Re: [RFC] Support projects including other projects

On Wed, 11 May 2005, Junio C Hamano wrote:

I think that the core of your idea of recording "required
version" of the depended project (core GIT) in the depending
project (Cogito) is a very sound one.  GNU Arch folks do
something similar in their "package-framework" stuff.  

I however do not think that belongs to the core GIT nor even to
Cogito for that matter.  To me, it feels like this is a pure
build infrastructure issue.

If you think about it as git and cogito being entirely separate projects,
where users would be expected to have the right version of git most of the
time (or ever), this is true. But I think that cogito is as closely tied
to git as the kernel is to kbuild or kconfig; the difference is that git
is not solely available with cogito, like kbuild is solely available with
the kernel.

I think you could arrange something like that with today's core
GIT tools, like this:

 - Tweak Cogito Makefile so that pure Cogito and core GIT are
   housed in separate subdirectories;

 - Add "required-git-pb" file to Cogito source as a tracked
   source file, and record the required version of git-pb there;

 - Arrange Cogito Makefile to make sure the subtree that has the
   core GIT side meets "required-git-pb" constraints.  The
   constraints could be "at least contains this one", "exactly
   this one".  The policy would be differnt from a depending
   project to another.  What happens if the requirements are not
   met is also up to the policy of that depending project.

When a particular cogito commit is made, it is impossible to tell whether
the next git-pb will work with it; the current set of patches could be
rejected in mainline git, and different support for the same functionality
added which requires something different from cogito.

This also means that Petr can't really test changes to git before
commiting them (and a new cogito with the constraint changed), because the
cogito build system would then require him to use a version he's not
testing.

Also, either the user has to keep track of two projects without any system
support in the same directory structure and figure out how to follow the
instructions from the build system in getting the right version checked
out in the right place, or the build system is tied to a particular
wrapper layer.

I think your idea is theoretically possible, but that it is just too
impractical for anyone to ever actually use it. It's something that people
could do with CVS (and it would actually work better, due to CVS's
limitations making the issues simpler), but people don't.

	-Daniel
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