RCR 211: Normalize filenames before requiring them

Filenames should be normalized before the file is required. Otherwise, requiring files by two different names will result in the file being loaded twice.

Consider the following code:

  a.rb:
  puts "a"

  b.rb:
  require 'a'
  puts "b"

  test.rb:
  require './a'
  require './b'

This will print:

  a
  a
  b

because a.rb is loaded first by the name 'a.rb' and then by the name './a.rb'. This is probably not what is intended, and the only workarounds are:

• be careful never to require a file by two different names, or
• put require guards inside a.rb (similar to #include guards in C)

The above code is probably not a good idea to begin with (requring files from the current directory is bad, in case the user runs from a different directory than expected), but it is a simple way to demonstrate the problem.

The require method should be changed to:

1. Find the file to require by searching the search path
2. Expand the pathanme of the file being required by using File#expand_path
3. Determine if the resulting pathname refers to a symlink; if it does, then read the link and go back to step 2.

This should make it possible to require a file by two different names without problems. It also allows the user to require a symlink and the real file, and the file will only be loaded once.

Sample Ruby code that does this:

def find_file_in_path(file, path=$:, extensions=['', '.so', '.rb'])
  path.each do |dir|
    pathname = File.join(dir, file)
    extensions.each do |ext|
      filename = pathname + ext
      if File.exists?(filename) then
        return File.expand_path(filename)
      end
    end
  end
  return file
end

def realpath(file)
  if not File.respond_to?(:readlink) then
    return file
  end

  total = ''
  File.expand_path(file).split(File::SEPARATOR).each do |piece|
    next if piece == ""
    if File.symlink?(total + File::SEPARATOR + piece) then
      total += File::SEPARATOR + piece
      begin
        total =  File.expand_path(
            File.readlink(total),
            File.dirname(total))
      end while File.symlink?(total)
    else
      total << File::SEPARATOR + piece
    end
  end

  return total
end

$__old_require__ = method(:require)

def require(file)
  file_to_require = find_file_in_path(file)
  $__old_require__.call(realpath(file_to_require))
end