Mercurial "vendor branches" from external repositories?

Question

I want to store a project in Mercurial that contains external code (which can be modified by me) coming from Git and SVN repositories. In SVN I would solve this with vendor branches and copy the code around, but I understood that in Mercurial it's better to have different repositories for different projects, and pull between them when needed.

The project layout will be like this:

 - externalLibraryA [comes from a SVN repo]
  - ...with some extra files from me
 - externalLibraryB [comes from a SVN repo]
  - ...with some extra files from me
  - externalPluginForExternalLibraryB [comes from a Git repo]

In Subversion I would create vendor dir and a trunk dir, copy all external libraries first in vendor, and then in the right place in trunk. (I think) I can do this in Mercurial too, with subrepositories, but is this the best way to do this?

I tried setting up different repositories for the external libraries, but then it seems I can't pull the externalLibraryARepo into the externalLibraryA directory of my main repository? It goes in the main directory, which is not what I want. I can also create a Mercurial mirror repository and include it as a subrepo in my main repository, but then the changes in this subdirectory go to the mirror repository, while I want them to stay in the main repository.

Answer 1

I'd probably just store this in one repository - note that in the link you give they are using their build system in the end to bring together the binary output from the different repos. I'm not clear on their rationale there.

If the underlying problem you're trying to solve is how to update the externals in a clean way, I'd probably use anonymous branching for that.

I.e. add the external lib to your project, and your modifications. Make sure it works. Tag with ExternalA-v1.0. Hack away on your actual project. Now ExternalA, Inc. has a new version of their stuff. Update your repo to ExternalA-v1.0 tag. Import their new version and apply your modifications on top. Commit. Now you have two heads: one with the latest version of your code (that works with ExternalA-v1.0) and one with the latest version of ExternalA (that does not work with your code, maybe). So then you merge and reconcile the two. Tag again, now with ExternalA-v2.0. Repeat as needed.

You can still keep your externals in separate repositories, but I assume that the project that is using those does not need to be up to date with changes there all the time - looks like the whole point of vendor branches is to have some point of isolation between dependee and dependants. Of course, moving the changes from the externalA project to the project that is using that will then be a manual affair (well, a copy, much like in SVN really).

Answer 2

It depends on whether your vendor code is going to be customized by your team or not. Our teams have had a great deal of success maintaining a named "vendor" branch on repositories with our own customizations on branches named by project name. This vendor code is then easily included in a project as a subrepository.

A caveat to this approach: if active development is going on in the subrepository, best keep it to directly editing the subrepository as a separate clone, otherwise it becomes necessary to pay close attention to the top-level repository so you don't inadvertantly bump your .hgsubstate forward to the wrong revision and break your build.

Watch out for merges of the top-level repository (your project) between versions which point to different named branches of your subrepository, as this can result in a merge between the "vendor" and "project" branches in the subrepository as it recurses, which may not be desirable.

Note that this functionality may change in the future as well, as some "warm" discussions have been taking place in recent months on the mercurial-devel mailing lists about the future of subrepository recursion.

edit: I just saw this discussion in the related links as well, which seems relevant: https://stackoverflow.com/a/3998791/1186771