Virtual Reality

While hacking a little bit with GConf I found what IMHO is an undesirable behaviour of GConf. I think most of developers will agree because you don’t want your application to run in a different way than the one you expected when you designed it. And this happens in the following situation:

I set up a GConfClient to listen to changes in keys under a specific directory. I wrote also a handler for this GConf notifications hoping that I’ll get every change in those keys (the API doc literally says “Any changes to keys below this directory will cause the “value_changed” signal to be emitted”). So I relied in this GConf notification handler some important code of my application. But, while executing the application I realized that I was not getting some notifications even tough I was completely sure that I was setting a new value in the GConf key. So, I had to take a deeper look…

I verified that the GConf server was issuing a notification for every change I made, so the problem should be in the client. Looking at the GConfClient code I found the following in the notify_from_server_callback function:

changed = gconf_client_cache (client, FALSE, entry, TRUE);

if (!changed)
return; /* don't do the notify */

gconf_client_queue_notify (client, entry->key);

What? This means that I won’t get notifications if the key is already updated in the client cache? What’s the reason for such a behaviour? I don’t think this is what a developer expects, specially for a system that uses idles for notifications (you could never be sure when you’ll get it), because it could cause things like the one that was happening to me:

1. As a response to a user action I was setting a new value to a GConf key
2. Before the GConfServer notified the GConfClient, there was some other code that was trying to get the new value of the same key
3. GConf cached the new value in the client while doing the get operation
4. The GConfClient received the GConfServer notification, but was not issuing a notification because the value was already updated
5. I was scratching my head trying to figure out what the hell was happening …

So, IMHO there are two possibilities

1. Always notify
2. Write in bold 46-point sized font that sometimes you could not get notifications because of that

Anyway, it’s always good to look into the guts of the software you use.