Provide a way to substitute values from volatile-status.txt

[aiuto]

Use case

• I have a build that creates binary with a versioned name. E.g. server.YYYYMMDD
• I want the .deb file to include that time stamp in the name
• Within the deb package, I have an /etc/init script that has a symlink to server.yymmdd.
• How can I get pkg_mklink() to do that? Likewise for pkg_mkdirs.

Obvious constraint:

• You can't read bazel-out/volatile-status.txt at analysis time, when much of mappings takes place

Idea 1

Have all the writers be able to read volatile-status and late substitute
Yeech. That fan out is infinite.

Idea 2

Add a manifest rewriter step that can do that. If any of the providers have a need substitute bit, invoke it

Idea 3

Do it at analysis time by

• having the tool that creates workspace status also write a .bzl file with a well known name.
• custom pkg_* rules that load the .bzl file and do the substitution.

Could we generalize that so people don't need their own rules?
Maybe:

• add PackageVariablesInfo as an input to more rules.
• user creates custom rule to load that .bzl file and return the provider.

[aiuto]

Idea 4 - import values from the environment in a controlled way.

• Create a repo rule to import select environment variables and make them available to rules.

  • PoC: https://github.com/DataDog/datadog-agent/blob/main/bazel/repo/env_vars.bzl

• wrap pkg_* rules to pull in only those things you wank.

  • Poc with pkg_mklink: https://github.com/DataDog/datadog-agent/blob/main/bazel/rules/dd_agent_pkg_mklink.bzl

• This does not pollute the build configuration

• It does materialize the values to disk, so it is OK for non-secrets like git commit or build id, but not for keys.