From bc1cfb346e78f520139201112d1109dad69b3011 Mon Sep 17 00:00:00 2001
From: Manuel Kaufmann <humitos@gmail.com>
Date: Wed, 31 Oct 2018 13:29:55 +0100
Subject: [PATCH 1/2] Avoid infinite redirection

---
 readthedocs/core/views/__init__.py            |  4 ++-
 readthedocs/rtd_tests/tests/test_redirects.py | 28 +++++++++++++++++++
 2 files changed, 31 insertions(+), 1 deletion(-)

diff --git a/readthedocs/core/views/__init__.py b/readthedocs/core/views/__init__.py
index e97a7247ba1..04838bbb328 100644
--- a/readthedocs/core/views/__init__.py
+++ b/readthedocs/core/views/__init__.py
@@ -116,7 +116,9 @@ 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:
+
+    if response and response.url != request.build_absolute_uri():
+        # check that we do have a response and avoid infinite redirect
         return response
     r = render(request, template_name)
     r.status_code = 404
diff --git a/readthedocs/rtd_tests/tests/test_redirects.py b/readthedocs/rtd_tests/tests/test_redirects.py
index 7595d147568..335c8feba86 100644
--- a/readthedocs/rtd_tests/tests/test_redirects.py
+++ b/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(

From 00fe39a5b13ffbd0d6f5da383ea52743f009f99b Mon Sep 17 00:00:00 2001
From: Manuel Kaufmann <humitos@gmail.com>
Date: Wed, 31 Oct 2018 14:42:55 +0100
Subject: [PATCH 2/2] Log a WARNING when the infinite redirect happens

---
 readthedocs/core/views/__init__.py | 12 +++++++++---
 1 file changed, 9 insertions(+), 3 deletions(-)

diff --git a/readthedocs/core/views/__init__.py b/readthedocs/core/views/__init__.py
index 04838bbb328..342eb530498 100644
--- a/readthedocs/core/views/__init__.py
+++ b/readthedocs/core/views/__init__.py
@@ -117,9 +117,15 @@ def server_error_404(request, exception=None, template_name='404.html'):  # pyli
     """
     response = get_redirect_response(request, path=request.get_full_path())
 
-    if response and response.url != request.build_absolute_uri():
-        # check that we do have a response and avoid infinite redirect
-        return response
+    if 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