(dispatch): Rwlocks can hand out read locks while a write lock is

wanted.  Unfair to writers, but possible.  This lets commands run with
!player->aborted, which can then block on input and thus let players
delay the update or shutdown indefinitely.  Avoid by spinning until
the update is done.

src/lib/player/dispatch.c

@@ -88,6 +88,16 @@ dispatch(char *buf, char *redir)
 	pr("Command not implemented\n");
 	return 0;
     }
+    /*
+     * Rwlocks can hand out read locks while a write lock is wanted.
+     * Unfair to writers, but possible.  This lets commands run with
+     * !player->aborted, which can then block on input and thus delay
+     * the update indefinitely.  We could avoid that by setting
+     * player->aborted here, but spinning until the update is done is
+     * nicer to players.
+     */
+    while (play_wrlock_wanted)
+	empth_yield();
     empth_rwlock_rdlock(play_lock);
     if (redir) {
 	prredir(redir);