[Feature] Fetch dependencies from yarn.lock only

[edsrzf]

• 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

As a developer maintaining a CI pipeline, I want to keep a dependency cache that invalidates only when yarn.lock changes. However, yarn install requires all workspace package.json files in addition to yarn.lock, which means that the cache invalidates more often than it needs to.

It should be possible to populate the cache using only yarn.lock and no package.json files. The documentation for the pnpm fetch command describes exactly my use case and is similar to the solution I'm proposing.

Describe the solution you'd like

I imagine a yarn fetch command. It would perform a version of dependency resolution using only yarn.lock and populate the cache. Later, dependencies could be linked with yarn install --immutable --immutable-cache.

The command could be an external plugin, but currently it's difficult to implement (though possible). Ideally the command would consist mainly of calling Project.resolveEverything, followed by Project.fetchEverything, but currently there are two challenges:

• resolveEverything requires reading all workspace package.json files. There's no way to avoid this except to reimplement the necessary pieces within the plugin, duplicating logic.
• fetchEverything depends on resolveEverything, but mostly works otherwise. WorkspaceFetcher relies on package.json information, but it's not too hard to provide a custom Fetcher implementation to work around this.

There may be other problems; I've only tried writing a plugin that works for my monorepo use case.

Describe the drawbacks of your solution

A little bit of added complexity and increased surface area for the core API.

Describe alternatives you've considered

I've written a plugin that works for my use case, but it has to reimplement various parts of resolveEverything itself in order for fetchEverything to work properly. It feels fragile and unnecessarily complicated. It's hard for me to be sure that I've implemented it correctly since I'm not very familiar with Yarn internals.

[rohit-gohri]

I created a small package that recreates the full workspace structure with dummy package.jsons just from the yarn.lock file. You can use it in Dockerfile to do a yarn install - /~https://github.com/rohit-gohri/yarn-lock-to-package-json

Use like this

COPY .yarn ./.yarn
COPY yarn.lock .yarnrc.yml ./

RUN yarn dlx yarn-lock-to-package-json

RUN yarn install --immutable

This way you don't need to create a special command since yarn install itself works

[devthejo]

here is my plugin (based on yarn-lock-to-package-json):
/~https://github.com/devthejo/yarn-plugin-fetch