Efficiently read arbitrary directories of an arbitrary git revision from a git repo into memory, without affecting the working directory.

git_revision_entries.rb

# BEWARE! This code is largely untested and not ready for production!

Result  = Struct.new(:entries, :raw_error, :status)
class Entry
  HeaderUnpackFormat  = "Z100A8A8A8A12A12A8aZ100A6A2Z32Z32A8A8Z155".freeze
  EmptyString         = "".freeze
  OctalFields         = [1,2,3,4,5,6,10,13,14]

  def self.from_stream(io)
    header = io.read(512)
    return nil unless header

    fields = header.unpack(HeaderUnpackFormat)
    OctalFields.each do |octal_field|
      fields[octal_field] = fields[octal_field].oct
    end

    new(fields, io.read(fields[4]))
  end

  attr_reader :name
  attr_reader :mode
  attr_reader :uid
  attr_reader :gid
  attr_reader :size
  attr_reader :mtime
  attr_reader :checksum
  attr_reader :typeflag
  attr_reader :linkname
  attr_reader :magic
  attr_reader :version
  attr_reader :uname
  attr_reader :gname
  attr_reader :devmajor
  attr_reader :devminor
  attr_reader :prefix
  attr_reader :data

  def initialize(fields, data)
    @name      = fields[0]
    @mode      = fields[1]
    @uid       = fields[2]
    @gid       = fields[3]
    @size      = fields[4]
    @mtime     = fields[5]
    @checksum  = fields[6]
    @typeflag  = fields[7]
    @linkname  = fields[8]
    @magic     = fields[9]
    @version   = fields[10]
    @uname     = fields[11]
    @gname     = fields[12]
    @devmajor  = fields[13]
    @devminor  = fields[14]
    @prefix    = fields[15]
    @data      = data
  end

  def skip
    (512 - (size % 512)) % 512
  end

  def full_name
    if @prefix != EmptyString
      File.join(@prefix, @name)
    else
      @name
    end
  end
end

def git_archive_entries(revision, *paths)
  sor,sow   = IO.pipe
  ser,sew   = IO.pipe
  pid       = Process.spawn('git', 'archive', '--format=tar', revision, *paths, out: sow, err: sew)
  sew.close
  sow.close
  entries   = []

  while entry=Entry.from_stream(sor)
    entries << entry
    sor.read(entry.skip)
  end

  stderr    = ser.read
  _, status = Process.wait2(pid)

  Result.new(entries, stderr, status)
end

revision  = 'version-2.8.0-beta'
paths     = ['config/locales']
result    = git_archive_entries(revision, *paths)