pezra / filequeue Goto Github PK

The length method doesn't protect from a file that doesn't exist yet:

$ irb
irb(main):001:0> require 'filequeue'
=> true
irb(main):002:0> queue = FileQueue.new 'bob.txt'
=> #<FileQueue:0x007fb2cc2c10b0 @delimiter="\n", @file_name="bob.txt">
irb(main):003:0> queue.empty?
Errno::ENOENT: No such file or directory - bob.txt
from /Library/Ruby/Gems/2.0.0/gems/filequeue-0.0.2/lib/filequeue.rb:51:in initialize' from /Library/Ruby/Gems/2.0.0/gems/filequeue-0.0.2/lib/filequeue.rb:51:inopen'
from /Library/Ruby/Gems/2.0.0/gems/filequeue-0.0.2/lib/filequeue.rb:51:in safe_open' from /Library/Ruby/Gems/2.0.0/gems/filequeue-0.0.2/lib/filequeue.rb:35:inlength'
from /Library/Ruby/Gems/2.0.0/gems/filequeue-0.0.2/lib/filequeue.rb:42:in empty?' from (irb):3 from /usr/bin/irb:12:in

I may be wrong but it seems like the following code has a potential race condition:

    safe_open 'r' do |file|
      value = file.gets @delimiter
      rest = file.read
    end
    safe_open 'w+' do |file|
      file.write rest
    end

e.g:

A obtains lock; A reads line; A caches rest
B obtains lock; B reads line; B caches rest
B obtains lock; B writes rest
A obtains lock; A writes rest

Fixed by not releasing the lock until rest is written back to the file:

FileQueue.class_eval do
  def pop
    value = nil
    safe_open 'r+' do |file|
      value = file.gets @delimiter
      rest = file.read
      file.rewind
      file.truncate(0)
      file.write(rest)
    end
    value ? value[0..-(@delimiter.length) - 1] : nil
  end
end