RFC: a mechanism to declare runtime environment variables at build time

[robbkidd]

Problem:

$(hab pkg path origin/thingie) in a hook does not guarantee that that the path returned is the version the hook's package was built against. The presence of this command in a hook should be considered a code smell.

habitat-sh/core-plans > ack --files-with-matches "hab pkg path" | ack "hooks" | wc -l
      10

First notion I can find on this problem is a forum post from @elliott-davis.

Affects:

Anyone running multiple habitat packages on a single host, the bare-metal scenario. Should not affect those running habitat packages exported to containers.

Solutions Today:

• Paths can be computed at build time and ...
  • written out to wrapper scripts that act like binstubs for the real binaries, see this pattern in wrap_with_cert_path (used for cargo-nightly and rust) and write_shim (used for github_changelog_generator)
  • written out to a file that is sourced by hooks, see notional work on packaging Supermarket with Habitat (write out in plan and then source it in a hook)

Solution Idea

Throwing a noodle at the wall:

• A do_pkg_exports function that would write out given exports from a plan to an EXPORTS file at $pkg_prefix.
  • The generator for the default run hook would source this EXPORTS file.
  • A make_binstub (similar to fix_interpreter) to be called during build/install to rename an original binary (like in wrap_with_cert_path & write_shim) and write out a binstub/wrapper/shim to the original binary's location that sources the EXPORTS file.

[smurawski]

Currently the hab pkg path problem should be helped with a new helper pkgPathFor, but the greater problem remains and is one that we need to solve.

[robbkidd]

@christophermaier 👋 Heeeey ... runtime env vars!