depext: opam tries to be intelligent but fails; permit to choose the system package manager

[jbulow]

How to bypass the failing check of host dependencies in opam?

 >which emacs
/usr/bin/emacs
>opam install conf-emacs
The following actions will be performed:
  ∗ install conf-emacs 1

The following system packages will first need to be installed:
    emacs

<><> Handling external dependencies <><><><><><><><><><><><><><><><><><><><><><>
This command should get the requirements installed:

    apt-get install emacs

You may now install the packages on your system.
When you are done: check again and continue? [Y/n] y

The following remain to be installed: emacs
When you are done: check again and continue? [Y/n] y

The following remain to be installed: emacs
When you are done: check again and continue? [Y/n]

[jbulow]

I just found --no-depexts as a way to bypass this check. Maybe mention this in the info-output when not using this flag?

I.e

You may now install the packages on your system. Use --no-depexts to ignore this check.
When you are done: check again and continue? [Y/n] y

[rjbou]

Seems that opam fails to check that emacs defined package is installed. It doesn't check is the binary exists, but according to what is defined in the conf- package checks install status of system package, eg for conf-emacs

  ["emacs-nox"] {os-family = "debian" & os-distribution != "ubuntu"}
  ["emacs"] {os-distribution = "ubuntu"}
  ["emacs-nox"] {os-distribution = "centos"}
  ["emacs-nox"] {os-distribution = "rhel"}
  ["emacs-nox"] {os-distribution = "fedora"}
  ["emacs-nox"] {os-distribution = "alpine"}
  ["emacs-nox"] {os-family = "suse"}
  ["emacs-nox"] {os-distribution = "ol"}
  ["emacs"] {os-distribution = "arch"}

What is your distribution? Did you installed emacs via your distribution package manager? It will help us to see why opam didn't find that package.

On the bypass, it's better to use the --asume-depexts option or directly its setting opam option depext-bypass than disable the whole checking mechanism for a single blocking package.

[jbulow]

I use ubuntu 18.04.6 LTS. Emacs is installed as:

dpkg --get-selections | grep emacs
emacs-snapshot                                  install
emacs-snapshot-common                           install
emacs-snapshot-el                               install
emacsen-common                                  install

In general, it feels weird to to ask the default package manager what software is available. apt is only one way to install softwre. Compiling from source is another. Using snap is yet another option. Using alternative ppa:s with different builds and version (as I did) is also common.

[rjbou]

In most common cases (from all distro point of view), users use their system package manager. It is a good feature to permit user to choose its system package manager between handled ones: snap versus apt for example ; and store it in the config.

[AltGr]

We definitely need to improve the prompts, you're not the first one stumbling here:

When you are done: check again and continue? [Y/n] y

You just had to answer n at that point...

When you are done: check again and continue? [Y/n] n
Do you want to attempt to proceed anyway? [y/N] y
[... goes on assuming emacs is installed and registering it as with `--assume-depexts`]

Maybe something like Do you want to check again (y) or continue (n) ?

[AltGr]

A possibility would be to "flatten" all these embedded y/n questions, resulting in a menu like:

The package needs to be installed with the following command: `sudo apt-get xxx`
1. (y) Let opam do the installation
2. Let me do the installation (then select this to check again)
3. The package is installed by other means, bypass the check and go on (the exception will be registered upon success)
4. Abort

It's a little bit of reading but at least everything is on the table...

[jbulow]

Please, just keep it simple. If a requirement is missing on the host, just tell the user. Don't try to be smart and assume a specific package manager etc.

When checking for a requirement, check the actual requirement and do not assume or guess how a host requirement is registered in the system. If emacs is a requirement, just run "emacs --version" or similar to check the existence of emacs.

The worst solution is when one package manager invokes another package manager to install requirements.

[dra27]

Thoughts from today's dev meeting:

• Generally reducing the number of questions, as proposed in depext: opam tries to be intelligent but fails; permit to choose the system package manager #5026 (comment) is a good idea - then the option would have been visible
• It is already possible to turn off depext support already, however it's on by default because that's something that most users did seem to want! But we could move the turning of that to opam init (on by default) and similarly collapse the number of questions asked by default by opam init

Possible UI issues to look at for 2.3, therefore.

[rjbou]

Fixed by #5053