Reinstall plugin package if the symlink is missing

[dra27]

This is a partial fix for #4619 (the other part would be to remove the plugin symlinks at format upgrade).

This tweaks the plugin detection slightly so that if the symlink is missing, the command is still resolved, but package will be reinstalled in the current switch. This will recreate the symlink and also apply any changes to dependencies (e.g. after an opam client upgrade).

It's demonstrated by this slightly convoluted Dockerfile - commenting out the final rm command shows the existing error.

FROM ocaml/opam:debian-ocaml-4.12
WORKDIR /src
RUN sudo chown opam /src
RUN git clone https://github.com/dra27/opam.git
WORKDIR /src/opam
RUN opam pin add -n dose3 5.0.1-1
RUN git checkout reinstall-plugins ; opam install dose3 opam-file-format mccs cmdliner
RUN opam exec -- ./configure && opam exec -- make
WORKDIR /src/test
RUN sudo chown opam /src/test
RUN opam install dune
RUN opam exec -- dune init project foo
RUN echo > dune-project '(lang dune 2.8)\n(generate_opam_files true)(source (github foo/bar))(package (name foo))'
RUN opam exec -- dune build
RUN opam install opam-state.2.0.8
RUN opam install opam-dune-lint --deps-only
RUN opam pin -yn opam-dune-lint 'https://github.com/ocurrent/opam-dune-lint.git#bf451c8cf2d4f2be3c7d979b613c79588b55a172'
RUN opam dune-lint
#RUN sudo rm /usr/bin/opam && sudo ln /usr/bin/opam-2.1 /usr/bin/opam
RUN sudo rm /usr/bin/opam && sudo cp /src/opam/opam /usr/bin/opam
RUN opam update
RUN rm ~/.opam/plugins/bin/*
RUN opam dune-lint

[AltGr]

Nice!

[AltGr]

Won't this cause problems if the currently installed version is no longer available ? This could happen e.g. with available: depending on opam-version after an opam root upgrade, where an upgrade of the plugin becomes mandatory, and cause an error during resolution, requiring a manual opam upgrade.
I don't remember why this code used to force the latest version (within availability) though

[dra27]

It turns out it doesn't, but I think that's a bug - the availability check is not done for opam reinstall!

The alternative would be to upgrade the package by name instead?

[dra27]

I don't remember why this code used to force the latest version (within availability) though

OPAM 1.x days? (easing or even avoiding the solver?)

[rjbou]

The candidates package already have the availability check, so if a plugin is no more available it won't be in installed and its latest version will be "newly" installed:

$ opam install opam-depext.1.1.3 #without opam < 2.1 condition
$ opam update # reintroduce opam < 2.1 condition
$ rm <link>
$ opam depext
Opam plugin "depext" is not installed. Install it on the current switch? [Y/n] y
opam-depext.1.1.5 is not installed. Install it? [Y/n] y
The following actions will be performed:
  ↗ upgrade opam-depext 1.1.3 to 1.1.5

<><> Processing actions <><><><><><><><><><><><><><><><><><><><><><><><><><><><>
⬇ retrieved opam-depext.1.1.5  (cached)
⊘ removed   opam-depext.1.1.3
∗ installed opam-depext.1.1.5
Done.

On the availability check for reinstall, this (specific) case is handled by the orphans mechanism, and can't be reinstalled:

$ opam install opam-depext.1.1.3 #without opam < 2.1 condition
$ opam update # reintroduce opam < 2.1 condition
$ opam reinstall opam-depext
[ERROR] Sorry, these packages are no longer available from the repositories: opam-depext (= 1.1.3)

[dra27]

Need to double-check the reinstall question above, but otherwise this is good to go.

[rjbou]

A rebase and good to merge for me!

[dra27]

I agree - the reinstall question is already fixed - IIRC I tested on an old opam!