test: add test for DNS-SD D-Bus registration lifetime

Verify that a DNS-SD service registered via the D-Bus RegisterService()
call is bound to the lifetime of the client's bus connection: since
busctl disconnects right after the call returns, the service must be
unregistered again shortly after. Wait for resolved's vanish handler
log message as proof that the client tracking triggered the
unregistration, check that a subsequent UnregisterService() fails
specifically because the service is not known anymore, and verify that
the identifier can be registered again — previously it stayed taken by
the vanished client's service forever, failing with
DNSSD_SERVICE_EXISTS.

A dummy link with mDNS enabled makes sure the per-scope goodbye/zone
teardown path actually runs, and since resolved would be restarted
automatically if it crashed in it, letting the D-Bus level checks pass
vacuously, verify the InvocationID stayed the same throughout.

test/units/TEST-75-RESOLVED.sh

@@ -436,6 +436,78 @@ manual_testcase_02_mdns_llmnr() {
     assert_in 'no' "$(resolvectl llmnr hoge)"
 }
 
+# Test for the D-Bus RegisterService()/UnregisterService() API
+manual_testcase_03_register_service() {
+    local cursor_file="" invocation_id
+
+    # Cleanup
+    # shellcheck disable=SC2317,SC2329
+    cleanup() {
+        rm -f /run/systemd/resolved.conf.d/90-mdns.conf "${cursor_file:-}"
+        ip link del mdns99 2>/dev/null || :
+        systemctl reload systemd-resolved.service
+    }
+
+    trap cleanup RETURN ERR
+
+    mkdir -p /run/systemd/resolved.conf.d
+    {
+        echo "[Resolve]"
+        echo "MulticastDNS=yes"
+    } >/run/systemd/resolved.conf.d/90-mdns.conf
+    restart_resolved
+    resolvectl log-level debug
+
+    # resolved is restarted automatically if it crashes (Restart=always), which would let the checks below
+    # pass for the wrong reason, so remember the invocation and verify it stayed the same at the end.
+    invocation_id=$(systemctl show -P InvocationID systemd-resolved.service)
+
+    # Make sure at least one link carries mDNS scopes, so that unregistration actually runs the
+    # per-scope goodbye/zone teardown.
+    ip link add mdns99 type dummy
+    ip link set mdns99 up
+    resolvectl mdns mdns99 yes
+
+    cursor_file=$(mktemp)
+    journalctl -n 0 --cursor-file="$cursor_file"
+
+    # The lifetime of a registered service is bound to the bus connection of the client that registered
+    # it. busctl disconnects right after the call, so the service should be unregistered again shortly
+    # after. Wait for the vanish handler log message as proof that the client tracking triggered the
+    # unregistration. See testcase_15_wait_online_dns() for why journalctl -f | grep -m1 is not used here.
+    assert_in '/org/freedesktop/resolve1/dnssd/testservice' \
+              "$(busctl call org.freedesktop.resolve1 /org/freedesktop/resolve1 org.freedesktop.resolve1.Manager \
+                     RegisterService 'sssqqqaa{say}' testservice '%H' _testservice._tcp 1234 0 0 0)"
+
+    timeout 30 bash -c "until journalctl --after-cursor=\"\$(cat \"$cursor_file\")\" SYSLOG_IDENTIFIER=systemd-resolved --grep \"Client of DNS-SD service 'testservice' vanished, unregistering.\" >/dev/null 2>&1; do sleep .5; done"
+
+    assert_not_in 'dnssd/testservice' "$(busctl tree org.freedesktop.resolve1)"
+
+    # Unregistering it again should fail specifically because the service is gone (busctl prints the
+    # D-Bus error message, not the error name), i.e. not because resolved crashed or the bus connection
+    # failed.
+    assert_in "DNS-SD service 'testservice' not known" \
+              "$(busctl call org.freedesktop.resolve1 /org/freedesktop/resolve1 org.freedesktop.resolve1.Manager \
+                     UnregisterService 'o' /org/freedesktop/resolve1/dnssd/testservice 2>&1 || :)"
+
+    # Re-registering the same identifier must succeed: before client tracking worked, the identifier
+    # stayed taken by the vanished client's service and this failed with DNSSD_SERVICE_EXISTS. Take a
+    # fresh cursor so the wait below cannot match the first round's identical message.
+    rm -f "$cursor_file"
+    journalctl -n 0 --cursor-file="$cursor_file"
+    assert_in '/org/freedesktop/resolve1/dnssd/testservice' \
+              "$(busctl call org.freedesktop.resolve1 /org/freedesktop/resolve1 org.freedesktop.resolve1.Manager \
+                     RegisterService 'sssqqqaa{say}' testservice '%H' _testservice._tcp 1234 0 0 0)"
+
+    timeout 30 bash -c "until journalctl --after-cursor=\"\$(cat \"$cursor_file\")\" SYSLOG_IDENTIFIER=systemd-resolved --grep \"Client of DNS-SD service 'testservice' vanished, unregistering.\" >/dev/null 2>&1; do sleep .5; done"
+
+    assert_not_in 'dnssd/testservice' "$(busctl tree org.freedesktop.resolve1)"
+
+    # resolved must not have crashed and gotten restarted along the way.
+    assert_eq "$(systemctl show -P InvocationID systemd-resolved.service)" "$invocation_id"
+    systemctl is-active systemd-resolved.service
+}
+
 testcase_03_23951() {
     : "--- nss-resolve/nss-myhostname tests"
 
@@ -1725,6 +1797,7 @@ systemctl enable --now systemd-resolved.service
 # Need to be run before SETUP, otherwise things will break
 manual_testcase_01_resolvectl
 manual_testcase_02_mdns_llmnr
+manual_testcase_03_register_service
 
 # Run setup
 setup