Record which keg is linked

Will be useful for a variety of reasons, but for now, I'm just using it to ensure install won't install again if something is already installed (use brew upgrade instead).

But means that brew switch and that can work properly etc.

.gitignore

@@ -7,3 +7,4 @@
 !/bin/brew
 !/share/man/man1/brew.1
 .DS_Store
+/Library/LinkedKegs

Library/Homebrew/cmd/install.rb

@@ -9,6 +9,12 @@ module Homebrew extend self
       raise "No available formula for #{name}\n#{msg}" if msg
     end unless ARGV.force?
 
+    ARGV.formulae.each do |f|
+      if File.directory? HOMEBREW_REPOSITORY/"Library/LinkedKegs/#{f.name}"
+        raise "#{f} already installed\nTry: brew upgrade #{f}"
+      end
+    end
+
     if Process.uid.zero? and not File.stat(HOMEBREW_BREW_FILE).uid.zero?
       # note we only abort if Homebrew is *not* installed as sudo and the user
       # calls brew as root. The fix is to chown brew to root.

Library/Homebrew/keg.rb

@@ -33,10 +33,21 @@ class Keg < Pathname
       n+=1
       Find.prune if src.directory?
     end
+    linked_keg_record.unlink if linked_keg_record.exist?
     n
   end
 
+  def fname
+    parent.basename.to_s
+  end
+
+  def linked_keg_record
+    @linked_keg_record ||= HOMEBREW_REPOSITORY/"Library/LinkedKegs"/fname
+  end
+
   def link
+    raise "Cannot link #{fname}\nAnother version is already linked: #{linked_keg_record.realpath}" if linked_keg_record.directory?
+
     $n=0
     $d=0
 
@@ -68,6 +79,8 @@ class Keg < Pathname
       end
     end
 
+    (HOMEBREW_REPOSITORY/"Library/LinkedKegs"/fname).make_relative_symlink(self)
+
     return $n+$d
   end