Create active symlinks for installed formula

Similar to the LinkedKegs record, we write a symlink for installed kegs to PREFIX/opt.

Unlike the linked-keg record, unlinking doesn't remove the link, only uninstalling, and keg-only formula have a record too.

The reason for this addition is so that formula that depend on keg-only formula can build against the opt directory and not the cellar keg. Thus surviving upgrades.

To enforce this fix_install_names and built were adapted to use the opt path.

Standard kegs also create an opt symlink so that caveats can now refer to the opt directory and thus provide steps that survive upgrades too.

Thus the choice of /opt. It is short, neat and the right choice: POSIX dictates that opt is for stand-alone prefixes of software.

Library/Homebrew/build.rb

@@ -61,14 +61,18 @@ def install f
   f.recursive_deps.uniq.each do |dep|
     dep = Formula.factory dep
     if dep.keg_only?
-      ENV.prepend 'LDFLAGS', "-L#{dep.lib}"
-      ENV.prepend 'CPPFLAGS', "-I#{dep.include}"
-      ENV.prepend 'PATH', "#{dep.bin}", ':'
+      opt = HOMEBREW_PREFIX/:opt/dep.name
 
-      pcdir = dep.lib/'pkgconfig'
+      raise "#{opt} not present\nReinstall #{dep}." unless opt.directory?
+
+      ENV.prepend 'LDFLAGS', "-L#{opt}/lib"
+      ENV.prepend 'CPPFLAGS', "-I#{opt}/include"
+      ENV.prepend 'PATH', "#{opt}/bin", ':'
+
+      pcdir = opt/'lib/pkgconfig'
       ENV.prepend 'PKG_CONFIG_PATH', pcdir, ':' if pcdir.directory?
 
-      acdir = dep.share/'aclocal'
+      acdir = opt/'share/aclocal'
       ENV.prepend 'ACLOCAL_PATH', acdir, ':' if acdir.directory?
     end
   end

Library/Homebrew/cmd/uninstall.rb

@@ -10,6 +10,7 @@ module Homebrew extend self
         puts "Uninstalling #{keg}..."
         keg.unlink
         keg.uninstall
+        rm_opt_link keg.fname
       end
     else
       ARGV.named.each do |name|
@@ -30,10 +31,18 @@ module Homebrew extend self
           end
           rack.rmtree
         end
+
+        rm_opt_link name
       end
     end
   rescue MultipleVersionsInstalledError => e
     ofail e
     puts "Use `brew remove --force #{e.name}` to remove all versions."
   end
+
+  def rm_opt_link name
+    optlink = HOMEBREW_PREFIX/:opt/name
+    optlink.unlink if optlink.symlink?
+  end
+
 end

Library/Homebrew/formula.rb

@@ -148,6 +148,8 @@ class Formula
     self.class.build
   end
 
+  def opt_prefix; HOMEBREW_PREFIX/:opt/name end
+
   # Use the @active_spec to detect the download strategy.
   # Can be overriden to force a custom download strategy
   def download_strategy

Library/Homebrew/formula_installer.rb

@@ -172,10 +172,18 @@ class FormulaInstaller
   def finish
     ohai 'Finishing up' if ARGV.verbose?
 
-    unless f.keg_only?
+    if f.keg_only?
+      begin
+        Keg.new(f.prefix).optlink
+      rescue Exception => e
+        onoe "Failed to create: #{f.opt_prefix}"
+        puts "Things that depend on #{f} will probably not build."
+      end
+    else
       link
-      check_PATH
+      check_PATH unless f.keg_only?
     end
+
     fix_install_names
 
     ohai "Summary" if ARGV.verbose? or show_summary_heading

Library/Homebrew/keg.rb

@@ -42,7 +42,7 @@ class Keg < Pathname
         Find.prune if src.directory?
       end
     end
-    linked_keg_record.unlink if linked_keg_record.exist?
+    linked_keg_record.unlink if linked_keg_record.symlink?
     n
   end
 
@@ -126,7 +126,25 @@ class Keg < Pathname
 
     linked_keg_record.make_relative_symlink(self) unless mode == :dryrun
 
+    optlink unless mode == :dryrun
+
     return $n + $d
+  rescue Exception
+    opoo "Could not link #{fname}. Unlinking..."
+    unlink
+    raise
+  end
+
+  def optlink
+    from = HOMEBREW_PREFIX/:opt/fname
+    if from.symlink?
+      from.delete
+    elsif from.directory?
+      from.rmdir
+    elsif from.exist?
+      from.delete
+    end
+    from.make_relative_symlink(self)
   end
 
 protected

Library/Homebrew/keg_fix_install_names.rb

@@ -56,8 +56,13 @@ class Keg
     end
 
     # the shortpath ensures that library upgrades don’t break installed tools
-    shortpath = HOMEBREW_PREFIX + Pathname.new(file).relative_path_from(self)
-    id = if shortpath.exist? then shortpath else file end
+    relative_path = Pathname.new(file).relative_path_from(self)
+    shortpath = HOMEBREW_PREFIX.join(relative_path)
+    id = if shortpath.exist?
+      shortpath
+    else
+      "#{HOMEBREW_PREFIX}/opt/#{fname}/#{relative_path}"
+    end
 
     yield id, install_names
   end