MacOS: refactor sdk_path lookup

* Pull SDK lookup code into a new `locator` class, which caches its
  results
* SDKLocator only queries one SDK location, not all SDK locations
* Build a map of all installed SDKs inside that location, instead of
  just the requested SDK
* Ask xcrun for --show-sdk-platform-path first so that all SDKs can be
  found, instead of asking xcodebuild for a specific SDK
* Add a new `SDK` class, which tracks the version and the prefix; add a
  new `MacOS.sdk` method which returns an `SDK` instance instead of a
  bare path; MacOS.sdk_path still returns a bare path

Library/Homebrew/os/mac.rb

@@ -3,6 +3,7 @@ require "os/mac/version"
 require "os/mac/xcode"
 require "os/mac/xquartz"
 require "os/mac/pathname"
+require "os/mac/sdk"
 
 module OS
   module Mac
@@ -79,19 +80,24 @@ module OS
       @active_developer_dir ||= Utils.popen_read("/usr/bin/xcode-select", "-print-path").strip
     end
 
-    def sdk_path(v = version)
-      (@sdk_path ||= {}).fetch(v.to_s) do |key|
-        opts = []
-        # First query Xcode itself
-        opts << Utils.popen_read(locate("xcodebuild"), "-version", "-sdk", "macosx#{v}", "Path").chomp
-        # Xcode.prefix is pretty smart, so lets look inside to find the sdk
-        opts << "#{Xcode.prefix}/Platforms/MacOSX.platform/Developer/SDKs/MacOSX#{v}.sdk"
-        # Xcode < 4.3 style
-        opts << "/Developer/SDKs/MacOSX#{v}.sdk"
-        @sdk_path[key] = opts.map { |a| Pathname.new(a) }.detect(&:directory?)
+    # Returns the requested SDK, if installed.
+    # If the requested SDK is not installed returns either:
+    # a) The newest SDK (if any SDKs are available), or
+    # b) nil
+    def sdk(v = version)
+      @locator ||= SDKLocator.new
+      begin
+        @locator.sdk_for v
+      rescue SDKLocator::NoSDKError
       end
     end
 
+    # Returns the path to an SDK or nil, following the rules set by #sdk.
+    def sdk_path(v = version)
+      s = sdk(v)
+      s.path unless s.nil?
+    end
+
     def default_cc
       cc = locate "cc"
       cc.realpath.basename.to_s rescue nil

Library/Homebrew/os/mac/sdk.rb

@@ -0,0 +1,59 @@
+require "os/mac/version"
+
+module OS
+  module Mac
+    class SDK
+      attr_reader :version, :path
+
+      def initialize(version, path)
+        @version = OS::Mac::Version.new version
+        @path = Pathname.new(path)
+      end
+    end
+
+    class SDKLocator
+      class NoSDKError < StandardError; end
+
+      def sdk_for(v)
+        path = sdk_paths[v]
+        raise NoSDKError if path.nil?
+
+        SDK.new v, path
+      end
+
+      def latest_sdk
+        return if sdk_paths.empty?
+
+        v, path = sdk_paths.max {|a, b| OS::Mac::Version.new(a[0]) <=> OS::Mac::Version.new(b[0])}
+        SDK.new v, path
+      end
+
+      private
+
+      def sdk_paths
+        @sdk_paths ||= begin
+          # Xcode.prefix is pretty smart, so let's look inside to find the sdk
+          sdk_prefix = "#{Xcode.prefix}/Platforms/MacOSX.platform/Developer/SDKs"
+          # Xcode < 4.3 style
+          sdk_prefix = "/Developer/SDKs" unless File.directory? sdk_prefix
+          # Finally query Xcode itself (this is slow, so check it last)
+          sdk_prefix = File.join(Utils.popen_read(OS::Mac.locate("xcrun"), "--show-sdk-platform-path").chomp, "Developer", "SDKs") unless File.directory? sdk_prefix
+
+          # Bail out if there is no SDK prefix at all
+          if !File.directory? sdk_prefix
+            {}
+          else
+            paths = {}
+
+            Dir[File.join(sdk_prefix, "MacOSX*.sdk")].each do |sdk_path|
+              version = sdk_path[/MacOSX(\d+\.\d+)u?.sdk$/, 1]
+              paths[version] = sdk_path unless version.nil?
+            end
+
+            paths
+          end
+        end
+      end
+    end
+  end
+end