[Feature] Tell lifecycle scripts when pack or publish is a dry run #7323

• I'd be willing to implement this feature (contributing guide)
• This feature is important to have in this repository; a contrib plugin wouldn't do

Describe the user story

Some prepack scripts modify files in the working tree. For example, clean-pkg-json removes development-only fields from package.json before packing. During a dry run, these scripts should leave the working tree unchanged.

yarn pack --dry-run and yarn npm publish --dry-run run prepack, but the script can't tell that it's a dry run. So a dry run modifies files the same way a real pack does.

Reproduction with Yarn 4.18.1:

{
  "name": "dry-run-env",
  "version": "1.0.0",
  "packageManager": "[email protected]",
  "scripts": {
    "prepack": "node -p process.env.npm_config_dry_run > prepack-output.txt"
  }
}

yarn pack --dry-run
cat prepack-output.txt # undefined

The full environment of prepack is the same for yarn pack --dry-run and yarn pack, except for per-run temporary paths. In the source, prepareForPack runs prepack and postpack without the command's dryRun option (packUtils.ts#L56-L68). This is unchanged on master.

Describe the solution you'd like

When yarn pack or yarn npm publish runs with --dry-run, set npm_config_dry_run=true in the environment of the prepack and postpack scripts.

npm sets this variable for npm pack --dry-run and npm publish --dry-run, and pnpm 10 did too. Using the same name means existing scripts work with Yarn without changes.

Describe the drawbacks of your solution

• It adds an npm_config_* variable, while Yarn otherwise sets only a small set of well-known npm_* variables. A Yarn-specific name would avoid that, but scripts would then need to check a different variable for each package manager.
• It is one more variable to keep in sync with the --dry-run flags of both commands.

Describe alternatives you've considered

• A contrib plugin: a plugin can add variables with the setupScriptEnvironment hook, but the hook receives only the project and the environment. It can't see the --dry-run flag of the core pack and npm publish commands.
• The beforeWorkspacePacking hook: a plugin can change the packed manifest without touching the file on disk. That covers package.json changes, but it is Yarn-specific, so tools that support npm, pnpm, and Yarn run from prepack.
• Reading the parent process's arguments from the script: this depends on the platform, and it breaks when Yarn runs through Corepack or another wrapper.
• Asking users to set their own variable, such as MY_DRY_RUN=1 yarn pack --dry-run: users have to remember it, and forgetting it modifies their files.