ruby implementation of find that follows symbolic links; depth first order; raises if broken links or cycles are met

find_follow.rb

=begin
* ruby implementation of find that follows symbolic directory links
* tested on ruby 1.9.3, ruby 2.0 and jruby on Fedora 20 linux
* you can use Find.prune
* detect symlinks to dirs by path "/" suffix; does nothing with files so `symlink?` method is working fine
* depth first order
* detects cycles and raises an error
* raises on broken links
* uses recursion in the `do_find` proc when directory links are met (takes a lot of nested links until SystemStackError, that's practically never)

* use like: find_follow(".") {|f| puts f}

Copyright (c) 2014 Red Hat inc

Permission is hereby granted, free of charge, to any person obtaining a copy
of this software and associated documentation files (the "Software"), to deal
in the Software without restriction, including without limitation the rights
to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
copies of the Software, and to permit persons to whom the Software is
furnished to do so, subject to the following conditions:

The above copyright notice and this permission notice shall be included in
all copies or substantial portions of the Software.

THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
THE SOFTWARE.
=end

require 'find'
require 'pathname'

def find_follow(*paths)
  block_given? or return enum_for(__method__, *paths)

  link_cache = {}
  link_resolve = lambda { |path|
    # puts "++ link_resolve: #{path}" # trace
    if link_cache[path]
      return link_cache[path]
    else
      return link_cache[path] = Pathname.new(path).realpath.to_s
    end
  }
  # this lambda should cleanup `link_cache` from unnecessary entries
  link_cache_reset = lambda { |path|
    # puts "++ link_cache_reset: #{path}" # trace
    # puts link_cache.to_s # trace
    link_cache.select! do |k,v|
      path == k || k == "/" || path.start_with?(k + "/")
    end
    # puts link_cache.to_s # trace
  }
  link_is_recursive = lambda { |path|
    # puts "++ link_is_recursive: #{path}" # trace
    # the ckeck is useless if path is not a link but not our responsibility

    # we need to check full path for link cycles
    pn_initial = Pathname.new(path)
    unless pn_initial.absolute?
      # can we use `expand_path` here? Any issues with links?
      pn_initial = Pathname.new(File.join(Dir.pwd, path))
    end

    # clear unnecessary cache
    link_cache_reset.call(pn_initial.to_s)

    link_dst = link_resolve.call(pn_initial.to_s)

    pn_initial.ascend do |pn|
      if pn != pn_initial && link_dst == link_resolve.call(pn.to_s)
        return {:link => path, :dst => pn}
      end
    end

    return false
  }

  do_find = proc { |path|
    Find.find(path) do |path|
      if File.symlink?(path) && File.directory?(File.realpath(path))
        if path[-1] == "/"
          # probably hitting https://github.com/jruby/jruby/issues/1895
          yield(path.dup)
          Dir.new(path).each { |subpath|
            do_find.call(path + subpath) unless [".", ".."].include?(subpath)
          }
        elsif is_recursive = link_is_recursive.call(path)
          raise "cannot handle recursive links: #{is_recursive[:link]} => #{is_recursive[:dst]}"
        else
          do_find.call(path + "/")
        end
      else
        yield(path)
      end
    end
  }

  while path = paths.shift
    do_find.call(path)
  end
end