`depext` installation failure results in error code 0

[Sudha247]

When the command to install only depexts fails, the shell command produces an error code 0, not classifying it as an error. For example, on ocaml-cohttp, this is what happens:

$ opam install ./ --depext-only --with-test --with-doc
[NOTE] Package cohttp-lwt-unix is already installed (current version is 6.1.0).
[NOTE] Package cohttp-lwt is already installed (current version is 6.1.0).
[NOTE] Package cohttp is already installed (current version is 6.1.0).
[NOTE] Package http is already installed (current version is 6.1.0).
[ERROR] Package conflict!
  * No agreement on the version of ocaml-base-compiler:
    - (invariant) -> ocaml-base-compiler >= 5.3.0
    - cohttp-async -> ocaml < 5.3.0 -> ocaml-base-compiler < 5.2.2~
    You can temporarily relax the switch invariant with `--update-invariant'
  * Incompatible packages:
    - (invariant) -> ocaml-base-compiler >= 5.3.0
    - cohttp-async -> ocaml < 5.3.0 -> dkml-base-compiler
  * Incompatible packages:
    - (invariant) -> ocaml-base-compiler >= 5.3.0
    - cohttp-async -> ocaml < 5.3.0 -> ocaml-system >= 3.09.1
  * Incompatible packages:
    - (invariant) -> ocaml-base-compiler >= 5.3.0
    - cohttp-async -> ocaml < 5.3.0 -> ocaml-variants
  * Missing dependency:
    - cohttp-async -> ocaml < 5.3.0 -> ocaml-variants -> xenbigarray
    unknown package
  * Missing dependency:
    - cohttp-async -> ocaml < 5.3.0 -> ocaml-variants -> ocaml-beta
    unmet availability conditions: 'enable-ocaml-beta-repository'
  * Missing dependency:
    - cohttp-async -> ocaml < 5.3.0 -> ocaml-variants -> system-msvc
    unmet availability conditions: 'os = "win32"'

And looking at the error code:

$ echo $?
0

It would be useful if opam could classify this as an error and assign an error code.

[kit-ty-kate]

This is specifically the current buggy behaviour of --depext-only:

$ opam install core_unix.v0.17.0 ; echo $?
[ERROR] Package conflict!
[...]

No solution found, exiting
20
$ opam install core_unix.v0.17.0 --depext-only ; echo $?
[ERROR] Package conflict!
[...]

0

The behaviour has been there the since the introduction of the option in 345b475.
This commit introduces an explicit exception to the rule of checking for the result, just for the option and i'm not sure why. cc @rjbou?

Actually i'm not sure why --depext-only tries to install (even simulated) the package. Shouldn't we just need to list the depexts, install them and exit?

[rjbou]

The behaviour has been there the since the introduction of the option in 345b475. This commit introduces an explicit exception to the rule of checking for the result, just for the option and i'm not sure why. cc @rjbou?

It is indeed introduced intentionally in #4238 (see #4238 (comment)). I think it was a mistake/misunderstanding at the time, as the checked solution in case of conflict is just a print of conflict & exit with not found.

Actually i'm not sure why --depext-only tries to install (even simulated) the package. Shouldn't we just need to list the depexts, install them and exit?

To list the depexts to install, we need to compute a solution, which is done here, we don't simulate or install the solution. We use install_t but with depext_only parameter to true it only computes the solution and installs the depexts of the packages in the solution.