[Feature Request] Use shallow clone for git repositories

[kingsamchen]

CPM currently clones full history for git-repository dependencies, which takes more time than necessary (in most cases) on downloading, and consumes more disk space.

Maybe use git shallow clone by default for those dependencies?

There is a GIT_SHALLOW option to enable --depth=1 with clone, which does what we exactly desire.

If I may, I would like to make a PR on it, :-)

[TheLartians]

Hey thanks for the suggestion and PR! I agree that it would be great to have GIT_SHALLOW enabled by default! However, as you can see from the CI failure, enabling GIT_SHALLOW can lead to unexpected failures as CMake fails to checkout certain branches or commits for some reason.

Until that is resolved, imo it's safer to keep the current approach, enabling GIT_SHALLOW explicitly on a per-dependency basis, with knowledge that CMake will be able to successfully check out that specific commit. (which can be activated by providing the GIT_SHALLOW ON argument together with the dependency source parameters in CPMAddPackage)

[kingsamchen]

According to this answer https://stackoverflow.com/questions/31278902/how-to-shallow-clone-a-specific-commit-with-depth-1 clone with depth 1 won't work on a specific commit HASH. That is why the yaml example failed.

Maybe making GIT_SHALLOW a per-dependency option is a more practical approach.

[TheLartians]

According to this answer https://stackoverflow.com/questions/31278902/how-to-shallow-clone-a-specific-commit-with-depth-1 clone with depth 1 won't work on a specific commit HASH. That is why the yaml example failed.

Shallow fetching an actual git tag seems to work afaict, perhaps we can enable shallow clones by default behaviour when we detect the GIT_TAG to be an actual tag as opposed to a hash. E.g. by checking the length and format.

[kingsamchen]

A wonderful idea.

And maybe it is easier to identify out a hash from tag/branch value, because commit hash has fixed length and contains no characters other than hex digits.

While tags from different projects could use varied formats.

E.g for standalone ASIO, its release tag is in the form of asio-1-23-4; it doesn't even have prefix v, and yet uses - instead of conventional . as the version delimiter.

[kingsamchen]

@TheLartians Hi, I just made another commit on enabling shallow clone conditionally. Could you please review the commits when you are free?

Any suggestion is welcome

[TheLartians]

Feature added in #130.