add /r/ redirect endpoint

for #16, #32. cc @swentel. more to come.

app.yaml

@@ -67,6 +67,10 @@ handlers:
   script: logs.app
   secure: always
 
+- url: /r/.+
+  script: redirect.app
+  secure: always
+
 - url: /wm/.+
   script: add_webmention.app
   secure: always

redirect.py

@@ -0,0 +1,34 @@
+"""Simple endpoint that redirects to the embedded fully qualified URL.
+
+Used to wrap ActivityPub ids with the fed.brid.gy domain so that Mastodon
+accepts them. Background:
+
+https://github.com/snarfed/bridgy-fed/issues/16#issuecomment-424799599
+https://github.com/tootsuite/mastodon/pull/6219#issuecomment-429142747
+"""
+import logging
+
+import webapp2
+
+import appengine_config
+import common
+
+
+class RedirectHandler(webapp2.RequestHandler):
+    """301 redirects to the embedded fully qualified URL.
+
+    e.g. redirects /r/https://foo.com/bar?baz to https://foo.com/bar?baz
+    """
+
+    def get(self):
+        assert self.request.path_qs.startswith('/r/')
+        to = self.request.path_qs[3:]
+        if not to.startswith('http://') and not to.startswith('https://'):
+            common.error(self, 'Expected fully qualified URL; got %s' % to)
+        logging.info('redirecting to %s', to)
+        self.redirect(to)
+
+
+app = webapp2.WSGIApplication([
+    (r'/r/.+', RedirectHandler),
+], debug=appengine_config.DEBUG)

tests/test_redirect.py

@@ -0,0 +1,20 @@
+"""Unit tests for redirect.py.
+"""
+from redirect import app
+import testutil
+
+
+class ActivityPubTest(testutil.TestCase):
+
+    def test_redirect(self):
+        got = app.get_response('/r/https://foo.com/bar?baz=baj&biff')
+        self.assertEqual(302, got.status_int)
+        self.assertEqual('https://foo.com/bar?baz=baj&biff', got.headers['Location'])
+
+    def test_redirect_scheme_missing(self):
+        got = app.get_response('/r/asdf.com')
+        self.assertEqual(400, got.status_int)
+
+    def test_redirect_url_missing(self):
+        got = app.get_response('/r/')
+        self.assertEqual(404, got.status_int)