Re: impure renames / history tracking

On Wed, 1 Mar 2006, Paul Jakma wrote:

The thing is, it does better than anything that _tries_ to be "reliable".

I can pretty much _guarantee_ that you can't do it better.

Tracking "inodes" - aka file identities - (which is what BK does, and I assume what SVN does) is fundamentally problematic. I particular, it's a horrible problem when two inodes "meet" under the same name. You now have two identities for the same file, and you're fundamentally screwed.

And don't tell me it doesn't happen. It _does_ happen, and it did happen with the kernel under BK.

It doesn't even need renames to be a problem. JUST THE FACT THAT YOU TRY TO TRACK FILE "IDENTITY" HISTORY IS BROKEN. For example, take CVS, which doesn't actually try to do renames, but _does_ try to track the identity of a file, since all the history is tied into that identity: think about what happens in Attic when a file is deleted. Completely broken model.

Now, CVS doesn't tend to show the problems very much, because people don't actually use branches that much (they are a pain in the neck), and they sure as hell try to avoid deleting and creating the same filename under a branch and on HEAD. I'm sure you can do it, but I'm also pretty sure there's a lot of old projects around that have ended up moving the ,v files around to play rename/delete games.

And that's really fundamental. CVS doesn't show the problems so much, because CVS actively tries to make it hard to do these things.

With renames-tracking-file-identities, it's _really_ easy to get some major confusion going. What happens when one branch creates a file, and another one renames a file to that same name, and they merge?

Don't tell me it doesn't happen. It happened under BK. The way BK "solved" it was to keep the two separate identities: one of them got resolved to the new filename, the other one went into the "deleted" directory. Guess what happens when the side that got merged into "deleted" continues to edit the file? That's right - their edits happen on the deleted file, and never show up in the real tree in a subsequent merge ever again.

And as far as I can tell, BK really did the best you can do. Following file identities really _is_ fundamentally broken. It sounds like a nice idea, but while you migth solve a few problems, you create a whole raft of much more fundamental problems.

So next time you think about a merge that migt have been improved by tracking renames, please also think about a merge where one of the filenames came from two or more different sources through an earlier merge, and thank your benevolent Gods that they instructed me to make git be based purely on file contents.

		Linus