Fix support for Yarn Plug'n Play in the plugins execution

[ehmicky]

@netlify/build runs yarn on two instances, which are both opt-in behavior:

• When the @netlify/plugin-functions-install-core plugin is used, we run yarn in the functions directory
• When the @netlify/plugin-local-install-core plugin is used, we run yarn in any local plugins directory

In both cases, we detect yarn by looking for the presence of a yarn.lock. This does not work in many cases, which is why we might want to also check for the presence of a USE_YARN environment, as suggested in netlify/build-image#440

Also, the command we are running is yarn install --no-progress --non-interactive --cache-folder=/opt/buildhome/.yarn_cache. According to netlify/build-image#440, this has few issues:

• --cache-folder was replaced with the environment variable YARN_GLOBAL_FOLDER in Yarn v2
• --no-progress and --non-interactive do not exist anymore

[Embraser01]

Thanks for looking at this!

A few notes I forgot to mention yesterday:

• YARN_GLOBAL_FOLDER is the folder where everything global to yarn 2 is stored. For now I think it is only a cache folder so not really a problem.
• You can force disable progress bars with this option (YARN_ENABLE_PROGRESS_BARS=false) even though it is already disabled on CI env
• Omitting the --non-interactive option should not cause any problems as yarn install should never ask for anything
• Even though caching $YARN_GLOBAL_FOLDER/cache might be enough, I'd like to note that some install could have a "local" cache folder (aka Offline mirrors in v1) that is or isn't committed in the repo. It is used mainly for PnP installs (the default in v2) and caching this folder (when not committed in vcs) instead of the global one should be more efficient. node_modules installs should use enableGlobalCache: true so it shouldn't matter much for those.

Also, how could we detect the usage of Yarn v2? Another env var?

Actually the easiest and more reliable way would be too run yarn --version and parse result to check version

[ehmicky]

Thanks a lot for this information, this is very helpful! ❤️

[gregberge]

I am using yarn v2 on xstyled project, it works fine but it looks like caching is not really effective. See this build: https://app.netlify.com/sites/xstyled/deploys/60584152ce7ae70008522e1f

I saw that I can configure a custom cache folder using environment variables but I think dependencies caching should work out of the box for common package managers like yarn.

[mosesoak]

Hi @gregberge and the Netlify team. We were considering trying out Yarn 2, which was released at the beginning of 2020, but are concerned about how little support Netlify seems to have for it.

Wanted to draw your attention to the fact that there's been a few posts to your forums over the last year without any actual movement. One of them claims that adding an extra path configuration file to the repo makes it at least buildable with Yarn 2. https://answers.netlify.com/t/using-the-new-yarn-release-2-0-0-berry/8270

But the issue notes above indicate that there's v1-specific yarn install code baked into Netlify, so I sort of doubt those workarounds do all that much. (I don't even know if you're supposed to run yarn install in v2, since the idea with it is that deps are pre-bundled in the repo.)

All of these things make our team wary of even trying it out while using Netlify, although we'd like to -- Yarn 2 is a major version of one of the most popular package managers after all. Any progress to report here? Thanks!

[panmona]

@mosesoak
I agree with your general points just wanted to add something to the following:

One of them claims that adding an extra path configuration file to the repo makes it at least buildable with Yarn 2.

I was able to verify that this works for one of my projects.

---

I don't even know if you're supposed to run yarn install in v2, since the idea with it is that deps are pre-bundled in the repo.

You're probably referring to zero installs here and that is something that is completely optional (see more here). Netlify should probably verify if a project is using zero installs and then don't execute yarn install.