sd-event: refuse running default event loops in any other thread than the one they are default for

TODO

@@ -701,7 +701,6 @@ Features:
   - allow multiple signal handlers per signal?
   - document chaining of signal handler for SIGCHLD and child handlers
   - define more intervals where we will shift wakeup intervals around in, 1h, 6h, 24h, ...
-  - generate a failure of a default event loop is executed out-of-thread
 
 * investigate endianness issues of UUID vs. GUID
 

src/libsystemd/sd-event/sd-event.c

@@ -3422,6 +3422,11 @@ _public_ int sd_event_prepare(sd_event *e) {
         assert_return(e->state != SD_EVENT_FINISHED, -ESTALE);
         assert_return(e->state == SD_EVENT_INITIAL, -EBUSY);
 
+        /* Let's check that if we are a default event loop we are executed in the correct thread. We only do
+         * this check here once, since gettid() is typically not cached, and thus want to minimize
+         * syscalls */
+        assert_return(!e->default_event_ptr || e->tid == gettid(), -EREMOTEIO);
+
         if (e->exit_requested)
                 goto pending;