fix(*): expose Bazel INSTALL_DESTDIR to Lua code and fix admin_gui in dev [KM-117]

[sumimakito]

Summary

Exposing INSTALL_DESTDIR in Bazel build via an auto-generated Lua module kong.kong_usr_path.

This fixes an issue where Admin GUI is not correctly prepared for the configured prefix while developing with Bazel's vdev environment.

Checklist

• The Pull Request has tests
• A changelog file has been created under changelog/unreleased/kong or skip-changelog label added on PR if changelog is unnecessary. README.md
• There is a user-facing docs PR against https://github.com/Kong/docs.konghq.com - PUT DOCS PR HERE

KM-117

[fffonion]

I'm slightly worried about the use of LIBRARY_PREFIX here as it's currently only used by dev scripts but now it could has effect in production as well. Some idea to improve it:

• Prefix it with KONG_ to avoid surprises. We will then need to set the new env var in our dev script too.
• Use a hardcoded path when a env var indicating we are in dev env, instead using directly from the value of LIBRARY_PREFIX. This could help us prevent path traversal even if malicious user injected this env var in production.

[sumimakito]

@fffonion I agree with you. I'm converting this PR into a draft and will refactor it. Meanwhile, do we have best practices (e.g., env vars, helpers) to tell if we are in the dev env?

Update: It appears that we cannot hardcode the venv path as it is subject to user overrides on BUILD_NAME.

[fffonion]

We can use BUILD_NAME and use that to construct path. Or it just occurs to me that we can also produce a lua file to populate the prefix, and require that from the file that you change in this PR. So you don't need to rely on the
fact that BUILD_NAME or some other vars are populated, which is done by the venv scripts/may not work in other tooling.

It can be a simple genrule that produce a lua file, something like

genrule(
    name = "kong_path_config",
    outs = ["path/to/the/kong_path_config.lua"],
    cmd = "echo 'return \"%s/kong\"' >$@" %  KONG_VAR["INSTALL_DESTDIR"],
)

then in lua we can just

local the_path = require "path.to.the.kong_path_config"

[fffonion]

Or to take a completely different path, that to start using of datafile to ship admin_gui assets, like the saml plugin did for xml assets would also be a good idea.

[sumimakito]

Thanks for the suggestion. I've appended the echo command to the kong target which would be more straightforward.