Avoid infinite redirection

readthedocs/core/views/__init__.py

@@ -116,8 +116,16 @@ def server_error_404(request, exception=None, template_name='404.html'):  # pyli
         Marking exception as optional to make /404/ testing page to work.
     """
     response = get_redirect_response(request, path=request.get_full_path())
+
     if response:
-        return response
+        if response.url == request.build_absolute_uri():
+            # check that we do have a response and avoid infinite redirect
+            log.warning(
+                'Infinite Redirect: FROM URL is the same than TO URL. url=%s',
+                response.url,
+            )
+        else:
+            return response
     r = render(request, template_name)
     r.status_code = 404
     return r

readthedocs/rtd_tests/tests/test_redirects.py

@@ -127,6 +127,34 @@ def setUp(self):
         self.pip = Project.objects.get(slug='pip')
         self.pip.versions.create_latest()
 
+    @override_settings(USE_SUBDOMAIN=True)
+    def test_redirect_prefix_infinite(self):
+        """
+        Avoid infinite redirects.
+
+        If the URL hit is the same that the URL returned for redirection, we
+        return a 404.
+
+        These examples comes from this issue:
+          * https://github.com/rtfd/readthedocs.org/issues/4673
+        """
+        Redirect.objects.create(
+            project=self.pip, redirect_type='prefix',
+            from_url='/',
+        )
+        r = self.client.get('/redirect', HTTP_HOST='pip.readthedocs.org')
+        self.assertEqual(r.status_code, 302)
+        self.assertEqual(
+            r['Location'], 'http://pip.readthedocs.org/en/latest/redirect.html')
+
+        r = self.client.get('/redirect/', HTTP_HOST='pip.readthedocs.org')
+        self.assertEqual(r.status_code, 302)
+        self.assertEqual(
+            r['Location'], 'http://pip.readthedocs.org/en/latest/redirect/')
+
+        r = self.client.get('/en/latest/redirect/', HTTP_HOST='pip.readthedocs.org')
+        self.assertEqual(r.status_code, 404)
+
     @override_settings(USE_SUBDOMAIN=True)
     def test_redirect_root(self):
         Redirect.objects.create(