Fixes #390

Problem

npx turbo currently fails when in an npm-powered monorepo on Vercel and Netlify because of a user permissions issue related to its use of postinstall script. it works fine with yarn- and pnpm-powered monorepos

Background

The actual turbo npm package is a small wrapper around a postinstall npm script that is responsible for downloading the correct os-specific version of turbo and copying it into position. I (@jaredpalmer) initially copied the script directly from esbuild around March 2021.

Package managers have since started to correctly handle optionalDependencies. esbuild has thus updated its script and its latest iterations fixes the above issue

High Level Solution

• Update/copy turbo's postinstall script to the latest iteration of esbuild's postinstall ( we only care about node.js, as we don't have a browser target) (using optionalDependencies)
  • Copy it to JS so we don't have a new build step
• Ensure that each of the os-target package's is correct and that their package.json have the correctly specified keys
• A custom script will likely be needed when bumping versions of turbo to alter the turbo package's optionalDependencies that should set the versions of each of os-specific packages. This script should also run in the platform-neutral command in turbo's makefile https://github.com/vercel/turborepo/blob/main/cli/Makefile#L112

Resources

• The turbo package's posinstall script
  • For reference, see esbuild's old postinstall script to see differences (they are very very small)
• The turbo package's package.json
• esbuild's current postinstall script (note that it is now split in multiple files)
• esbuild's current package.json
• NPM docs on optionalDependencies
• Yarn 2 docs on optionalDependencies (even though we don't officially support it yet, this script should, since esbuild's script already does)
• Yarn 1 docs on optionalDependencies

Non-goals

• Improve on esbuild's script in any way. Make as few changes as possible.