Index: use a bare repository rather than a non-bare repository without checkout

[joshtriplett]

Ever since #4015 , Cargo avoids checking out the index as a working directory; however, cargo still creates the repository as a non-bare repository with a working directory that just doesn't have any files checked out.

Ideally, I'd love to store this as a bare repository instead, where git doesn't expect to have a working directory.

This would have a few advantages:

• Git tools would understand the repository better, making it easier to explore and understand and debug.
• Tools that want to check .cargo into git (for caching or testing or other purposes) would be able to do so; git can track a repo.git bare repository, but cannot track a .git directory.

(This was discussed briefly in a Cargo team meeting; I'm filing this issue as a follow-up.)

If we don't worry about a one-time re-download of the index, we could solve this by just switching to a bare repository.

[ehuss]

Just to be clear, is the only difference that .git/config would have core.bare = true in it? And I guess the directory layout would also remove the .git directory? That might be something that would cause a lot of disruption that may be quite tricky to handle, particularly when using older versions of cargo. There could be a parallel directory, but that would use a very large amount of disk space.

[joshtriplett]

The main difference would be having .cargo/registry/index/github.com-1ecc6299db9ec823.git rather than .cargo/registry/index/github.com-1ecc6299db9ec823/.git; both that and the configuration tell git that it's a bare repository with no working directory. (The current configuration looks to git like a working directory where all the files have been deleted.)

[weihanglo]

I originally felt like this can only be done when we stabilize shallow-clone feature, though now with cache.auto-clean-frequency it should be fine to have git checkouts directories in parallel.

[weihanglo]

Just also note that we might not want to update this too often: #t-cargo/build-integration > Changes to SourceId hashing