Adding way to mount unusual path in bwrap sandbox

[ErwanGa]

- add usage OPAM_USER_PATH_RO environment variable for setting
unusual paths in mount bind_ro list

- remove some unusual path from the script (/nix/store /rw/usrlocal)

- fix problem of build in the sandbox
    Some modules generates an error if the complete infra of {opam_root}
    is not seen in the sandbox
         -> [ERROR] /work/CG-2.1/_build/_opam exists, but does not appear
            to be a valid opam root.....
     They also need to write in $(dirname $OPAM_SWITCH_PREFIX)/log
    =>
        Need to mount $(dirname $OPAM_SWITCH_PREFIX) in ro
    but need to mount $(dirname $OPAM_SWITCH_PREFIX)/log in rw

[rjbou]

I'm afraid there is some confusion between OPAMROOT which contains global configuration files and OPAM_SWITCH_PREFIX which contains local switch configuration and installation structure. It's the opam root that contains those files, not the switch prefix. Plus, at build stage, opam don't need to access to its opam root (if so, it's a packaging issue).
I've checked and I can confirm that it works as is: having opam root and opam switch in an unlisted directory, launching an install from another one unrelated.
What is you configuration? You can have internal values of root & switch with opam config list.

[ErwanGa]

Ok
Seems to be package issue.
Shall I do another pr without this part of the patch ?

[rjbou]

Seems to be package issue.

What is the package you tried to install?

Shall I do another pr without this part of the patch ?

No need to create a new one, you can update this PR by updating the branch you used. You can squash commits in order to have a clean history.

[ErwanGa]

I've such issue with package cpuid @ version 0.1.1.

I'll update the pr asap

[rjbou]

Usually, cpuid can be installed as $HOME is ro mounted and it uses opam to access configuration (via ocb-stubblr, cf. above links). The ocb-stubblr PR is not yet merged fixing it, so meanwhile it still need to access opam root, but just in ro.
I tried using the new OPAM_USER_PATH_RO variable set to an unusual opam root and it installed with no problem.

[ErwanGa]

Understood...
PR has been updated. It now only contains the OPAM_USER_PATH_RO patch

[rjbou]

Thanks for the contribution!

I'll ping @reynir to inform about the /rw/usrlocal change.

PR related to #3534.

[rjbou]

Thanks!