Set OPAMCLI=2.0 by default during compilation/install/remove phases

[kit-ty-kate]

People who want to use the 2.1 features should be using --cli=2.1 or build-env: OPAMCLI = "2.1"

Tested successfully using the following opam files:

opam-version: "2.0"
build: ["env"]
install: ["env"]
remove: ["env"]

opam-version: "2.0"
build-env: [OPAMCLI = "2.1"]
build: ["env"]
install: ["env"]
remove: ["env"]

Fixes #4491

[kit-ty-kate]

I've been trying to add tests for this, but I'm unsure where I should put it. Any idea?

[rjbou]

I've added a test in cli versioning one, can you check that it is exhaustive, what you had in mind to test ?
I'm not completely convinced with those test as they need an opam system installed with cli versioning, I'd prefer not to depend on that (even if it will be the usual case).

[dra27]

LGTM, thank you!

Note that OPAMCLI=2.1 should never go in a released file (apart from anything else, it generates a warning). All new package descriptions should expect to use --cli=2.1

[dra27]

I haven't reviewed the reftests PR yet - do they use the system-installed opam at any point rather than the one just built?

[rjbou]

@dra27 I don't think. In reftests, it's the newly compiled opam that is used to launch command to test, a substitution is done. But in test added in this PR, I want to test an opam call inside an opam package, and these call use the system installed opam. The requirement for the installed opam is to have cli versioning working because test output is based on its error messages.

[kit-ty-kate]

I've changed the tests. Feel free to add more if you feel like it should but as is I think this should be good to merge.

[AltGr]

This seems sensible; my worry is that we really don't want to encourage calls to opam from within opam packages.

[kit-ty-kate]

my worry is that we really don't want to encourage calls to opam from within opam packages.

I think it is way too late to worry about this right now. People started using it this way years ago.
This PR is about adding a sane default to opam cli system and allow hundreds of packages in opam-repository to compile again with opam.2.1.0~beta4 (coming today?)

[avsm]

I agree with @kit-ty-kate -- it's use is common (even though we want to discourage it). I'm in favour of this change.

[avsm]

Just one note: this definitely needs to be documented in the manual somewhere.

[kit-ty-kate]

Good point. There was no section documenting the default environment variables so I've documented this in #4496. (separately because I'd rather avoid blocking this PR and the release on documentation)

[dra27]

I also agree - the stable door is definitely off its hinges! CLI versioning and configuration information in the build are orthogonal features (even if CLI versioning does make using the opam command in build scripts less brittle)

[dra27]

Thanks both for the idea and the PR, @kit-ty-kate!