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require 'ftools'
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# DirInit::Manager handles directory initialization and clean-up when
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# there are many concurrent processes that wants to modify the
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# directory in the same way.
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#
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# An example usage is when each process wants to copy some temporary
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# files to the directory and delete these files after finishing its
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# job. Problems may occur when the first process delete the files
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# while the second process is still using the files.
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#
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# This library maintain a reference counter on the processes using the
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# directory. It locks the dir to manage critical section when
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# updating the reference counter.
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#
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# Example usage:
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#
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# dman = DirInit::Manager.new("mydir")
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#
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# dman.setup do
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# # do some initialization
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# end
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#
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# #... do anything you want
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#
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# dman.teardown do
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# # clean up
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# end
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#
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# DirInit::Manager ensures that the block passed to <tt>setup</tt>
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# only runs once by the first process in the concurrent dir usage and
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# block passed to <tt>teardown</tt> runs once by the last process in
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# that concurrent activities leaving that dir.
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module DirInit
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class Manager
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def initialize(dir_name, usage_filename='.usage_counter')
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@dir_name = dir_name
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@usage_filename = usage_filename
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end
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# Check if someone has initialized the dir. If not, call block.
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def setup # :yields: block
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dir = File.new(@dir_name)
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dir.flock(File::LOCK_EX)
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begin
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counter_filename = get_counter_filename
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if File.exist? counter_filename
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# someone is here
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f = File.new(counter_filename,"r+")
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counter = f.read.to_i
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f.seek(0)
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f.write("#{counter+1}\n")
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f.close
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else
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# i'm the first, create the counter file
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counter = 0
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f = File.new(counter_filename,"w")
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f.write("1\n")
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f.close
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end
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# if no one is here
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if counter == 0
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if block_given?
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yield
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end
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end
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rescue
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raise
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ensure
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# make sure it unlock the directory
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dir.flock(File::LOCK_UN)
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end
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end
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# Check if I am the last one using the dir. If true, call block.
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def teardown
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dir = File.new(@dir_name)
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dir.flock(File::LOCK_EX)
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begin
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counter_filename = get_counter_filename
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if File.exist? counter_filename
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# someone is here
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f = File.new(counter_filename,"r+")
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counter = f.read.to_i
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f.seek(0)
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f.write("#{counter-1}\n")
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f.close
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if counter == 1
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# i'm the last one
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File.delete(counter_filename)
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if block_given?
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yield
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end
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end
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else
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# This is BAD
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raise "Error: reference count missing"
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end
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rescue
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raise
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ensure
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# make sure it unlock the directory
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dir.flock(File::LOCK_UN)
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end
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end
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protected
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def get_counter_filename
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return File.join(@dir_name,@usage_filename)
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end
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end
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end
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jittat-