[opam 2.1~beta2] opam 2.1 does not seem to have an equivalant for opam depext -u

[kit-ty-kate]

In #3975 opam integrated opam-depext into opam.
However currently there doesn't seem to be possible to get the same behaviour as opam depext with the -u argument that was used to have a system agnostic way to update the system repositories. This was a rather useful command to have in some CI situations.

OpamSysInteract.update still exists but is not used anywhere.

[AltGr]

Ah, seems that has been overlooked, should be fairly easy to remedy...
Not sure where it would fit, though, because we probably don't want to do it before all queries to the OS packages (e.g. when the switch state is loaded).
Maybe just opam update --system-packages ?

[kit-ty-kate]

What about reintroducing the opam depext subcommand? Except now it's builtin.

This would bring a more smooth user experience upgrading from opam 2.0 to opam 2.1, would bring back opam depext -l <pkg> to get the name of the system packages to install – useful to, for example, remove them later (this is used in the Travis script in ocaml-repository currently for instance).

[dra27]

The downside of bringing opam-depext in as a builtin, is that the gamut of opam-depext's CLI then has to be emulated, so it's potentially better to put the commands more logically elsewhere. I haven't checked - which options are missing apart from -u and -l?

opam-update seems a logical home, but perhaps with a name that more clearly links it to depexts, than system switches? Would opam update --depexts be better?

[AltGr]

opam depext -l <pkg> is just a wrapper over opam list --external --resolve <pkg>, so that would be for compatibility or convenience, but not a requirement feature-wise.
I still agree with @rjbou's point on this that, would you really need backwards-compatibility, you should remain able to install the plugin, that should actually still work on 2.1, even if it's not needed anymore (it was marked incompatible with opam 2.1 to avoid confusion, though).

I know the CLI is not as clear and streamlined as it could be, but I don't have a good feeling about duplicating features in different CLI areas just for better compatibility with an older plugin;
of course if there are other missing features like depext -u, we should find a way to re-introduce them.

[avsm]

I agree that we shouldn't duplicate CLI functionality.

Putting this in opam update seems logical. Although it's the reverse of the semantics of opam install, we should probably make this not the default, so opam update --with-depexts activates the functionality. The reason is that it's only necessary for CI to have this option, as in the desktop case the user has other mechanisms to ensure their system packages are updated.