test: add a test for resolved and wait-online interactions

Specifically, add a test case that ensures systemd-networkd-wait-online --dns
is robust against (a) systemd-resolved absence, and (b) systemd-resolved
restarts.

test/units/TEST-75-RESOLVED.sh

@@ -1241,6 +1241,66 @@ testcase_14_refuse_record_types() {
     (! run varlinkctl call /run/systemd/resolve/io.systemd.Resolve io.systemd.Resolve.ResolveService '{"name":"","type":"_mysvc._tcp","domain":"signed.test"}')
 }
 
+# Test systemd-networkd-wait-online interactions with systemd-resolved
+testcase_15_wait_online_dns() {
+    # Cleanup
+    # shellcheck disable=SC2317
+    cleanup() {
+        echo "===== journalctl -u $unit ====="
+        journalctl -b --no-pager --no-hostname --full -u "$unit"
+        echo "=========="
+        rm -f "$override"
+        restart_resolved
+        resolvectl revert dns0
+    }
+
+    trap cleanup RETURN ERR
+
+    local unit
+    local override
+
+    unit="wait-online-dns-$(systemd-id128 new -u).service"
+    override="/run/systemd/resolved.conf.d/90-global-dns.conf"
+
+    # Clear global and per-interface DNS before monitoring the configuration change.
+    mkdir -p "$(dirname "$override")"
+    {
+        echo "[Resolve]"
+        echo "DNS="
+        echo "FallbackDNS="
+    } > "$override"
+    systemctl reload systemd-resolved.service
+    resolvectl dns dns0 ""
+    resolvectl domain dns0 ""
+
+    # Stop systemd-resolved before calling systemd-networkd-wait-online. It should retry connections.
+    systemctl stop systemd-resolved.service
+
+    # Begin systemd-networkd-wait-online --dns
+    systemd-run -u "$unit" -p "Environment=SYSTEMD_LOG_LEVEL=debug" -p "Environment=SYSTEMD_LOG_TARGET=journal" --service-type=exec \
+        /usr/lib/systemd/systemd-networkd-wait-online --timeout=20 --dns --interface=dns0
+
+    # Wait until it blocks waiting for updated DNS config
+    timeout 10 bash -c "journalctl -b -u $unit -f | grep -q -m1 'dns0: No.*DNS server is accessible'"
+
+    # Update the global configuration. Restart rather than reload systemd-resolved so that
+    # systemd-networkd-wait-online has to re-connect to the varlink service.
+    {
+        echo "[Resolve]"
+        echo "DNS=10.0.0.1"
+    } > "$override"
+    systemctl restart systemd-resolved.service
+
+    # Wait for the monitor to exit gracefully.
+    timeout 10 bash -c "while systemctl --quiet is-active $unit; do sleep 0.5; done"
+
+    # Check that a disconnect happened, and was handled.
+    journalctl -b -u "$unit" --grep="DNS configuration monitor disconnected, reconnecting..." > /dev/null
+
+    # Check that dns0 was found to be online.
+    journalctl -b -u "$unit" --grep="dns0: link is configured by networkd and online." > /dev/null
+}
+
 # PRE-SETUP
 systemctl unmask systemd-resolved.service
 systemctl enable --now systemd-resolved.service