Disable caching in OpamSystem.resolve_command

[dra27]

OpamSystem.resolve_command caches results. This cannot work - the ocamlfind package, for example, installs an ocaml script which is intended to wrap the system-installed ocaml command. This script is not invoked if ocaml has been previously executed, because the path to the system-installed ocaml command is cached.

There are various shims which could mitigate this for opam-installed binaries/scripts only, but it seems better just not to cache the results at all.

[AltGr]

Nicely spotted!
The cache was indeed initially planned for commands used internally (e.g. tar, patch, git, etc.) where it really should be harmless (and makes the output of strace a bit tidier). But yes, applying it to packages' command scripts is a big mistake indeed.

Could it be worth adding an optional argument to disable the cache only for commads from scripts ?

[dra27]

I think it's correct to view it as an unnecessary optimisation (i.e. running those commands will take vastly more time than locating them). Personally, I'd drop it completely - it's already been a subtle issue, and it feels like it could be a subtle one in future (imagine if/when ocaml-git starts installing a git command and we expect that to be used).

If opam's internal invocations want to cache the location of tar, patch, git and so forth then I think it should do that at opam init time and record the location of the tool in a config file/cache (invalidated only if the tool subsequently can't be found). i.e. it should be more like a configurable option (like MAKE=make, etc.)

[avsm]

If this PR could be merged soon, it'll make a huge usability difference to me. Right now local switches are very unreliable on the Mac with a system switch due to this.

[AltGr]

Yup, merging, I'll consider re-adding for non-script commands later. Thanks!