yarn incorrectly uses $XDG_DATA_HOME for linkFolder

[asottile-sentry]

yarn's default for linkFolder is in the default for $XDG_CONFIG_HOME:

$ yarn config current | grep linkFolder
  "linkFolder": "/Users/asottile/.config/yarn/link",

the default for XDG_CONFIG_HOME is ~/.config

applying XDG_DATA_HOME changes the value of linkFolder even if XDG_DATA_HOME is set to the default:

$ XDG_DATA_HOME=$HOME/.local/share yarn config current | grep linkFolder
  "linkFolder": "/Users/asottile/.local/share/yarn/link",

this appears to be two bugs -- fixing either would resolve this:

• yarn/src/constants.js

  Line 63 in 158d96d

  export const LINK_REGISTRY_DIRECTORY = path.join(DATA_DIRECTORY, 'link');

  (this is using the data home as the joined value for linkFolder)

• yarn/src/util/user-dirs.js

  Line 22 in 158d96d

  return FALLBACK_CONFIG_DIR;

  getDataDir falls back to ~/.config

[Daniel15]

Good point, and in fact there's a comment in the code that alludes to this:

yarn/src/util/user-dirs.js

Lines 19 to 21 in 158d96d

	// Instead, use our prior fallback. Some day this could be
	// path.join(userHome, '.local', 'share', 'yarn')
	// or return path.join(WIN32_APPDATA_DIR, 'Data') on win32

I think the old fallback was kept for backwards compatibility.

The XDG spec says:

$XDG_DATA_HOME defines the base directory relative to which user-specific data files should be stored. If $XDG_DATA_HOME is either not set or empty, a default equal to $HOME/.local/share should be used.

So it should be falling back to .local/share/yarn/link.