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The destructor triggered by dbwrap_watched_watch_recv() will remove the watcher instance via a dedicated dbwrap_do_locked(), just calling dbwrap_watched_watch_remove_instance() inside. But the typical caller triggers a dbwrap_do_locked() again after dbwrap_watched_watch_recv() returned. Which means we call dbwrap_do_locked() twice. We now allow dbwrap_watched_watch_recv() to return the existing instance id (if it still exists) and removes the destructor. That way the caller can pass the given instance id to dbwrap_watched_watch_remove_instance() from within its own dbwrap_do_locked(), when it decides to leave the queue, because it's happy with the new state of the record. In order to get the best performance dbwrap_watched_watch_remove_instance() should be called before any dbwrap_record_storev() or dbwrap_record_delete(), because that will only trigger a single low level storev/delete. If the caller found out that the state of the record doesn't meet the expectations and the callers wants to continue watching the record (from its current position, most likely the first one), dbwrap_watched_watch_remove_instance() can be skipped and the instance id can be passed to dbwrap_watched_watch_send() again, in order to resume waiting on the existing instance. Currently the watcher instance were always removed (most likely from the first position) and re-added (to the last position), which may cause unfair latencies. In order to improve the overhead of adding a new watcher instance the caller can call dbwrap_watched_watch_add_instance() before any dbwrap_record_storev() or dbwrap_record_delete(), which will only result in a single low level storev/delete. The returned instance id is then passed to dbwrap_watched_watch_send(), within the same dbwrap_do_locked() run. It also adds a way to avoid alerting any callers during the current dbwrap_do_locked() run. Layers above may only want to wake up watchers during specific situations and while it's useless to wake others in other situations. This will soon be used to add more fairness to the g_lock code. Note that this commit only prepares the api for the above to be useful, the instance returned by dbwrap_watched_watch_recv() is most likely 0, which means the watcher entry was already removed, but that will change in the following commits. BUG: https://bugzilla.samba.org/show_bug.cgi?id=15125 Signed-off-by: Stefan Metzmacher <metze@samba.org> Reviewed-by: Jeremy Allison <jra@samba.org> Reviewed-by: Ralph Boehme <slow@samba.org>