Create intermediate symlink for switching between build and launch modules

[thitch97]

Summary

Avoids changing /workspace permissions, creating instead a symlink in /tmp which points to build modules during build and launch modules at launch. The /tmp symlink is pointed to by the node_modules symlink in /workspace (i.e. /workspace/node_modules -> /tmp/node_modules -> {build, launch}-modules layer).

The exec.d script is now only run when node_modules is required for both build and launch.

Use Cases

This should resolve paketo-buildpacks/nodejs#602, while still conforming to the Secure Runtime RFC requirements.

Checklist

• I have viewed, signed, and submitted the Contributor License Agreement.
• I have linked issue(s) that this PR should close using keywords or the Github UI (See docs)
• I have added an integration test, if necessary.
• I have reviewed the styleguide for guidance on my code quality.
• I'm happy with the commit history on this PR (I have rebased/squashed as needed).

[thitch97]

It's worth capturing here that Angular apps have been observed to fail on stacks with distinct build and run uids as the CLI attempts to create/modify a cache in /workspace. This can be rectified by setting the cache path in the project's configuration (angular.json).

[ryanmoran]

The exec.d script is now only run when node_modules is required for both build and launch.

@thitch97 Can you explain this choice? What happens if only launch is specified?

[thitch97]

Can you explain this choice? What happens if only launch is specified?

@ryanmoran My thinking here is that if the setup-symlinks exec.d script is solely responsible for ensuring that /workspace/node_modules points to /layers/.../launch-modules/node_modules instead of /layers/.../build-modules/node_modules at launch, then there should be no need to run it if only build OR launch is specified. If only launch is specified, for example, /workspace/node_modules will already point to /layers/.../launch-modules/node_modules at runtime (the build-modules layer wouldn't even exist).

[ryanmoran]

@thitch97 thanks for including that context.