Hi Christian. Before you spend lots more time and effort on discussing how to implement "this thing" efficiently ...
I think this feature is exactly the sort of thing that would make sense as a plug-in to Subversion, and not as part of the core product, but Subversion does not (yet) have a plug-in architecture that would support this. It would be valuable to determine what kind of plug-in support would be needed to enable this. If you want this to be a "feature" rather than a "hack", or if you are thinking in terms of submitting it to the central Subversion code base, then as well as an efficient implementation it also needs to have a good, simple, high-level behavioural definition. It would not be enough just to specify how the "update" command would behave. For example, it could be something like ... "The HeadRev keyword gives the head revision number of the repository. Every Subversion command that contacts the repository and potentially modifies the WC (including commit and export) will update the HeadRev keyword value in every file in which it occurs in the scope of the command." ... but with more detail. I think the lack of a good high-level definition is the more significant reason why this has not been done before. I do not even know if a sensible definition is possible. Picture the scene when you are working on a project that is one of many projects in a busy repository. Every time you update to "head", even if no changes have been made to your files the "head" revision number changes because several commits have been made to other projects in the repository. And then a minute after you have updated, the number is already out of date, so what is the point of this information? Or rather, why is this information related to files in your working copy? It isn't, it's related to the repository. If I wanted a number that indicates which revision of my project I'm building, then maybe I would want the highest revision number in which any file in my project was committed, not the head of the repository. At least that number would only change when my project changes. If you just want to hack up something that works for you and build your own copy of Subversion from your source code, that's totally fine and I think you can achieve that, although it might be harder than you expect. We would be glad to advise you on implementation details. - Julian
