1 files changed, 99 insertions, 0 deletions
diff --git a/lib/rake/promise.rb b/lib/rake/promise.rb
new file mode 100644
index 0000000000..3258b91139
--- /dev/null
+++ b/lib/rake/promise.rb
@@ -0,0 +1,99 @@
+module Rake
+
+  # A Promise object represents a promise to do work (a chore) in the
+  # future. The promise is created with a block and a list of
+  # arguments for the block. Calling value will return the value of
+  # the promised chore.
+  #
+  # Used by ThreadPool.
+  #
+  class Promise               # :nodoc: all
+    NOT_SET = Object.new.freeze # :nodoc:
+
+    attr_accessor :recorder
+
+    # Create a promise to do the chore specified by the block.
+    def initialize(args, &block)
+      @mutex = Mutex.new
+      @result = NOT_SET
+      @error = NOT_SET
+      @args = args.collect { |a| begin; a.dup; rescue; a; end }
+      @block = block
+    end
+
+    # Return the value of this promise.
+    #
+    # If the promised chore is not yet complete, then do the work
+    # synchronously. We will wait.
+    def value
+      unless complete?
+        stat :sleeping_on, :item_id => object_id
+        @mutex.synchronize do
+          stat :has_lock_on, :item_id => object_id
+          chore
+          stat :releasing_lock_on, :item_id => object_id
+        end
+      end
+      error? ? raise(@error) : @result
+    end
+
+    # If no one else is working this promise, go ahead and do the chore.
+    def work
+      stat :attempting_lock_on, :item_id => object_id
+      if @mutex.try_lock
+        stat :has_lock_on, :item_id => object_id
+        chore
+        stat :releasing_lock_on, :item_id => object_id
+        @mutex.unlock
+      else
+        stat :bailed_on, :item_id => object_id
+      end
+    end
+
+    private
+
+    # Perform the chore promised
+    def chore
+      if complete?
+        stat :found_completed, :item_id => object_id
+        return
+      end
+      stat :will_execute, :item_id => object_id
+      begin
+        @result = @block.call(*@args)
+      rescue Exception => e
+        @error = e
+      end
+      stat :did_execute, :item_id => object_id
+      discard
+    end
+
+    # Do we have a result for the promise
+    def result?
+      ! @result.equal?(NOT_SET)
+    end
+
+    # Did the promise throw an error
+    def error?
+      ! @error.equal?(NOT_SET)
+    end
+
+    # Are we done with the promise
+    def complete?
+      result? || error?
+    end
+
+    # free up these items for the GC
+    def discard
+      @args = nil
+      @block = nil
+    end
+
+    # Record execution statistics if there is a recorder
+    def stat(*args)
+      @recorder.call(*args) if @recorder
+    end
+
+  end
+
+end