[FIXES #2096] fix relative symlinks of symlinks

[stefanpenner]

Summary

This makes yarn link work when $HOME/.config/yarn/link/ is itself a symlink.

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The following commit forces symlinks to be relative: 36d73cc

This isn't ideal, although it enables some portability it breaks others. For example, it is not uncommon for $HOME/.config itself to be be a symlink: $HOME/.config -> $HOME/src/stefanpenner/dotfiles.

Now when running yarn link inside $HOME/src/ember-cli/ember-cli we end up with:

path.relative('/Users/spenner/.config/yarn/link/ember-cli', '/Users/spenner/src/ember-cli/ember-cli');
=> '../../../src/ember-cli/ember-cli'

Which results in the link located at: /Users/spenner/.config/yarn/link/ember-cli pointing to
/Users/spenner/.config/src/ember-cli/ember-cli rather then /Users/spenner/src/ember-cli/ember-cli

This is because /Users/spenner/.config is actually a symlink pointing to /Users/spenner/src/stefanpenner/dotfiles/.config/

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This PR mitigates he issue raised in #2096, but reifying the paths via realpathSync before deriving the relative path. This ensures that at the time of linking, the paths are correct. This doesn't fix all issues, e.g. symlinks cannot always be changed after this reification.

Test plan

src/lib/fs.js appears currently untested... if this is an approach you all would like to take I can add some tests.

[arcanis]

I've just stumbled upon this PR, but I don't understand it.

path.relative('/Users/spenner/.config/yarn/link/ember-cli', '/Users/spenner/src/ember-cli/ember-cli');
=> '../../../src/ember-cli/ember-cli'

Which results in the link located at: /Users/spenner/.config/yarn/link/ember-cli pointing to
/Users/spenner/.config/src/ember-cli/ember-cli rather then /Users/spenner/src/ember-cli/ember-cli

I don't understand this last sentence. If /Users/spenner/.config/yarn/link/ember-cli is a symlink to ../../../src/ember-cli/ember-cli, then realpath("/Users/spenner/.config/yarn/link/ember-cli") will yield /Users/spenner/src/ember-cli/ember-cli as it should be, not /Users/spenner/.config/src/ember-cli/ember-cli as you report. The realpath doesn't change anything here.

@stefanpenner Do you recall more details? This realpath doesn't make sense to me and cause issues when one of the path component is invalid for whatever reason.

[stefanpenner]

I don’t really recall. I believe my pr was simply the best of two evils. IMHO the entire endeavor of changing how fs symlink works is fraught with peril, and should be truely reconsidered