFoundError):
+        server.get("/hmm")
+
+  def test_composition(self):
+    app = self._echo_app
+    app = path_prefix.PathPrefixMiddleware(app, "/bar")
+    app = path_prefix.PathPrefixMiddleware(app, "/foo")
+    server = werkzeug.test.Client(app, werkzeug.BaseResponse)
+
+    response = server.get("/foo/bar/baz/quux")
+    self._assert_ok(response, path="/baz/quux", script="/foo/bar")
+
+
+if __name__ == "__main__":
+  tb_test.main()