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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 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 = 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_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 !@thread_safe && @lock.locked?
@lock.synchronize do
@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 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
rescue ThreadError
raise PStore::Error, "nested transaction"
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)
#
# 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 =~ /bccwin/ ||
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.instance_method(:marshal_dump_supports_canonical_option?)
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
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