fix(instances): the create wizard reaches the page of any architecture

The first page of the create wizard built the next page's URL from a
reversed ".../x86_64/pc/" by replacing "x86_64" with the chosen
architecture and then the first "pc" with the machine type. An
architecture containing "pc" got the machine type inside its name:
ppc64le with pseries went to /instances/create/1/ppseries64le/pc/, so
VMs could not be created on PowerPC hosts.

The URL is now reversed with the placeholders __arch__ and __machine__,
which the script replaces with the URL-encoded values.

The test renders the page, runs the script with node and fails on the
previous template for ppc64le.

instances/templates/create_instance_w1.html

@@ -131,7 +131,10 @@
             let arch = $("#select_archs").val();
             let machine = $("#select_chipset").val();
             //create_machine_url = "/computes/" + compute + "/create/archs/" + arch + "/machines/" + machine;
-            url = "{% url 'instances:create_instance' compute.id 'x86_64' 'pc' %}".replace(/x86_64/, arch).replace(/pc/, machine);
+            // placeholders, not "x86_64"/"pc": an arch such as ppc64 contains "pc"
+            url = "{% url 'instances:create_instance' compute.id '__arch__' '__machine__' %}"
+                .replace("__arch__", encodeURIComponent(arch))
+                .replace("__machine__", encodeURIComponent(machine));
             window.location.href = url;//create_machine_url;
         }
 

instances/test_create_wizard_url.py

@@ -0,0 +1,36 @@
+"""The create wizard's first page sends the browser to the next page for the
+chosen architecture and machine type; the URL is built from placeholders."""
+
+import re
+import subprocess
+import shutil
+import unittest
+
+from computes.models import Compute
+from django.contrib.auth import get_user_model
+from django.template.loader import render_to_string
+from django.test import RequestFactory, TestCase
+
+
+class CreateWizardUrlTests(TestCase):
+    def render_script(self):
+        compute = Compute.objects.create(name="cw", hostname="10.0.0.6", login="u", password="p", type=1)
+        request = RequestFactory().get("/")
+        request.user = get_user_model().objects.create_superuser("cw-admin", "cw@example.com", "pw")
+        page = render_to_string("create_instance_w1.html", {"compute": compute}, request=request)
+        # the statement that builds the URL from arch and machine
+        return re.search(r"\burl = (.*?);", page, re.S).group(1)
+
+    @unittest.skipUnless(shutil.which("node"), "node is not installed")
+    def test_arch_and_machine_names_land_in_their_segments(self):
+        script = self.render_script()
+        cases = {
+            ("x86_64", "q35"): "x86_64/q35/",
+            ("ppc64le", "pseries"): "ppc64le/pseries/",  # contains "pc"
+            ("aarch64", "virt"): "aarch64/virt/",
+        }
+        for (arch, machine), tail in cases.items():
+            js = "const arch = %r, machine = %r; console.log(%s);" % (arch, machine, script)
+            out = subprocess.run(["node", "-e", js], capture_output=True, text=True, timeout=30)
+            with self.subTest(arch=arch):
+                self.assertTrue(out.stdout.strip().endswith(f"/{tail}"), out.stdout + out.stderr)