fix: a form libvirt refuses goes back to its page with the reason

A libvirt error that a view left to ExceptionMiddleware showed the "500
Internal Server Error" page, which blames the server, at the form's
action URL; its only way on was the browser's Back. Adding a NIC, a new
or existing disk, editing a disk, setting a NIC's QoS and creating a
volume end there when libvirt refuses, as do the other VM actions.

For a form (any method but GET or HEAD) the middleware now goes back to
the page the form was on, with the error in the messages, as it already
does for a busy compute; a page that cannot be shown keeps the error
page. same_origin_referer moves from the instances views to the
middleware module, next to error_text, for both to use.

webvirtcloud/middleware.py

@@ -1,6 +1,7 @@
 from django.contrib import messages
 from django.http import JsonResponse
 from django.shortcuts import redirect, render
+from django.utils.http import url_has_allowed_host_and_scheme
 from django.utils.translation import gettext_lazy as _
 from libvirt import libvirtError
 from operations import claims, jobs
@@ -11,6 +12,16 @@ API_PREFIX = "/api/"
 CONTENT_SECURITY_POLICY = "object-src 'none'; base-uri 'self'; form-action 'self'; frame-ancestors 'none'"
 
 
+def same_origin_referer(request, default):
+    """The Referer without its fragment if it is on this site, else default."""
+    referer = request.META.get("HTTP_REFERER")
+    if referer and url_has_allowed_host_and_scheme(
+        url=referer, allowed_hosts={request.get_host()}, require_https=request.is_secure()
+    ):
+        return referer.split("#")[0]
+    return default
+
+
 def error_text(request, exception):
     """
     The libvirt error to show. Administrators (superusers and the global
@@ -63,5 +74,8 @@ class ExceptionMiddleware:
                 request,
                 _("libvirt Error - %(exception)s") % {"exception": error_text(request, exception)},
             )
+            if request.method not in ("GET", "HEAD"):
+                # a refused form: back to the page it was on, with the reason
+                return redirect(same_origin_referer(request, "/"))
             return render(request, "500.html", {"libvirt_error": True}, status=500)
             # TODO: check connecting to host via VPN

webvirtcloud/tests/test_connection_errors.py

@@ -33,8 +33,8 @@ class ConnectionErrorPageTests(TestCase):
         self.viewer.user_permissions.add(Permission.objects.get(codename="view_instances"))
         self.viewer = User.objects.get(pk=self.viewer.pk)
 
-    def handle(self, user, exception, path="/instances/1/poweron/"):
-        request = RequestFactory().post(path)
+    def handle(self, user, exception, path="/instances/1/poweron/", method="post", **headers):
+        request = getattr(RequestFactory(), method)(path, **headers)
         SessionMiddleware(lambda r: None).process_request(request)
         request._messages = FallbackStorage(request)
         request.user = user
@@ -48,10 +48,26 @@ class ConnectionErrorPageTests(TestCase):
         ):
             with self.subTest(error=str(error)):
                 response, messages = self.handle(self.owner, error)
-                self.assertEqual(response.status_code, 500)
                 self.assertNotIn("10.0.0.9", " ".join(messages))
                 self.assertTrue(any("cannot be reached" in m for m in messages), messages)
 
+    def test_a_refused_form_goes_back_to_its_page_with_the_error(self):
+        error = libvirt_error(libvirt.VIR_FROM_QEMU, "Requested operation is not valid: domain is not running",
+                              libvirt.VIR_ERR_OPERATION_INVALID)
+        response, messages = self.handle(self.admin, error, HTTP_REFERER="http://testserver/instances/1/#network")
+        self.assertEqual((response.status_code, response["Location"]), (302, "http://testserver/instances/1/"))
+        self.assertTrue(any("domain is not running" in m for m in messages), messages)
+        for referer in ({}, {"HTTP_REFERER": "https://evil.example/"}):
+            with self.subTest(referer=referer):
+                response, _ = self.handle(self.admin, error, **referer)
+                self.assertEqual(response["Location"], "/")
+
+    def test_a_page_that_cannot_be_shown_is_the_error_page(self):
+        response, messages = self.handle(self.admin, libvirt_error(libvirt.VIR_FROM_RPC, HOST_DETAIL),
+                                         path="/instances/1/", method="get")
+        self.assertEqual(response.status_code, 500)
+        self.assertTrue(messages)
+
     def test_administrators_see_connection_details(self):
         for user in (self.admin, self.viewer):
             with self.subTest(user=user.username):