Get rid of calls to opam when using opam

[kit-ty-kate]

Currently using opam 2.1~alpha (this is probably a bug but it might break in the future), mirage-solo5 fails to install because it uses opam config var while already being inside opam.

This PR gets rid of the issue by using dune variables. However I didn't found a way to directly get the PREFIX (e.g. ~/.opam/<switch>/) with dune other than relying on the "hack" %{lib:pkgconfig:}/../pkgconfig, so I'm tagging this PR as a draft for now to start a discussion. cc @jeremiedimino @rgrinberg is there a cleaner way to get the opam prefix with dune?

[rgrinberg]

This seems like the best workaround for now. We could add an %{install:lib} variable, but it seems like that's very specialized.

[samoht]

This looks like a bug in opam 2.1~alpha (did you report it upstream?). This mechanism will go away in mirage4 so I'm not sure we really want to workaround that bug.

[hannesm]

I agree with @samoht here, that opam config var {prefix,lib} should not open a log file (and be fine to be called under opam - would be great to re-introduce this behaviour in opam 2.1). The use of opam config var prefix/lib is present in for MirageOS3 "cross"-compilation in roughly a dozen opam packages.

[mato]

What is the status of this and the related mirage/ocaml-solo5#79? Do I understand this correctly, namely that functionality Mirage has been relying on (and will continue to rely on, at least for the lifetime of Mirage 3.x), i.e. the use of $(opam config var XXX) while "inside" opam, has been removed in opam 2.1?

[samoht]

@mato That's a bug in opam 2.1~alpha and should be fixed there. /cc @rjbou

[rjbou]

Calling opam during build is still permitted (with --safe restriction). It is indeed a bug in 2.1.0~alpha that under some circumstances (switch state), a syscall is made. Cf. the corresponding opam issue ocaml/opam#4188.

[hannesm]

I'll close this PR. With #79, opam is only called if available (as in other cross-compilation Makefiles). Furthermore, the "calling opam inside opam" is allowed. And opam var prefix is used instead of opam config var prefix (that is now deprecated). Please reopen if you need further adjustments.