sd-bus: introduce API for re-enqueuing incoming messages

When authorizing via PolicyKit we want to process incoming method calls
twice: once to process and figure out that we need PK authentication,
and a second time after we aquired PK authentication to actually execute
the operation. With this new call sd_bus_enqueue_for_read() we have a
way to put an incoming message back into the read queue for this
purpose.

This might have other uses too, for example debugging.

src/libsystemd/libsystemd.sym

@@ -685,6 +685,7 @@ global:
 
 LIBSYSTEMD_245 {
 global:
+        sd_bus_enqeue_for_read;
         sd_bus_message_dump;
         sd_bus_message_sensitive;
         sd_event_add_child_pidfd;

src/libsystemd/sd-bus/sd-bus.c

@@ -4207,3 +4207,27 @@ _public_ int sd_bus_get_close_on_exit(sd_bus *bus) {
 
         return bus->close_on_exit;
 }
+
+_public_ int sd_bus_enqeue_for_read(sd_bus *bus, sd_bus_message *m) {
+        int r;
+
+        assert_return(bus, -EINVAL);
+        assert_return(bus = bus_resolve(bus), -ENOPKG);
+        assert_return(m, -EINVAL);
+        assert_return(m->sealed, -EINVAL);
+        assert_return(!bus_pid_changed(bus), -ECHILD);
+
+        if (!BUS_IS_OPEN(bus->state))
+                return -ENOTCONN;
+
+        /* Re-enqeue a message for reading. This is primarily useful for PolicyKit-style authentication,
+         * where we want accept a message, then determine we need to interactively authenticate the user, and
+         * when we have that process the message again. */
+
+        r = bus_rqueue_make_room(bus);
+        if (r < 0)
+                return r;
+
+        bus->rqueue[bus->rqueue_size++] = bus_message_ref_queued(m, bus);
+        return 0;
+}

src/systemd/sd-bus.h

@@ -207,6 +207,7 @@ int sd_bus_process(sd_bus *bus, sd_bus_message **r);
 int sd_bus_process_priority(sd_bus *bus, int64_t max_priority, sd_bus_message **r);
 int sd_bus_wait(sd_bus *bus, uint64_t timeout_usec);
 int sd_bus_flush(sd_bus *bus);
+int sd_bus_enqeue_for_read(sd_bus *bus, sd_bus_message *m);
 
 sd_bus_slot* sd_bus_get_current_slot(sd_bus *bus);
 sd_bus_message* sd_bus_get_current_message(sd_bus *bus);