class Rake::Promise

:nodoc: all
Used by ThreadPool.
the promised chore.
arguments for the block. Calling value will return the value of
future. The promise is created with a block and a list of
A Promise object represents a promise to do work (a chore) in the

def chore

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

def complete?

Are we done with the promise 
def complete?
  result? || error?
end

def discard

free up these items for the GC 
def discard
  @args = nil
  @block = nil
end

def error?

Did the promise throw an error 
def error?
  !@error.equal?(NOT_SET)
end

def initialize(args, &block)

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
  @block = block
end

def result?

Do we have a result for the promise 
def result?
  !@result.equal?(NOT_SET)
end

def stat(*args)

Record execution statistics if there is a recorder 
def stat(*args)
  @recorder.call(*args) if @recorder
end

def value

synchronously. We will wait.
If the promised chore is not yet complete, then do the work

Return the value of this promise. 
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

def work

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