fix desktop tests

desktop/tests/test_resolver.py

@@ -231,18 +231,23 @@ def test_junk_on_the_socket_does_not_take_the_stub_down(stub):
 
 
 def test_a_dead_upstream_becomes_servfail(tmp_path):
-    resolver = StubResolver(ports=(0,), upstream_path=str(tmp_path / "empty.conf"), timeout=0.3)
-    assert resolver.start()
-    try:
-        # The file does not exist and /etc/resolv.conf points at resolved, which
-        # is filtered out — so there is nowhere to forward to.
-        with socket.socket(socket.AF_INET, socket.SOCK_DGRAM) as sock:
-            sock.settimeout(2)
-            sock.sendto(query("example.com"), ("127.0.0.1", resolver.port))
-            response, _ = sock.recvfrom(4096)
-        assert response[3] & 0x0F == 2  # SERVFAIL, not a lie about the address
-    finally:
-        resolver.stop()
+    # A socket that is bound and listening but never answers. The upstream must
+    # be named explicitly: an empty or missing file falls back to the machine's
+    # /etc/resolv.conf, which in CI is Docker's resolver and *does* answer.
+    with socket.socket(socket.AF_INET, socket.SOCK_DGRAM) as dead:
+        dead.bind(("127.0.0.1", 0))
+        conf = tmp_path / "resolv.conf"
+        conf.write_text(f"nameserver 127.0.0.1:{dead.getsockname()[1]}\n")
+        resolver = StubResolver(ports=(0,), upstream_path=str(conf), timeout=0.3)
+        assert resolver.start()
+        try:
+            with socket.socket(socket.AF_INET, socket.SOCK_DGRAM) as sock:
+                sock.settimeout(2)
+                sock.sendto(query("example.com"), ("127.0.0.1", resolver.port))
+                response, _ = sock.recvfrom(4096)
+            assert response[3] & 0x0F == 2  # SERVFAIL, not a lie about the address
+        finally:
+            resolver.stop()
 
 
 def test_upstreams_are_read_from_resolv_conf_and_loops_are_dropped(tmp_path):