TailFromSentinel

Parses a file, looking for a sentinel line that satisfies a block, and returns
all lines from there until the end of the file (including the sentinel itself).

This can be handy when you are looking for information in a log file only after
a certain condition (eg. a date) has been met.

You can either use it on it's own, or subclass it to build your own custom
tail-like classes.

Standalone:
  File.open('/var/log/messages', 'r') do |file|
    tfs=TailFromSentinel::Base.new(file) { |line| line =~ /Dec 20 11:2\d+/ }
  end
  tfs.data # => [ 'Dec 20 11:20:00 ...', 'Dec 20 11:21:00 ...', ... ]

Subclassed:
  class TailMyFile < TailFromSentinel::Base
    DATA_ROOT="/path/to/data/"

    def initialize(data_file)
      file=File.open(File.join(DATA_ROOT, data_file), 'r') do |file|
        super(file, &select_sentinel_criteria(data_file))
      end
    end

    def select_sentinel_criteria(data_file)
      case data_file
        when ''production.log'
          Proc.new { |line| line =~ /pattern/ }
        when 'development.log'
          Proc.new { |line| line =~ /another pattern/ }
        else
          raise "Unhandled file: #{data_file}"
      end
    end
  end

  t=TailMyFile.new('production.log')
  t.data # => [ 'Started GET "/pattern ...", "  Processing ... ", ... ]