The problem comes if two people have changed the same file in different ways, and you want both changes. What are you going to do? There's no 'wrong' set of changes that you can discard. If you lose one and keep the other, that means somebody's work gets lost, and they have to redo it - so you need to merge. Will you sit there in Photoshop, carefully pasting together the modified bits? Will you open 2 copies of 3D Studio or Maya or what have you, and painstakingly join the two models together?? Well... I'd hope not.

Fortunately, as with many problems, there is a perfect solution, which is to (try to) not have that problem in the first place. It just needs a bit of process. Locking is a nice and easy way of introducing a process that works, and that anybody can understand, all without their needing to know the first thing about how the mysterious computer-ma-bob actually works. You can do without it, but it just makes life difficult if the team is distributed, tiresome if they are not, and error-prone in either case.

I don't have any real idea how you'd go about adding such a thing to git. Perhaps it wouldn't even be possible. That doesn't make it any less useful, though. If you don't like the idea of SVN doing something useful that git doesn't even try to support, consider the two as solving somewhat different problems.

> The problem comes if two people have changed the same file in different ways, and you want both changes. What are you going to do? There's no 'wrong' set of changes that you can discard.

Well, as you say ...

> there is a perfect solution, which is to (try to) not have that problem in the first place

Why are two people both returning the same work? That's a process control issue you're having and a filesystem hack isn't going to solve that.

> Locking is a nice and easy way of introducing a process that works, and that anybody can understand, all without their needing to know the first thing about how the mysterious computer-ma-bob actually works.

Not in the slightest. With locking you'd have a confused person trying to save two hours of their changes, wondering why it isn't working. Eventually they'll save to a temp file and still have all the same merging work to do, but they can't do it with the versioning tool so you're pretending it's not there.

> Will you sit there in Photoshop, carefully pasting together the modified bits?

Yeah yeah, binary files. No simple merge.

But you wouldn't want the change from the person who shouldn't have made it, so you wouldn't take it and there wouldn't be a problem. If you do want their work they'll have to redo it in turn, based on the latest set of changes.

> I don't have any real idea how you'd go about adding such a thing to git.

Simple, you'd have a list of files that shouldn't be edited, or with the name of who should be editing them. A script would run at checkout and set appropriate permissions for each. I'm sure there are a few other ways too.

But why? Are your teams really just working on whatever files aren't locked? Do you really need this tool to do this?

> Perhaps it wouldn't even be possible. That doesn't make it any less useful, though.

Perhaps git already does it. That doesn't make it useful.

Do your team communication and synchronization with a communication tool and leave the filesystem permissions alone. There is no central repo that needs protection, contention isn't a failure.

> If you don't like the idea of SVN doing something useful that git doesn't even try to support, consider the two as solving somewhat different problems.

I'm sure there are many valid capabilities of most programs that I don't know of, or don't fully appreciate the uses for. This seems like a misfeature though.

"If you do want their work they'll have to redo it in turn, based on the latest set of changes." - and this is exactly the problem that locking is trying to help solve. I don't see how you can state that contention isn't a failure, if people are then to have to redo work! These are real people, presumably working to some kind of deadline. They have better things to do (and their paymasters have better things to spend their money on) than spend time redoing changes that they've already done. (It sort of sounds like you don't think non-programmers would ever have the need to edit a file at the same time as anybody else might be editing it! Sadly that is not the case...)

As for using an external tool or process to manage the locking, perhaps one has to actually experience this approach in person to appreciate its limitations? Or maybe what's in my mind's eye is different from what's in yours? Anyway, I would not recommend doing this myself.

Did you miss my discussion of the problems of permission-based locking? You dismiss "using external tools" when this is a communication issue, not a permission issue.

What better achieves the desired results?

1) Having many files, only some of which you can save changes to, but working on those files otherwise totally unguided when you can.

2) Having many files, all of which you can edit, but only some of which you have been told via email or IM to work on.

Obviously (I think) #1 is a joke and #2 is the only practical way to go.

> As for using an external tool or process to manage the locking

You're thinking of it from the wrong point of view. From the POV of the development manager using gantt charts, using the CVS permissions to convey intent is that horrid external tool.

> I don't see how you can state that contention isn't a failure, if people are then to have to redo work!

I didn't say that. I said it's not a failure of the VCS. It's a failure of the process that led the people to doing two inherently unmergeable things.

As I said in that post, they'd still have done the useless work but they'd simply have saved it under a temp filename after finding out the file was write protected. Then - as you read, I said they'd have to do the painful merge anyways - or watch their work be totally wasted.

> These are real people [...] better things to do [...] paymasters have better things [...] than spend time redoing changes that they've already done.

Yeah yeah, I get it. But you're missing the point. I'm saying developers waste more time doing this your way, despite the locking.

btw, redoing changes they've already done is redundant.

> It sort of sounds like you don't think non-programmers would ever have the need to edit a file at the same time as anybody else might be editing it!

Not at all. But the non-programmers are less likely to notice the permissions and deduce their ultimate reason and impact and do the right workflow thing.

Falling back on the false security of a broken process instead of fixing it is like using broken crypto - you think you're doing well but you'd be better off if someone painfully opened your eyes and realized you were totally insecure all along.

This is all so contrary to my experience, that I can only assume we are actually talking about completely different things. At best, I must simply be explaining myself poorly. (See, e.g., http://en.wikipedia.org/wiki/Revision_control#File_locking for a brief description. I tried to keep my description brief, but perhaps it came too close to 'cryptic'... apologies...)

Anyway, should you ever meet one of these systems in practice, rest assured that the problems you suggest will happen, won't.

No, I'm pretty sure I'm with you. One person 'checks out' a file to edit and the rest of the people see it as locked.

But I remember conflicts when working with that style of locking. The biggest was people forgetting to check their work in before leaving and kept anyone else from using it, but also people not checking the permissions before editing, and people who didn't have the lock editing it anyways knowingly because they had to fix a bug and didn't have time to do it right.

If we were using a dvcs (and no locking mechanism) we'd have been using our communication tool (then just email and calendaring) more and posting things like "I'm going to fix bug #... - this is going to require a rearchitecting foo.c and touching all of foo_*.c and all accompanying headers". This way it's not preventing someone from working on things, it's warning them of potential conflicts that would have to be merged.

As long as these conflicts seem small people are welcome to keep developing next to me. There's no reason to use a giant lock on a whole file. But also, this schedules my intent in a semantic way - people can see it and say "Whoa, you'll need to break bar.c as well and Dave's doing delicate work there." whereas if I'd just locked files nobody would have known to warn me and I'd have gotten half-way to a working change to be stalled by Dave's project. Worse, Dave would probably have needed my files to finish his work leading to a nasty conflict.