The Desura client gives you option to download either the installer standalone or download & install the game via client.When an update is available, it notifies you and you have option to uninstall the current installation and download and install again. If a GOG client comes into being id prefer

1) it gives you option to download the installer standalone and stuff as it does now
2) gives you option to download & install for you but also keeps the installer intact (not exactly sure what happens to the desura one during this process) . This stops customer from wasting bandwidth having to redownload if they want a copy to backup etc.
3) option to download incremental patches AND updated installer. that would keep most people happy who wanted either.

Probably asking for too much, but it would save bandwidth for both customer & GOG.

Yes, it makes sense to provide all kind of paths for updates / backups at the same time, where user can control what to do.

1. Normal incremental update.
2. Downloadable standalone package/installer which includes the game with all updates up to the current point (that's what GOG offers now).
3. Downladable update package (delta) which can be applied to the installation performed by the previous full package from the option #2.

#2 and #3 will cover DRM free backups any way users might want, and #1 will offer convenient updater. The client can offer convenient way to download #2 and #3 and they can be as well offered through the site.

Sorry if I am not fully up to date about open source, but is it so that GOG wouldn't be able to control to which direction such open source client would be heading?

Say, if a number of client users/developers thought it to be a great idea that the client would also let you share your purchased GOG games directly with other users (the client could e.g. include p2p functionality for that), if you feel so. If GOG opposed that idea (as it would make the game publishers quite anxious, I believe), would GOG have any power to block such feature from the client?

Sounds like a neat idea that will never happen and that's okay.

I would love it if the Steam client was open source. At least if it would give the power to the userbase to make the offline mode work 100% (not just "kind of"), or maybe even change it to a completely DRM-free client? :)

But Valve is such a bunch of control freaks that they'd never let others meddle with their business.

I think it would be a nice move of GOG although it's not really necessary. After all even Windows is a black box and I'm using it. On the other hand maybe GOG just don't want to give control away. And if the API is unknown, it would be hard for the community to come up with their own solution.

I actually already trust GOG enough that when they say they aren't spying on us, then they don't. But then I also use Windows and Steam.

They can control the main client directly by deciding which community patches to approve and merge into it.

Here's the more complicated part of it all. By making it open-source you expose yourself to that (anybody can just fork your project and do whatever they want), but you can have your own license which for example disallows certain things to be done to the code (for example you can disallow commercial use so others can't sell their clients, or distribution so they can't share their modifications based on your code and so on).

You can have the code available for contributors but a license which restricts what they can do with them. Open-source code doesn't necessarily mean that you can't control your code.

You can go here for a brief description of the most popular open-source licenses and imagine GOG having a mix between the No License one with parts from the others.

Open source means that everybody can make his or her own version. However it would still be a breach of contract and you can already now download the files and share them with another service which is similarly easy. However GOG could make their own official client and then they could perfectly well control what's in it.

In short Open Source gives maximum freedom to everyone. And this means everyone.

I think that is irrelevant. Having such "piracy feature" in a GOG client, and being able to torrent GOG games from torrent sites separately, are still two completely different things in the eyes of potential game publishers.

Would Valve or EA allow and endorse an open-source Steam/Origin client, and if not, why not?

But if GOG has full control over it (its features etc.), could it be considered open source anymore?

EDIT: AndrewC wrote something about it, so apparently the answer is "maybe" or "sort of". :)

Anyways, if GOG says yes to open source, naturally I will not oppose it. Free labor, just like Steam forum moderators or people doing freeware mods for Steamworks games. :)

The first part is very relevant because in practice the two methods would be nearly identical. Sharing GOGs is so simple, everyone can do it already now. Your GOG downloader already now packs all the files in one folder. Just share it. I would even say that it is so easy that there wouldn't be any demand for an integrated solution.

But of course I see the potentially bad publicity by the press.

For the second thing: GOG would have only full control about their own version. Every version of their version could also be the basis for a "mean" version of somebody else at every time. As I said: open source means full freedom to everyone. The design principle is always that everybody can do whatever he wants but must allow others to do the same. Copyleft instead of Copyright.

If the API is openly documented, the official client itself doesn't need to be open source. Third-party implementations will appear (admittedly, they already have, through black-box observation).

No, open source just means that you have access to the source code, and that's it. The degree of access/freedom/whatever you want to call it is decided by the author of the code with the license he chooses.

I can have my code open-source but have the license say that you can't modify it, can't distribute it, can't resell it, basically you can't do anything with it legally except look at the source code and maybe compile it.
I'm with you on this one. I see an open-source client as a waste of resources (having to do code reviews for merges, later even have to fix bugs introduced by community code) if you have an available API.

But... What if you don't want to change the name?
To be fair, I think OP is asking for the code to be released for peer (and internet rage idiots) review, not necessarily community expansion.

That's what I gather from the original post, but afterwards he adds:

Though I see that potential keyword in there.

The problem is that maintaining an open-source project is not something I'd think of "losing nothing by doing this" because there's a lot of bureaucracy, especially if you accept community contributions/forks.