Add error view for error handling and error view testing (#11263)

* Add error view for error handling and error view testing

I was having trouble testing error views and templates locally and found
at some point we added a `/500` for testing the 500 error page. I
expanded on this to create a view that could emit any error status code
and relevant template. Afterwards, it seemed like we had a few redundant
views doing similar things, so I expanded the dashboard error handlers
to all use the same view as well.

Changes here are:

- When using the new dashboard, dashboard error templates are in the
  `/errors/dashboard/` template path. There will be a duplicate set of
  templates for Proxito errors and documentation errors, in
  `/errors/proxito/` eventually.
- This swaps out the URL `handler500`, and defines the missing handlers
  in the URL module. Mostly, this loads these error templates from a
  more structured path instead of top level `/404.html` templates.
- Adds a debug view/URL for testing arbitrary error views/templates in
  debug mode.

This does not touch proxito error pages yet.

* More linting

* Support all request methods in ErrorView

* Add support for spam project response/410

This is just the dashboard response view for spam projects.

* Symlink 403 to 401 template

The 401 template has copy closer to 403 already, and I don't believe we
are emitting a templated 403 response anywhere either.

readthedocs/core/views/__init__.py

@@ -8,7 +8,7 @@
 import structlog
 from django.conf import settings
 from django.http import JsonResponse
-from django.shortcuts import redirect, render
+from django.shortcuts import redirect
 from django.urls import reverse
 from django.views.generic import TemplateView, View
 
@@ -84,6 +84,62 @@ def get_context_data(self, **kwargs):
         return context
 
 
+class ErrorView(TemplateView):
+
+    """
+    Render templated error pages.
+
+    This can be used both for testing and as a generic error view. This supports
+    multiple subpaths for errors, as we need to show application themed errors
+    for dashboard users and minimal error pages for documentation readers.
+
+    View arguments:
+
+    status_code
+        This can also be a kwarg, like in the case of a testing view for all
+        errors. Set through ``as_view(status_code=504)``, this view will always
+        render the same template and status code.
+
+    base_path
+        Base path for templates. Dashboard templates can be loaded from a
+        separate path from Proxito error templates.
+    """
+
+    base_path = "errors/dashboard/"
+    status_code = 500
+
+    def get_status_code(self):
+        status_code = self.status_code
+        try:
+            status_code = int(self.kwargs["status_code"])
+        except (ValueError, KeyError):
+            pass
+        return status_code
+
+    def get_template_names(self):
+        status_code = self.get_status_code()
+        if settings.RTD_EXT_THEME_ENABLED:
+            # First try to load the template for the specific HTTP status code
+            # and fall back to a generic 400/500 level error template
+            status_code_class = int(status_code / 100)
+            generic_code = f"{status_code_class}xx"
+            return [
+                f"{self.base_path}/{code}.html" for code in [status_code, generic_code]
+            ]
+        # TODO the legacy dashboard has top level path errors, as is the
+        # default. This can be removed later.
+        return f"{status_code}.html"
+
+    def dispatch(self, request, *args, **kwargs):
+        context = self.get_context_data(**kwargs)
+        status_code = self.get_status_code()
+        return self.render_to_response(
+            context,
+            status=status_code,
+        )
+
+
+# TODO replace this with ErrorView and a template in `errors/` instead
 class TeapotView(TemplateView):
     template_name = "core/teapot.html"
 
@@ -92,13 +148,6 @@ def get(self, request, *args, **kwargs):
         return self.render_to_response(context, status=418)
 
 
-def server_error_500(request, template_name="500.html"):
-    """A simple 500 handler so we get media."""
-    r = render(request, template_name)
-    r.status_code = 500
-    return r
-
-
 def do_not_track(request):
     dnt_header = request.headers.get("Dnt")
 

readthedocs/projects/views/base.py

@@ -103,6 +103,9 @@ def get(self, request, *args, **kwargs):
             )
 
             if is_show_dashboard_denied(self.get_project()):
-                return render(request, template_name="spam.html", status=410)
+                template_name = "spam.html"
+                if settings.RTD_EXT_THEME_ENABLED:
+                    template_name = "errors/dashboard/410.html"
+                return render(request, template_name=template_name, status=410)
 
         return super().get(request, *args, **kwargs)

readthedocs/templates/403.html

@@ -0,0 +1 @@
+401.html

readthedocs/urls.py

@@ -8,17 +8,15 @@
 from django.urls import include, path, re_path
 from django.views.generic.base import RedirectView, TemplateView
 
-from readthedocs.core.views import (
-    HomepageView,
-    SupportView,
-    do_not_track,
-    server_error_500,
-)
+from readthedocs.core.views import ErrorView, HomepageView, SupportView, do_not_track
 from readthedocs.search.views import GlobalSearchView
 
 admin.autodiscover()
 
-handler500 = server_error_500
+handler400 = ErrorView.as_view(status_code=400)
+handler403 = ErrorView.as_view(status_code=403)
+handler404 = ErrorView.as_view(status_code=404)
+handler500 = ErrorView.as_view(status_code=500)
 
 basic_urls = [
     path("", HomepageView.as_view(), name="homepage"),
@@ -51,8 +49,6 @@
     path("invitations/", include("readthedocs.invitations.urls")),
     # For redirects
     path("builds/", include("readthedocs.builds.urls")),
-    # For testing the 500's with DEBUG on.
-    path("500/", handler500),
     # Put this as a unique path for the webhook, so we don't clobber existing Stripe URL's
     path("djstripe/", include("djstripe.urls", namespace="djstripe")),
 ]
@@ -125,6 +121,11 @@
         "style-catalog/",
         TemplateView.as_view(template_name="style_catalog.html"),
     ),
+    # For testing error responses and templates
+    path(
+        "error/<int:status_code>/",
+        ErrorView.as_view(base_path="errors/dashboard"),
+    ),
     # This must come last after the build output files
     path(
         "media/<path:remainder>",