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Edit File : pstore.rb \| Size : 14.70 KB
# frozen_string_literal: true # = PStore -- Transactional File Storage for Ruby Objects # # pstore.rb - # originally by matz # documentation by Kev Jackson and James Edward Gray II # improved by Hongli Lai # # See PStore for documentation. require "digest" # # PStore implements a file based persistence mechanism based on a Hash. User # code can store hierarchies of Ruby objects (values) into the data store file # by name (keys). An object hierarchy may be just a single object. User code # may later read values back from the data store or even update data, as needed. # # The transactional behavior ensures that any changes succeed or fail together. # This can be used to ensure that the data store is not left in a transitory # state, where some values were updated but others were not. # # Behind the scenes, Ruby objects are stored to the data store file with # Marshal. That carries the usual limitations. Proc objects cannot be # marshalled, for example. # # == Usage example: # # require "pstore" # # # a mock wiki object... # class WikiPage # def initialize( page_name, author, contents ) # @page_name = page_name # @revisions = Array.new # # add_revision(author, contents) # end # # attr_reader :page_name # # def add_revision( author, contents ) # @revisions << { :created => Time.now, # :author => author, # :contents => contents } # end # # def wiki_page_references # [@page_name] + @revisions.last[:contents].scan(/\b(?:[A-Z]+[a-z]+){2,}/) # end # # # ... # end # # # create a new page... # home_page = WikiPage.new( "HomePage", "James Edward Gray II", # "A page about the JoysOfDocumentation..." ) # # # then we want to update page data and the index together, or not at all... # wiki = PStore.new("wiki_pages.pstore") # wiki.transaction do # begin transaction; do all of this or none of it # # store page... # wiki[home_page.page_name] = home_page # # ensure that an index has been created... # wiki[:wiki_index] \|\|= Array.new # # update wiki index... # wiki[:wiki_index].push(home_page.wiki_page_references) # end # commit changes to wiki data store file # # ### Some time later... ### # # # read wiki data... # wiki.transaction(true) do # begin read-only transaction, no changes allowed # wiki.roots.each do \|data_root_name\| # p data_root_name # p wiki[data_root_name] # end # end # # == Transaction modes # # By default, file integrity is only ensured as long as the operating system # (and the underlying hardware) doesn't raise any unexpected I/O errors. If an # I/O error occurs while PStore is writing to its file, then the file will # become corrupted. # # You can prevent this by setting <em>pstore.ultra_safe = true</em>. # However, this results in a minor performance loss, and only works on platforms # that support atomic file renames. Please consult the documentation for # +ultra_safe+ for details. # # Needless to say, if you're storing valuable data with PStore, then you should # backup the PStore files from time to time. class PStore RDWR_ACCESS = {mode: IO::RDWR \| IO::CREAT \| IO::BINARY, encoding: Encoding::ASCII_8BIT}.freeze RD_ACCESS = {mode: IO::RDONLY \| IO::BINARY, encoding: Encoding::ASCII_8BIT}.freeze WR_ACCESS = {mode: IO::WRONLY \| IO::CREAT \| IO::TRUNC \| IO::BINARY, encoding: Encoding::ASCII_8BIT}.freeze # The error type thrown by all PStore methods. class Error < StandardError end # Whether PStore should do its best to prevent file corruptions, even when under # unlikely-to-occur error conditions such as out-of-space conditions and other # unusual OS filesystem errors. Setting this flag comes at the price in the form # of a performance loss. # # This flag only has effect on platforms on which file renames are atomic (e.g. # all POSIX platforms: Linux, MacOS X, FreeBSD, etc). The default value is false. attr_accessor :ultra_safe # # To construct a PStore object, pass in the _file_ path where you would like # the data to be stored. # # PStore objects are always reentrant. But if _thread_safe_ is set to true, # then it will become thread-safe at the cost of a minor performance hit. # def initialize(file, thread_safe = false) dir = File::dirname(file) unless File::directory? dir raise PStore::Error, format("directory %s does not exist", dir) end if File::exist? file and not File::readable? file raise PStore::Error, format("file %s not readable", file) end @filename = file @abort = false @ultra_safe = false @thread_safe = thread_safe @lock = Thread::Mutex.new end # Raises PStore::Error if the calling code is not in a PStore#transaction. def in_transaction raise PStore::Error, "not in transaction" unless @lock.locked? end # # Raises PStore::Error if the calling code is not in a PStore#transaction or # if the code is in a read-only PStore#transaction. # def in_transaction_wr in_transaction raise PStore::Error, "in read-only transaction" if @rdonly end private :in_transaction, :in_transaction_wr # # Retrieves a value from the PStore file data, by _name_. The hierarchy of # Ruby objects stored under that root _name_ will be returned. # # WARNING: This method is only valid in a PStore#transaction. It will # raise PStore::Error if called at any other time. # def [](name) in_transaction @table[name] end # # This method is just like PStore#[], save that you may also provide a # _default_ value for the object. In the event the specified _name_ is not # found in the data store, your _default_ will be returned instead. If you do # not specify a default, PStore::Error will be raised if the object is not # found. # # WARNING: This method is only valid in a PStore#transaction. It will # raise PStore::Error if called at any other time. # def fetch(name, default=PStore::Error) in_transaction unless @table.key? name if default == PStore::Error raise PStore::Error, format("undefined root name `%s'", name) else return default end end @table[name] end # # Stores an individual Ruby object or a hierarchy of Ruby objects in the data # store file under the root _name_. Assigning to a _name_ already in the data # store clobbers the old data. # # == Example: # # require "pstore" # # store = PStore.new("data_file.pstore") # store.transaction do # begin transaction # # load some data into the store... # store[:single_object] = "My data..." # store[:obj_hierarchy] = { "Kev Jackson" => ["rational.rb", "pstore.rb"], # "James Gray" => ["erb.rb", "pstore.rb"] } # end # commit changes to data store file # # WARNING: This method is only valid in a PStore#transaction and it cannot # be read-only. It will raise PStore::Error if called at any other time. # def []=(name, value) in_transaction_wr @table[name] = value end # # Removes an object hierarchy from the data store, by _name_. # # WARNING: This method is only valid in a PStore#transaction and it cannot # be read-only. It will raise PStore::Error if called at any other time. # def delete(name) in_transaction_wr @table.delete name end # # Returns the names of all object hierarchies currently in the store. # # WARNING: This method is only valid in a PStore#transaction. It will # raise PStore::Error if called at any other time. # def roots in_transaction @table.keys end # # Returns true if the supplied _name_ is currently in the data store. # # WARNING: This method is only valid in a PStore#transaction. It will # raise PStore::Error if called at any other time. # def root?(name) in_transaction @table.key? name end # Returns the path to the data store file. def path @filename end # # Ends the current PStore#transaction, committing any changes to the data # store immediately. # # == Example: # # require "pstore" # # store = PStore.new("data_file.pstore") # store.transaction do # begin transaction # # load some data into the store... # store[:one] = 1 # store[:two] = 2 # # store.commit # end transaction here, committing changes # # store[:three] = 3 # this change is never reached # end # # WARNING: This method is only valid in a PStore#transaction. It will # raise PStore::Error if called at any other time. # def commit in_transaction @abort = false throw :pstore_abort_transaction end # # Ends the current PStore#transaction, discarding any changes to the data # store. # # == Example: # # require "pstore" # # store = PStore.new("data_file.pstore") # store.transaction do # begin transaction # store[:one] = 1 # this change is not applied, see below... # store[:two] = 2 # this change is not applied, see below... # # store.abort # end transaction here, discard all changes # # store[:three] = 3 # this change is never reached # end # # WARNING*: This method is only valid in a PStore#transaction. It will # raise PStore::Error if called at any other time. # def abort in_transaction @abort = true throw :pstore_abort_transaction end # # Opens a new transaction for the data store. Code executed inside a block # passed to this method may read and write data to and from the data store # file. # # At the end of the block, changes are committed to the data store # automatically. You may exit the transaction early with a call to either # PStore#commit or PStore#abort. See those methods for details about how # changes are handled. Raising an uncaught Exception in the block is # equivalent to calling PStore#abort. # # If _read_only_ is set to +true+, you will only be allowed to read from the # data store during the transaction and any attempts to change the data will # raise a PStore::Error. # # Note that PStore does not support nested transactions. # def transaction(read_only = false) # :yields: pstore value = nil if !@thread_safe raise PStore::Error, "nested transaction" unless @lock.try_lock else begin @lock.lock rescue ThreadError raise PStore::Error, "nested transaction" end end begin @rdonly = read_only @abort = false file = open_and_lock_file(@filename, read_only) if file begin @table, checksum, original_data_size = load_data(file, read_only) catch(:pstore_abort_transaction) do value = yield(self) end if !@abort && !read_only save_data(checksum, original_data_size, file) end ensure file.close end else # This can only occur if read_only == true. @table = {} catch(:pstore_abort_transaction) do value = yield(self) end end ensure @lock.unlock end value end private # Constant for relieving Ruby's garbage collector. CHECKSUM_ALGO = %w[SHA512 SHA384 SHA256 SHA1 RMD160 MD5].each do \|algo\| begin break Digest(algo) rescue LoadError end end EMPTY_STRING = "" EMPTY_MARSHAL_DATA = Marshal.dump({}) EMPTY_MARSHAL_CHECKSUM = CHECKSUM_ALGO.digest(EMPTY_MARSHAL_DATA) # # Open the specified filename (either in read-only mode or in # read-write mode) and lock it for reading or writing. # # The opened File object will be returned. If _read_only_ is true, # and the file does not exist, then nil will be returned. # # All exceptions are propagated. # def open_and_lock_file(filename, read_only) if read_only begin file = File.new(filename, RD_ACCESS) begin file.flock(File::LOCK_SH) return file rescue file.close raise end rescue Errno::ENOENT return nil end else file = File.new(filename, RDWR_ACCESS) file.flock(File::LOCK_EX) return file end end # Load the given PStore file. # If +read_only+ is true, the unmarshalled Hash will be returned. # If +read_only+ is false, a 3-tuple will be returned: the unmarshalled # Hash, a checksum of the data, and the size of the data. def load_data(file, read_only) if read_only begin table = load(file) raise Error, "PStore file seems to be corrupted." unless table.is_a?(Hash) rescue EOFError # This seems to be a newly-created file. table = {} end table else data = file.read if data.empty? # This seems to be a newly-created file. table = {} checksum = empty_marshal_checksum size = empty_marshal_data.bytesize else table = load(data) checksum = CHECKSUM_ALGO.digest(data) size = data.bytesize raise Error, "PStore file seems to be corrupted." unless table.is_a?(Hash) end data.replace(EMPTY_STRING) [table, checksum, size] end end def on_windows? is_windows = RUBY_PLATFORM =~ /mswin\|mingw\|bccwin\|wince/ self.class.__send__(:define_method, :on_windows?) do is_windows end is_windows end def save_data(original_checksum, original_file_size, file) new_data = dump(@table) if new_data.bytesize != original_file_size \|\| CHECKSUM_ALGO.digest(new_data) != original_checksum if @ultra_safe && !on_windows? # Windows doesn't support atomic file renames. save_data_with_atomic_file_rename_strategy(new_data, file) else save_data_with_fast_strategy(new_data, file) end end new_data.replace(EMPTY_STRING) end def save_data_with_atomic_file_rename_strategy(data, file) temp_filename = "#{@filename}.tmp.#{Process.pid}.#{rand 1000000}" temp_file = File.new(temp_filename, WR_ACCESS) begin temp_file.flock(File::LOCK_EX) temp_file.write(data) temp_file.flush File.rename(temp_filename, @filename) rescue File.unlink(temp_file) rescue nil raise ensure temp_file.close end end def save_data_with_fast_strategy(data, file) file.rewind file.write(data) file.truncate(data.bytesize) end # This method is just a wrapped around Marshal.dump # to allow subclass overriding used in YAML::Store. def dump(table) # :nodoc: Marshal::dump(table) end # This method is just a wrapped around Marshal.load. # to allow subclass overriding used in YAML::Store. def load(content) # :nodoc: Marshal::load(content) end def empty_marshal_data EMPTY_MARSHAL_DATA end def empty_marshal_checksum EMPTY_MARSHAL_CHECKSUM end end Back

Edit File : pstore.rb | Size : 14.70 KB

# frozen_string_literal: true
# = PStore -- Transactional File Storage for Ruby Objects
#
# pstore.rb -
#   originally by matz
#   documentation by Kev Jackson and James Edward Gray II
#   improved by Hongli Lai
#
# See PStore for documentation.

require "digest"

#
# PStore implements a file based persistence mechanism based on a Hash.  User
# code can store hierarchies of Ruby objects (values) into the data store file
# by name (keys).  An object hierarchy may be just a single object.  User code
# may later read values back from the data store or even update data, as needed.
#
# The transactional behavior ensures that any changes succeed or fail together.
# This can be used to ensure that the data store is not left in a transitory
# state, where some values were updated but others were not.
#
# Behind the scenes, Ruby objects are stored to the data store file with
# Marshal.  That carries the usual limitations.  Proc objects cannot be
# marshalled, for example.
#
# == Usage example:
#
#  require "pstore"
#
#  # a mock wiki object...
#  class WikiPage
#    def initialize( page_name, author, contents )
#      @page_name = page_name
#      @revisions = Array.new
#
#      add_revision(author, contents)
#    end
#
#    attr_reader :page_name
#
#    def add_revision( author, contents )
#      @revisions << { :created  => Time.now,
#                      :author   => author,
#                      :contents => contents }
#    end
#
#    def wiki_page_references
#      [@page_name] + @revisions.last[:contents].scan(/\b(?:[A-Z]+[a-z]+){2,}/)
#    end
#
#    # ...
#  end
#
#  # create a new page...
#  home_page = WikiPage.new( "HomePage", "James Edward Gray II",
#                            "A page about the JoysOfDocumentation..." )
#
#  # then we want to update page data and the index together, or not at all...
#  wiki = PStore.new("wiki_pages.pstore")
#  wiki.transaction do  # begin transaction; do all of this or none of it
#    # store page...
#    wiki[home_page.page_name] = home_page
#    # ensure that an index has been created...
#    wiki[:wiki_index] ||= Array.new
#    # update wiki index...
#    wiki[:wiki_index].push(*home_page.wiki_page_references)
#  end                   # commit changes to wiki data store file
#
#  ### Some time later... ###
#
#  # read wiki data...
#  wiki.transaction(true) do  # begin read-only transaction, no changes allowed
#    wiki.roots.each do |data_root_name|
#      p data_root_name
#      p wiki[data_root_name]
#    end
#  end
#
# == Transaction modes
#
# By default, file integrity is only ensured as long as the operating system
# (and the underlying hardware) doesn't raise any unexpected I/O errors. If an
# I/O error occurs while PStore is writing to its file, then the file will
# become corrupted.
#
# You can prevent this by setting <em>pstore.ultra_safe = true</em>.
# However, this results in a minor performance loss, and only works on platforms
# that support atomic file renames. Please consult the documentation for
# +ultra_safe+ for details.
#
# Needless to say, if you're storing valuable data with PStore, then you should
# backup the PStore files from time to time.
class PStore
  RDWR_ACCESS = {mode: IO::RDWR | IO::CREAT | IO::BINARY, encoding: Encoding::ASCII_8BIT}.freeze
  RD_ACCESS = {mode: IO::RDONLY | IO::BINARY, encoding: Encoding::ASCII_8BIT}.freeze
  WR_ACCESS = {mode: IO::WRONLY | IO::CREAT | IO::TRUNC | IO::BINARY, encoding: Encoding::ASCII_8BIT}.freeze

# The error type thrown by all PStore methods.
  class Error < StandardError
  end

# Whether PStore should do its best to prevent file corruptions, even when under
  # unlikely-to-occur error conditions such as out-of-space conditions and other
  # unusual OS filesystem errors. Setting this flag comes at the price in the form
  # of a performance loss.
  #
  # This flag only has effect on platforms on which file renames are atomic (e.g.
  # all POSIX platforms: Linux, MacOS X, FreeBSD, etc). The default value is false.
  attr_accessor :ultra_safe

#
  # To construct a PStore object, pass in the _file_ path where you would like
  # the data to be stored.
  #
  # PStore objects are always reentrant. But if _thread_safe_ is set to true,
  # then it will become thread-safe at the cost of a minor performance hit.
  #
  def initialize(file, thread_safe = false)
    dir = File::dirname(file)
    unless File::directory? dir
      raise PStore::Error, format("directory %s does not exist", dir)
    end
    if File::exist? file and not File::readable? file
      raise PStore::Error, format("file %s not readable", file)
    end
    @filename = file
    @abort = false
    @ultra_safe = false
    @thread_safe = thread_safe
    @lock = Thread::Mutex.new
  end

# Raises PStore::Error if the calling code is not in a PStore#transaction.
  def in_transaction
    raise PStore::Error, "not in transaction" unless @lock.locked?
  end
  #
  # Raises PStore::Error if the calling code is not in a PStore#transaction or
  # if the code is in a read-only PStore#transaction.
  #
  def in_transaction_wr
    in_transaction
    raise PStore::Error, "in read-only transaction" if @rdonly
  end
  private :in_transaction, :in_transaction_wr

#
  # Retrieves a value from the PStore file data, by _name_.  The hierarchy of
  # Ruby objects stored under that root _name_ will be returned.
  #
  # *WARNING*:  This method is only valid in a PStore#transaction.  It will
  # raise PStore::Error if called at any other time.
  #
  def [](name)
    in_transaction
    @table[name]
  end
  #
  # This method is just like PStore#[], save that you may also provide a
  # _default_ value for the object.  In the event the specified _name_ is not
  # found in the data store, your _default_ will be returned instead.  If you do
  # not specify a default, PStore::Error will be raised if the object is not
  # found.
  #
  # *WARNING*:  This method is only valid in a PStore#transaction.  It will
  # raise PStore::Error if called at any other time.
  #
  def fetch(name, default=PStore::Error)
    in_transaction
    unless @table.key? name
      if default == PStore::Error
        raise PStore::Error, format("undefined root name `%s'", name)
      else
        return default
      end
    end
    @table[name]
  end
  #
  # Stores an individual Ruby object or a hierarchy of Ruby objects in the data
  # store file under the root _name_.  Assigning to a _name_ already in the data
  # store clobbers the old data.
  #
  # == Example:
  #
  #  require "pstore"
  #
  #  store = PStore.new("data_file.pstore")
  #  store.transaction do  # begin transaction
  #    # load some data into the store...
  #    store[:single_object] = "My data..."
  #    store[:obj_hierarchy] = { "Kev Jackson" => ["rational.rb", "pstore.rb"],
  #                              "James Gray"  => ["erb.rb", "pstore.rb"] }
  #  end                   # commit changes to data store file
  #
  # *WARNING*:  This method is only valid in a PStore#transaction and it cannot
  # be read-only.  It will raise PStore::Error if called at any other time.
  #
  def []=(name, value)
    in_transaction_wr
    @table[name] = value
  end
  #
  # Removes an object hierarchy from the data store, by _name_.
  #
  # *WARNING*:  This method is only valid in a PStore#transaction and it cannot
  # be read-only.  It will raise PStore::Error if called at any other time.
  #
  def delete(name)
    in_transaction_wr
    @table.delete name
  end

#
  # Returns the names of all object hierarchies currently in the store.
  #
  # *WARNING*:  This method is only valid in a PStore#transaction.  It will
  # raise PStore::Error if called at any other time.
  #
  def roots
    in_transaction
    @table.keys
  end
  #
  # Returns true if the supplied _name_ is currently in the data store.
  #
  # *WARNING*:  This method is only valid in a PStore#transaction.  It will
  # raise PStore::Error if called at any other time.
  #
  def root?(name)
    in_transaction
    @table.key? name
  end
  # Returns the path to the data store file.
  def path
    @filename
  end

#
  # Ends the current PStore#transaction, committing any changes to the data
  # store immediately.
  #
  # == Example:
  #
  #  require "pstore"
  #
  #  store = PStore.new("data_file.pstore")
  #  store.transaction do  # begin transaction
  #    # load some data into the store...
  #    store[:one] = 1
  #    store[:two] = 2
  #
  #    store.commit        # end transaction here, committing changes
  #
  #    store[:three] = 3   # this change is never reached
  #  end
  #
  # *WARNING*:  This method is only valid in a PStore#transaction.  It will
  # raise PStore::Error if called at any other time.
  #
  def commit
    in_transaction
    @abort = false
    throw :pstore_abort_transaction
  end
  #
  # Ends the current PStore#transaction, discarding any changes to the data
  # store.
  #
  # == Example:
  #
  #  require "pstore"
  #
  #  store = PStore.new("data_file.pstore")
  #  store.transaction do  # begin transaction
  #    store[:one] = 1     # this change is not applied, see below...
  #    store[:two] = 2     # this change is not applied, see below...
  #
  #    store.abort         # end transaction here, discard all changes
  #
  #    store[:three] = 3   # this change is never reached
  #  end
  #
  # *WARNING*:  This method is only valid in a PStore#transaction.  It will
  # raise PStore::Error if called at any other time.
  #
  def abort
    in_transaction
    @abort = true
    throw :pstore_abort_transaction
  end

#
  # Opens a new transaction for the data store.  Code executed inside a block
  # passed to this method may read and write data to and from the data store
  # file.
  #
  # At the end of the block, changes are committed to the data store
  # automatically.  You may exit the transaction early with a call to either
  # PStore#commit or PStore#abort.  See those methods for details about how
  # changes are handled.  Raising an uncaught Exception in the block is
  # equivalent to calling PStore#abort.
  #
  # If _read_only_ is set to +true+, you will only be allowed to read from the
  # data store during the transaction and any attempts to change the data will
  # raise a PStore::Error.
  #
  # Note that PStore does not support nested transactions.
  #
  def transaction(read_only = false)  # :yields:  pstore
    value = nil
    if !@thread_safe
      raise PStore::Error, "nested transaction" unless @lock.try_lock
    else
      begin
        @lock.lock
      rescue ThreadError
        raise PStore::Error, "nested transaction"
      end
    end
    begin
      @rdonly = read_only
      @abort = false
      file = open_and_lock_file(@filename, read_only)
      if file
        begin
          @table, checksum, original_data_size = load_data(file, read_only)

catch(:pstore_abort_transaction) do
            value = yield(self)
          end

if !@abort && !read_only
            save_data(checksum, original_data_size, file)
          end
        ensure
          file.close
        end
      else
        # This can only occur if read_only == true.
        @table = {}
        catch(:pstore_abort_transaction) do
          value = yield(self)
        end
      end
    ensure
      @lock.unlock
    end
    value
  end

private
  # Constant for relieving Ruby's garbage collector.
  CHECKSUM_ALGO = %w[SHA512 SHA384 SHA256 SHA1 RMD160 MD5].each do |algo|
    begin
      break Digest(algo)
    rescue LoadError
    end
  end
  EMPTY_STRING = ""
  EMPTY_MARSHAL_DATA = Marshal.dump({})
  EMPTY_MARSHAL_CHECKSUM = CHECKSUM_ALGO.digest(EMPTY_MARSHAL_DATA)

#
  # Open the specified filename (either in read-only mode or in
  # read-write mode) and lock it for reading or writing.
  #
  # The opened File object will be returned. If _read_only_ is true,
  # and the file does not exist, then nil will be returned.
  #
  # All exceptions are propagated.
  #
  def open_and_lock_file(filename, read_only)
    if read_only
      begin
        file = File.new(filename, RD_ACCESS)
        begin
          file.flock(File::LOCK_SH)
          return file
        rescue
          file.close
          raise
        end
      rescue Errno::ENOENT
        return nil
      end
    else
      file = File.new(filename, RDWR_ACCESS)
      file.flock(File::LOCK_EX)
      return file
    end
  end

# Load the given PStore file.
  # If +read_only+ is true, the unmarshalled Hash will be returned.
  # If +read_only+ is false, a 3-tuple will be returned: the unmarshalled
  # Hash, a checksum of the data, and the size of the data.
  def load_data(file, read_only)
    if read_only
      begin
        table = load(file)
        raise Error, "PStore file seems to be corrupted." unless table.is_a?(Hash)
      rescue EOFError
        # This seems to be a newly-created file.
        table = {}
      end
      table
    else
      data = file.read
      if data.empty?
        # This seems to be a newly-created file.
        table = {}
        checksum = empty_marshal_checksum
        size = empty_marshal_data.bytesize
      else
        table = load(data)
        checksum = CHECKSUM_ALGO.digest(data)
        size = data.bytesize
        raise Error, "PStore file seems to be corrupted." unless table.is_a?(Hash)
      end
      data.replace(EMPTY_STRING)
      [table, checksum, size]
    end
  end

def on_windows?
    is_windows = RUBY_PLATFORM =~ /mswin|mingw|bccwin|wince/
    self.class.__send__(:define_method, :on_windows?) do
      is_windows
    end
    is_windows
  end

def save_data(original_checksum, original_file_size, file)
    new_data = dump(@table)

if new_data.bytesize != original_file_size || CHECKSUM_ALGO.digest(new_data) != original_checksum
      if @ultra_safe && !on_windows?
        # Windows doesn't support atomic file renames.
        save_data_with_atomic_file_rename_strategy(new_data, file)
      else
        save_data_with_fast_strategy(new_data, file)
      end
    end

new_data.replace(EMPTY_STRING)
  end

def save_data_with_atomic_file_rename_strategy(data, file)
    temp_filename = "#{@filename}.tmp.#{Process.pid}.#{rand 1000000}"
    temp_file = File.new(temp_filename, WR_ACCESS)
    begin
      temp_file.flock(File::LOCK_EX)
      temp_file.write(data)
      temp_file.flush
      File.rename(temp_filename, @filename)
    rescue
      File.unlink(temp_file) rescue nil
      raise
    ensure
      temp_file.close
    end
  end

def save_data_with_fast_strategy(data, file)
    file.rewind
    file.write(data)
    file.truncate(data.bytesize)
  end

# This method is just a wrapped around Marshal.dump
  # to allow subclass overriding used in YAML::Store.
  def dump(table)  # :nodoc:
    Marshal::dump(table)
  end

# This method is just a wrapped around Marshal.load.
  # to allow subclass overriding used in YAML::Store.
  def load(content)  # :nodoc:
    Marshal::load(content)
  end

def empty_marshal_data
    EMPTY_MARSHAL_DATA
  end
  def empty_marshal_checksum
    EMPTY_MARSHAL_CHECKSUM
  end
end

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