Metadata for Mercurial

http://stackoverflow.com/questions/4443712/is-there-a-mercurial-extension-like-svn-propset/11679615#11679615

(I'm not saying anything that I haven't said before: metadata needs to be versioned (everything needs to be versioned), but metadata may be imagined as transcending versions in normal operation.  Looking at these Q&As helps me crystallize my thoughts.)

I'm going to add a really late quasi-answer to this question, since it is one that many, many, people using Mercurial, including me, ask - something we want to have, and expect to have, after using other tools like svn properties.

As far as I can tell, there is no such extension for Mercurial. yet.

Yes, the Mercurial way seems to be to track files and only files.  And it might be that they way to right such an extension is to put the necessary metadata into ...repo/.hg* files.

OK.  I've been playing around with that.  Doing stuff by hand, before trying to write tools.

The key weakness of the versioned .hg files approach is that if you check out a non-tip version, e.g. "hg update -r OLD-VERSION", you get an old version of the metadata.

I think the key thing, therefore, may be to put the metadata in ...repo/.hg* files.

But...  most operations should be performed with or upon the latest version of such files. I.e. I think such metadata files "transcend" versions - i.e. they want to be versioned, but in an ideal situation you might imagine them as overlaid upon the normally versioned files that you may have checked out an older version of.

Moreover, in many cases you want the branching of such metadata files to be handled separately.  E.g. imagine a file where you are trying to write a description of all branches, together. Perhaps comparing branches, like "branch1.1 is a more recent version of branch1".  You don't want that description to be on either branch; or, rather, you want it to be on both brancges, at the same time, and you want changes to be reflected on both branches.

Such a putative extension would either operate on "hg cat -r tip ...repo/.hg-my-new-metadata".  Or it would somehow overlay the versioned of the files with the normally version transcending metadata files.

I have made some progress on doing this with subrepos:

    superrepo
       files // normally-versioned-files <-- a subrepo
       metadata // version transcending metadata <-- a subrepo

this allows me to check out the latest metadata alongside an older version of the files

It's not quite there, because checking out a particular version of the superrepo may get old versions of the metadata subrepo. But at least the newer versions are in the subrepo.

Let me also note that, whether you put the metadata in a neighboring subrepo, or keep it in the same repo (but operate on the tip), there is a problem that you can do

    hg clone -r OLD-REVISION repo newrepo

This will strip out metadata later than OLD-REVISION.  Including the metadata that says that "OLD-REVISION passed all of the tests", i.e. it will strip out metadata from a later revision that might apply to OLD-REVISION.

This same problem happens with hg tags.

One might say "well, never do that" - never strip out history.  Unfortunately, that is often recommended as a way of "tidying up" the repository.

It seems hard to avoid this with Mercurial.