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# = 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 persistance 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 upated 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 limitations:
#
# 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_transactions = 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_transactions+ 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_transactions

  #
  # 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_transactions = 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
        if @ultra_safe_transactions && file_renames_are_atomic?
          value = run_transaction_with_atomic_file_rename_strategy(file, read_only, &block)
        else
          value = run_transaction_with_regular_strategy(file, read_only, &block)
        end
      else
        # This can only occur if read_only == true.
        @table = {}
        catch(:pstore_abort_transaction) do
          value = yield(self)
        end
      end
    end
  ensure
    @transaction = false
    value
  end

  private
  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

  def file_renames_are_atomic?
    # File renames are atomic on POSIX platforms. As far as I know, Windows is
    # the only non-POSIX platform that Ruby runs on.
    #
    # See also the following URL for information on atomic file writes on Windows:
    # http://blogs.msdn.com/adioltean/archive/2005/12/28/507866.aspx
    # This looks way too complicated: performance would go down the drain if we
    # implement this. Maybe it's a good idea to use TxF in the future, but at the
    # moment very few people use Vista and we'd need to write yet-another native
    # extension.
    @@file_renames_are_atomic ||= \
      RUBY_PLATFORM !~ /mswin/ &&
      RUBY_PLATFORM !~ /mingw/ &&
      RUBY_PLATFORM !~ /bbcwin/ &&
      RUBY_PLATFORM !~ /wince/
  end

  def run_transaction_with_atomic_file_rename_strategy(file, read_only)
    value = nil

    # Load PStore data file.
    begin
      @table = Marshal.load(file)
      if !@table.is_a?(Hash)
        raise Error, "PStore file seems to be corrupted."
      end
    rescue EOFError
      # This is a newly-created file.
      @table = {}
    end

    # Run the transaction block.
    catch(:pstore_abort_transaction) do
      value = yield(self)
    end

    # Write new data back to PStore file, if necessary. We write to a temporary file,
    # then rename that to the original file.
    if !@abort && !read_only
      temp_filename = "#{@filename}.tmp.#{Process.pid}.#{rand 1000000}"
      temp_file = File.new(temp_filename, WR_ACCESS)
      begin
        Marshal.dump(@table, temp_file)
        File.rename(temp_filename, @filename)
      rescue
        File.unlink(temp_file) rescue nil
        raise
      ensure
        temp_file.close
      end
    end
  ensure
    file.close
    value
  end

  def run_transaction_with_regular_strategy(file, read_only)
    value = nil

    # Load PStore data file.
    begin
      @table = Marshal.load(file)
      if !@table.is_a?(Hash)
        raise Error, "PStore file seems to be corrupted."
      end
    rescue EOFError
      # This is a newly-created file.
      @table = {}
    end

    # Run the transaction block.
    catch(:pstore_abort_transaction) do
      value = yield(self)
    end

    # Write new data back to PStore file, if necessary.
    if !@abort && !read_only
      # We must write the marshal data to a temporary buffer,
      # because Marshal.dump might fail and we don't want to
      # corrupt the file.
      data = Marshal.dump(@table)
      file.rewind
      file.truncate(0)
      file.write(data)
      # Free the marshal data; this is good for the garbage collector.
      data.replace("")
    end
  ensure
    file.close
    value
  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