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.

instances/views.py

@@ -28,7 +28,6 @@ from django.http import Http404, HttpResponse, JsonResponse
 from django.db.models import prefetch_related_objects
 from django.shortcuts import get_object_or_404, redirect, render
 from django.urls import reverse
-from django.utils.http import url_has_allowed_host_and_scheme
 from django.utils.translation import gettext_noop as _
 from django.views.decorators.http import require_POST
 from libvirt import (VIR_DOMAIN_UNDEFINE_KEEP_NVRAM,
@@ -40,7 +39,7 @@ from libvirt import (VIR_DOMAIN_UNDEFINE_KEEP_NVRAM,
 from logs.views import addlogmsg
 from operations import claims, jobs
 from operations.utils import busy_message, summary
-from webvirtcloud.middleware import error_text
+from webvirtcloud.middleware import error_text, same_origin_referer
 from vrtManager import util
 from vrtManager.create import wvmCreate
 from vrtManager.instance import USAGE_MAX_WINDOW
@@ -377,18 +376,8 @@ def sshkeys(request, pk):
     return JsonResponse(instance_keys, safe=False)
 
 
-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 get_safe_redirect(request, default=None):
-    return redirect(_same_origin_referer(request, default or reverse("instances:index")))
+    return redirect(same_origin_referer(request, default or reverse("instances:index")))
 
 
 def _is_int(value):
@@ -402,7 +391,7 @@ def _is_int(value):
 def _back(request, pk, tab):
     """Back to the page the form was on, at the given tab; the VM page if
     the Referer is missing or from another site."""
-    return redirect(_same_origin_referer(request, reverse("instances:instance", args=[pk])) + "#" + tab)
+    return redirect(same_origin_referer(request, reverse("instances:instance", args=[pk])) + "#" + tab)
 
 
 def get_instance(user, pk, perm_type="view"):
@@ -1318,7 +1307,7 @@ def unmount_iso(request, pk):
     else:
         msg = _("Unmount media: %(dev)s") % {"dev": dev}
         addlogmsg(request.user.username, instance.compute.name, instance.name, msg)
-    return redirect(_same_origin_referer(request, reverse("instances:instance", args=[pk])) + "#disks")
+    return redirect(same_origin_referer(request, reverse("instances:instance", args=[pk])) + "#disks")
 
 
 @require_POST
@@ -1335,7 +1324,7 @@ def mount_iso(request, pk):
     else:
         msg = _("Mount media: %(dev)s") % {"dev": dev}
         addlogmsg(request.user.username, instance.compute.name, instance.name, msg)
-    return redirect(_same_origin_referer(request, reverse("instances:instance", args=[pk])) + "#disks")
+    return redirect(same_origin_referer(request, reverse("instances:instance", args=[pk])) + "#disks")
 
 
 @require_POST
@@ -1562,7 +1551,7 @@ def set_video_model(request, pk):
 @serialize_instance_mutation
 def change_network(request, pk):
     instance = get_instance(request.user, pk)
-    back = _same_origin_referer(request, reverse("instances:instance", args=[pk])) + "#network"
+    back = same_origin_referer(request, reverse("instances:instance", args=[pk])) + "#network"
 
     # Each NIC has its own form; its fields end with that NIC's index.
     nums = [key[len("net-old-mac-"):] for key in request.POST if key.startswith("net-old-mac-")]

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):