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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
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