Change global prefix directory to /usr/local on OS X

[KishanBagaria]

When node is installed from Homebrew, node exists at /usr/local/Cellar/node/7.10.0/bin/node

So yarn's global bin directory (yarn global bin) becomes: /usr/local/Cellar/node/7.10.0/bin

$ node -e 'console.log(process.execPath)'
/usr/local/Cellar/node/7.10.0/bin/node
$ yarn global bin
/usr/local/Cellar/node/7.10.0/bin

This creates two problems:

1. /usr/local/Cellar/node/(version)/bin is not present in PATH by default. Users have to modify their profile to extend PATH with yarn global bin.

2. When node is upgraded, all linked global binaries become inaccessible since yarn global bin automatically gets changed to /usr/local/Cellar/node/(latest-version) from /usr/local/Cellar/node/(old-version). The global bin directory should remain constant on upgrades.

The fix is to simply use /usr/local/bin as the default global bin directory in OS X installations. NPM (npm --global prefix) also uses that directory (at least when installed from Homebrew).

This should also fix #1194.

Alternatives

1. Instead of all OS X installations, this change can be applied only to installations where node is installed from Homebrew, by checking if process.execPath begins with /usr/local/Cellar. Although, this issue would still be present with nvm or some other node-installer.

2. Instead of modifying yarn, a patch could be made in the Yarn formula to automatically run yarn config set prefix /usr/local. (EDIT: It's not possible atm: Yarn global packages with brew's node.js no longer work on node update. Homebrew/homebrew-core#7283 (comment)) Other node-installers like nvm will also require similar changes.

3. In the getGlobalPrefix function, instead of

let prefix = path.dirname(path.dirname(process.execPath)); //=> /usr/local/Cellar/node/7.10.0

this could be used:

const whichNode = execa.sync('which', ['node']).stdout; //=> /usr/local/bin/node
let prefix = path.dirname(path.dirname(whichNode)); //=> /usr/local

[rally25rs]

I believe this PR would also fix: #2064
There is a lot of discussion in that thread related to this kind of change.

[bestander]

Looping in @Daniel15 as I completely lost faith in trying to understand all those prefixes needed for every OS

[Daniel15]

I'm not very familiar with Mac OS, but the issue on Linux would be that regular users normally don't have write access to /usr/local. My preferred fix here would be to use /usr/local if it's writable by the current user, otherwise use ~/.yarn/bin or something similar.

It might be easier to just always use a user directory like ~/.yarn/bin. On Windows we always use a user directory: %LOCALAPPDATA%\Yarn\bin (eg. C:\Users\Daniel\AppData\Local\Yarn\bin)

[bestander]

To me it seems like a safe enough change for Mac OS, let's merge it.
Speak up if you want to revert it.

@KishanBagaria, you have expressed a few good ideas about paths in Linux.
Do you want to give it a try and help Yarn to provide a great experience with all those PATH problems?

[paulirish]

On Google corporate OS X machines /usr/local is owned by root, rather than the user. As a result, most Googlers with homebrew install to another location rather than only using homebrew via sudo (which is terrrible).

FWIW I've been manually adding my homebrew'd node path to PATH in order to make sure my yarn global installs work. And I rev this each time I upgrade node. It's kinda gross. :/

For me, it'd be perfect to install the global links into ~.yarn/bin (as I've expected them to be there multiple times). @Daniel15's proposal of falling back to there if /usr/local isn't writable is fine with me.

[bestander]

Thanks for feedback @paulirish.
I'll revert this now.

@KishanBagaria would you give it another try with @Daniel15's feedback?

[KishanBagaria]

@paulirish That must needlessly require sudo for a lot of install scripts. Maybe you should drop the suggestion of making /usr/local user-accessible in their issue tracker.

Until the next Yarn release, you can simply run yarn config set prefix ~/.yarn and get rid of the hacks in your dotfile.