yarn-plugin-cache-download.cjs

module.exports = {
  name: 'plugin-cache-download',
  factory: (require) => {
    const { BaseCommand } = require('@yarnpkg/cli');
    const { Command } = require('clipanion');
    const { Project, Configuration, Cache, ThrowReport } = require('@yarnpkg/core');

    class CacheDownloadCommand extends BaseCommand {
      static paths = [['cache', 'download']];

      static usage = Command.Usage({
        description: 'Download all packages from lockfile and cache them',
        details:
          'This command only needs a yarn.lock to work; it will download all packages listed in the lockfile and cache them. This is useful for Docker images to split the download and linking steps.',
      });

      async execute() {
        // Retrieve the active project
        const configuration = await Configuration.find(this.context.cwd, this.context.plugins);
        const { project } = await Project.find(configuration, this.context.cwd);

        // This is the thing that will do the fetching
        const fetcher = configuration.makeFetcher();
        const fetcherOptions = {
          checksums: project.storedChecksums,
          project,
          cache: await Cache.find(configuration),
          fetcher,
          report: new ThrowReport(),
        };

        // Get the list of all packages that can be downloaded
        const remotePackages = [...project.originalPackages.values()].filter(
          ({ reference }) => !reference.startsWith('workspace:'),
        );

        this.context.stdout.write('Starting all downloads in parallel...\n');

        // Fetch them all in parallel, without much consideration for limits
        await Promise.all(remotePackages.map((pkg) => fetcher.fetch(pkg, fetcherOptions)));

        this.context.stdout.write('All packages cached!\n');
      }
    }

    return { commands: [CacheDownloadCommand] };
  },
};

Yarn 4 plugin to provide the yarn cache download command as described in yarnpkg/berry#5998