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[ruby-svn.git] / lib / pstore.rb

# = 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 "fileutils"
require "digest/md5"
require "thread"

#
# 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
  binmode = defined?(File::BINARY) ? File::BINARY : 0
  RDWR_ACCESS = File::RDWR | File::CREAT | binmode
  RD_ACCESS = File::RDONLY | binmode
  WR_ACCESS = File::WRONLY | File::CREAT | File::TRUNC | binmode

  # 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
    @transaction = false
    @filename = file
    @abort = false
    @ultra_safe = false
    if @thread_safe
      @lock = Mutex.new
    else
      @lock = DummyMutex.new
    end
  end

  # Raises PStore::Error if the calling code is not in a PStore#transaction.
  def in_transaction
    raise PStore::Error, "not in transaction" unless @transaction
  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_heirarchy] = { "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, &block)  # :yields:  pstore
    value = nil
    raise PStore::Error, "nested transaction" if @transaction
    @lock.synchronize do
      @rdonly = read_only
      @transaction = true
      @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 if !file.closed?
        end
      else
        # This can only occur if read_only == true.
        @table = {}
        catch(:pstore_abort_transaction) do
          value = yield(self)
        end
      end
    end
    value
  ensure
    @transaction = false
  end
  
  private
  # Constant for relieving Ruby's garbage collector.
  EMPTY_STRING = ""
  EMPTY_MARSHAL_DATA = Marshal.dump({})
  EMPTY_MARSHAL_CHECKSUM = Digest::MD5.digest(EMPTY_MARSHAL_DATA)
  
  class DummyMutex
    def synchronize
      yield
    end
  end
  
  #
  # 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, an MD5 checksum of the data, and the size of the data.
  def load_data(file, read_only)
    if read_only
      begin
        table = load(file)
        if !table.is_a?(Hash)
          raise Error, "PStore file seems to be corrupted."
        end
      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.size
      else
        table = load(data)
        checksum = Digest::MD5.digest(data)
        size = data.size
        if !table.is_a?(Hash)
          raise Error, "PStore file seems to be corrupted."
        end
      end
      data.replace(EMPTY_STRING)
      [table, checksum, size]
    end
  end
  
  def on_windows?
    is_windows = RUBY_PLATFORM =~ /mswin/  ||
                 RUBY_PLATFORM =~ /mingw/  ||
                 RUBY_PLATFORM =~ /bbcwin/ ||
                 RUBY_PLATFORM =~ /wince/
    self.class.__send__(:define_method, :on_windows?) do
      is_windows
    end
    is_windows
  end
  
  # Check whether Marshal.dump supports the 'canonical' option. This option
  # makes sure that Marshal.dump always dumps data structures in the same order.
  # This is important because otherwise, the checksums that we generate may differ.
  def marshal_dump_supports_canonical_option?
    begin
      Marshal.dump(nil, -1, true)
      result = true
    rescue
      result = false
    end
    self.class.__send__(:define_method, :marshal_dump_supports_canonical_option?) do
      result
    end
    result
  end
  
  def save_data(original_checksum, original_file_size, file)
    # We only want to save the new data if the size or checksum has changed.
    # This results in less filesystem calls, which is good for performance.
    if marshal_dump_supports_canonical_option?
      new_data = Marshal.dump(@table, -1, true)
    else
      new_data = dump(@table)
    end
    new_checksum = Digest::MD5.digest(new_data)
    
    if new_data.size != original_file_size || new_checksum != 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.truncate(0)
    file.write(data)
  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

# :enddoc:

if __FILE__ == $0
  db = PStore.new("/tmp/foo")
  db.transaction do
    p db.roots
    ary = db["root"] = [1,2,3,4]
    ary[1] = [1,1.5]
  end

  1000.times do
    db.transaction do
      db["root"][0] += 1
      p db["root"][0]
    end
  end

  db.transaction(true) do
    p db["root"]
  end
end