Provide custom manifest name via JULIA_MANIFEST_NAME env var

[IanButterworth]

Allows providing a custom manifest name via the JULIA_MANIFEST_NAME env var, which can have placeholder # chars that will be replaced by the VERSION fields.

% JULIA_MANIFEST_NAME=Manifest.#.#.toml julia -e "import Pkg; Pkg.status(mode=Pkg.PKGMODE_MANIFEST)"
Status `~/.julia/environments/v1.9/Manifest.1.9.toml` (empty manifest)

Requires JuliaLang/Pkg.jl#3000

Discussed in #43845 (comment)

Closes #43845

[DilumAluthge]

I imagine that Pkg (and any other users of the Base.manifest_names tuple) will need to be patched to use the Base.manifest_names() function instead?

[IanButterworth]

Yes. Just added a link to the Pkg branch

[DilumAluthge]

Once JuliaLang/Pkg.jl#3000 is merged, we'll bump the Pkg version on master, and once that bump PR is merged, we'll rebase this PR. Then CI will be able to pass on this PR.

[DilumAluthge]

@vtjnash Since you originally requested this feature, what do you think about the new interface?

[DilumAluthge]

If the length of the tuple we return is going to be variable, then we should go back to vectors instead, so that the manifest_names function will be type-stable.

[fredrikekre]

I think this could be populated on startup instead, and still be a global constant called manifest_names. I would think that would help tooling which uses this too.

[fredrikekre]

Should this split on : and replace empty parts with the default like load path?

I also don't think this functionality is that necessary, what is the problem with falling back to Manifest.toml if a versioned one doesn't exist?

[IanButterworth]

Just checking, do we want to get this figured out for 1.9 given weak deps have landed, and 1.9 manifests with weakdeps will be quite different to prior julia versions?

Over time I've actually come to prefer the original idea of just ranking version-named manifests higher automatically rather than requiring the user to specify an env var.

so

const manifest_names = ("JuliaManifest$(VERSION.major).$(VERSION.minor).toml", 
                        "Manifest$(VERSION.major).$(VERSION.minor).toml",
                        "JuliaManifest.toml", 
                        "Manifest.toml")

[vchuravy]

Over time I've actually come to prefer the original idea of just ranking version-named manifests higher automatically rather than requiring the user to specify an env var.

Yeah agreed that seems to me to be sensible behavior.

[IanButterworth]

Ok. Reviving #43845 in place of this