Language::Node.npm_install_args: add helper method (#37)

* add Language::Node helper module

This adds a language module for Node module based formulas.
It contains the 2 public methods `std_npm_install_args(libexec)` and
`local_npm_install_args`:

* `std_npm_install_args` is intended to be used in formulas for
  standard node modules and returns `npm install` args for a global
  style module installation to libexec.
* `local_npm_install_args` is for formulas, in which the `npm install`
  step is only one of multiple parts of the installation process and
  returns `npm install` args for a default local installation in place.

Both methods have in common, that they are

* making sure that a working copy of npm and node-gyp from node's
  libexec is prepended to the PATH (to not rely of a user managed npm)
* seting the npm cache to HOMEBREW_CACHE/npm, which fixes issues caused
  by overriding $HOME resulting in long install times + high disk usage
  (see https://github.com/Homebrew/brew/pull/37#issuecomment-208840366)

* audit: update npm install check for Language::Node

* cleanup: remove npm_cache too

* doc: add Node-for-Formula-Authors.md

1 files changed, 35 insertions, 0 deletions

diff --git a/Library/Homebrew/language/node.rb b/Library/Homebrew/language/node.rb
new file mode 100644
index 000000000..15a2ddc10
--- /dev/null
+++ b/Library/Homebrew/language/node.rb
@@ -0,0 +1,35 @@
+module Language
+  module Node
+    def self.npm_cache_config
+      "cache=#{HOMEBREW_CACHE}/npm_cache\n"
+    end
+
+    def self.setup_npm_environment
+      npmrc = Pathname.new("#{ENV["HOME"]}/.npmrc")
+      # only run setup_npm_environment once per formula
+      return if npmrc.exist?
+      # explicitly set npm's cache path to HOMEBREW_CACHE/npm_cache to fix
+      # issues caused by overriding $HOME (long build times, high disk usage)
+      # https://github.com/Homebrew/brew/pull/37#issuecomment-208840366
+      npmrc.write npm_cache_config
+      # explicitly use our npm and node-gyp executables instead of the user
+      # managed ones in HOMEBREW_PREFIX/lib/node_modules which might be broken
+      ENV.prepend_path "PATH", Formula["node"].opt_libexec/"npm/bin"
+    end
+
+    def self.std_npm_install_args(libexec)
+      setup_npm_environment
+      # tell npm to not install .brew_home by adding it to the .npmignore file
+      # (or creating a new one if no .npmignore file already exists)
+      open(".npmignore", "a") { |f| f.write( "\n.brew_home\n") }
+      # npm install args for global style module format installed into libexec
+      ["--verbose", "--global", "--prefix=#{libexec}", "."]
+    end
+
+    def self.local_npm_install_args
+      setup_npm_environment
+      # npm install args for local style module format
+      ["--verbose"]
+    end
+  end
+end