Yarn 2: cache is deleted before pruning dev dependencies, making build slow

Describe the bug
If you use yarn berry, the buildpack deletes the cache before pruning dev dependencies:

heroku-buildpack-nodejs/lib/dependencies.sh

Line 183 in 70af836

rm -rf "$cache_dir"

The result is that my builds take way longer than they should because while the first cached build is super fast, when it prunes dev deps it ends up re-downloading all my deps. This sucks! I don't really understand the justification for deleting the cache there, but it adds around 2+ minutes to my builds for a task that should happen near instantly.

~~For now I'll just disable pruning with the YARN_PRODUCTION=false flag.~~ That doesn't work either, #906

Versions (please complete the following information):

Heroku Stack: heroku-18
Node Version: 14
NPM or Yarn Version: yarn 2
Buildpack Version: latest from git

The cache is deleted because running the prune command does not actually remove the devDependencies. Right now, the cost of a smaller slug size is a longer build, but we can continue to investigate this to figure out if there's a better way to do this.

Discussion on the yarn berry package suggests that yarn maintainers went the route of not building in this functionality but allowing people to offer it as a yarn plugin:

If you're open to using that plugin, could make it possible fairly easily!

@danielleadams, @heroku: is there a fix for this, or any movement? our build times have significantly decreased because of it, and i'm concious that it's just putting more pressure on yarn mirrors which are often provided out of generosity- downloading again because you've purged a cache is not very economical.

@imajes unfortunately, I left Heroku, so I'm no longer on this Issue.

You can work around this by uninstalling the workspace-tools plugin before this step.

heroku-buildpack-nodejs/lib/dependencies.sh

Lines 177 to 189 in 5b3fac9

    
           if has_yarn_workspace_plugin_installed "$build_dir"; then 
        
             echo "Running 'yarn workspaces focus --all --production'" 
        
             meta_set "workspace-plugin-present" "true" 
        
             # The cache is removed beforehand because the command is running an install on devDeps, and 
        
             # it will not remove the existing dependencies beforehand. 
        
             rm -rf "$cache_dir" 
        
             monitor "yarn-prune" yarn workspaces focus --all --production 
        
             meta_set "skipped-prune" "false" 
        
           else 
        
             meta_set "workspace-plugin-present" "false" 
        
             echo "Skipping because the Yarn workspace plugin is not present. Add the plugin to your source code with 'yarn plugin import workspace-tools'." 
        
           fi

Add in package.json scripts:

    "heroku-postbuild": "yarn plugin remove @yarnpkg/plugin-workspace-tools",

@joshwlewis I think this is the thing you mentioned that is still remaining to fix wrt Yarn 2 parity after #978 got merged.

Thanks for fixing this, v193 appears indeed to work. Hopefully a future v195 will contain the revert of the revert in v194!

@lizthegrey v195 release does contain #999. It's currently running for only 20% of builds but we'll roll it out more broadly if it appears to handle pruning without tripping over the edge case that caused us to revert #990.

I'm confused--this is still a problem, no? Can this issue be reopened? See #1056 as an example

	if has_yarn_workspace_plugin_installed "$build_dir"; then
	echo "Running 'yarn workspaces focus --all --production'"
	meta_set "workspace-plugin-present" "true"

	# The cache is removed beforehand because the command is running an install on devDeps, and
	# it will not remove the existing dependencies beforehand.
	rm -rf "$cache_dir"
	monitor "yarn-prune" yarn workspaces focus --all --production
	meta_set "skipped-prune" "false"
	else
	meta_set "workspace-plugin-present" "false"
	echo "Skipping because the Yarn workspace plugin is not present. Add the plugin to your source code with 'yarn plugin import workspace-tools'."
	fi