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# 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 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.
#
# There are three important concepts here (details at the links):
#
# - {Store}[rdoc-ref:PStore@The+Store]: a store is an instance of \PStore.
# - {Entries}[rdoc-ref:PStore@Entries]: the store is hash-like;
# each entry is the key for a stored object.
# - {Transactions}[rdoc-ref:PStore@Transactions]: each transaction is a collection
# of prospective changes to the store;
# a transaction is defined in the block given with a call
# to PStore#transaction.
#
# == About the Examples
#
# Examples on this page need a store that has known properties.
# They can get a new (and populated) store by calling thus:
#
# example_store do |store|
# # Example code using store goes here.
# end
#
# All we really need to know about +example_store+
# is that it yields a fresh store with a known population of entries;
# its implementation:
#
# require 'pstore'
# require 'tempfile'
# # Yield a pristine store for use in examples.
# def example_store
# # Create the store in a temporary file.
# Tempfile.create do |file|
# store = PStore.new(file)
# # Populate the store.
# store.transaction do
# store[:foo] = 0
# store[:bar] = 1
# store[:baz] = 2
# end
# yield store
# end
# end
#
# == The Store
#
# The contents of the store are maintained in a file whose path is specified
# when the store is created (see PStore.new).
# The objects are stored and retrieved using
# module Marshal, which means that certain objects cannot be added to the store;
# see {Marshal::dump}[rdoc-ref:Marshal.dump].
#
# == Entries
#
# A store may have any number of entries.
# Each entry has a key and a value, just as in a hash:
#
# - Key: as in a hash, the key can be (almost) any object;
# see {Hash Keys}[rdoc-ref:Hash@Hash+Keys].
# You may find it convenient to keep it simple by using only
# symbols or strings as keys.
# - Value: the value may be any object that can be marshalled by \Marshal
# (see {Marshal::dump}[rdoc-ref:Marshal.dump])
# and in fact may be a collection
# (e.g., an array, a hash, a set, a range, etc).
# That collection may in turn contain nested objects,
# including collections, to any depth;
# those objects must also be \Marshal-able.
# See {Hierarchical Values}[rdoc-ref:PStore@Hierarchical+Values].
#
# == Transactions
#
# === The Transaction Block
#
# The block given with a call to method #transaction#
# contains a _transaction_,
# which consists of calls to \PStore methods that
# read from or write to the store
# (that is, all \PStore methods except #transaction itself,
# #path, and Pstore.new):
#
# example_store do |store|
# store.transaction do
# store.keys # => [:foo, :bar, :baz]
# store[:bat] = 3
# store.keys # => [:foo, :bar, :baz, :bat]
# end
# end
#
# Execution of the transaction is deferred until the block exits,
# and is executed _atomically_ (all-or-nothing):
# either all transaction calls are executed, or none are.
# This maintains the integrity of the store.
#
# Other code in the block (including even calls to #path and PStore.new)
# is executed immediately, not deferred.
#
# The transaction block:
#
# - May not contain a nested call to #transaction.
# - Is the only context where methods that read from or write to
# the store are allowed.
#
# As seen above, changes in a transaction are made automatically
# when the block exits.
# The block may be exited early by calling method #commit or #abort.
#
# - Method #commit triggers the update to the store and exits the block:
#
# example_store do |store|
# store.transaction do
# store.keys # => [:foo, :bar, :baz]
# store[:bat] = 3
# store.commit
# fail 'Cannot get here'
# end
# store.transaction do
# # Update was completed.
# store.keys # => [:foo, :bar, :baz, :bat]
# end
# end
#
# - Method #abort discards the update to the store and exits the block:
#
# example_store do |store|
# store.transaction do
# store.keys # => [:foo, :bar, :baz]
# store[:bat] = 3
# store.abort
# fail 'Cannot get here'
# end
# store.transaction do
# # Update was not completed.
# store.keys # => [:foo, :bar, :baz]
# end
# end
#
# === Read-Only Transactions
#
# By default, a transaction allows both reading from and writing to
# the store:
#
# store.transaction do
# # Read-write transaction.
# # Any code except a call to #transaction is allowed here.
# end
#
# If argument +read_only+ is passed as +true+,
# only reading is allowed:
#
# store.transaction(true) do
# # Read-only transaction:
# # Calls to #transaction, #[]=, and #delete are not allowed here.
# end
#
# == Hierarchical Values
#
# The value for an entry may be a simple object (as seen above).
# It may also be a hierarchy of objects nested to any depth:
#
# deep_store = PStore.new('deep.store')
# deep_store.transaction do
# array_of_hashes = [{}, {}, {}]
# deep_store[:array_of_hashes] = array_of_hashes
# deep_store[:array_of_hashes] # => [{}, {}, {}]
# hash_of_arrays = {foo: [], bar: [], baz: []}
# deep_store[:hash_of_arrays] = hash_of_arrays
# deep_store[:hash_of_arrays] # => {:foo=>[], :bar=>[], :baz=>[]}
# deep_store[:hash_of_arrays][:foo].push(:bat)
# deep_store[:hash_of_arrays] # => {:foo=>[:bat], :bar=>[], :baz=>[]}
# end
#
# And recall that you can use
# {dig methods}[rdoc-ref:dig_methods.rdoc]
# in a returned hierarchy of objects.
#
# == Working with the Store
#
# === Creating a Store
#
# Use method PStore.new to create a store.
# The new store creates or opens its containing file:
#
# store = PStore.new('t.store')
#
# === Modifying the Store
#
# Use method #[]= to update or create an entry:
#
# example_store do |store|
# store.transaction do
# store[:foo] = 1 # Update.
# store[:bam] = 1 # Create.
# end
# end
#
# Use method #delete to remove an entry:
#
# example_store do |store|
# store.transaction do
# store.delete(:foo)
# store[:foo] # => nil
# end
# end
#
# === Retrieving Values
#
# Use method #fetch (allows default) or #[] (defaults to +nil+)
# to retrieve an entry:
#
# example_store do |store|
# store.transaction do
# store[:foo] # => 0
# store[:nope] # => nil
# store.fetch(:baz) # => 2
# store.fetch(:nope, nil) # => nil
# store.fetch(:nope) # Raises exception.
# end
# end
#
# === Querying the Store
#
# Use method #key? to determine whether a given key exists:
#
# example_store do |store|
# store.transaction do
# store.key?(:foo) # => true
# end
# end
#
# Use method #keys to retrieve keys:
#
# example_store do |store|
# store.transaction do
# store.keys # => [:foo, :bar, :baz]
# end
# end
#
# Use method #path to retrieve the path to the store's underlying file;
# this method may be called from outside a transaction block:
#
# store = PStore.new('t.store')
# store.path # => "t.store"
#
# == Transaction Safety
#
# For transaction safety, see:
#
# - Optional argument +thread_safe+ at method PStore.new.
# - Attribute #ultra_safe.
#
# Needless to say, if you're storing valuable data with \PStore, then you should
# backup the \PStore file from time to time.
#
# == An Example Store
#
# require "pstore"
#
# # A mock wiki object.
# class WikiPage
#
# attr_reader :page_name
#
# def initialize(page_name, author, contents)
# @page_name = page_name
# @revisions = Array.new
# add_revision(author, contents)
# end
#
# 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 wiki page.
# home_page = WikiPage.new("HomePage", "James Edward Gray II",
# "A page about the JoysOfDocumentation..." )
#
# wiki = PStore.new("wiki_pages.pstore")
# # Update page data and the index together, or not at all.
# wiki.transaction do
# # Store page.
# wiki[home_page.page_name] = home_page
# # Create page index.
# wiki[:wiki_index] ||= Array.new
# # Update wiki index.
# wiki[:wiki_index].push(*home_page.wiki_page_references)
# end
#
# # Read wiki data, setting argument read_only to true.
# wiki.transaction(true) do
# wiki.keys.each do |key|
# puts key
# puts wiki[key]
# end
# end
#
class PStore
VERSION = "0.1.3"
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 an unlikely error (such as memory-error or filesystem error) occurs:
#
# - +true+: changes are posted by creating a temporary file,
# writing the updated data to it, then renaming the file to the given #path.
# File integrity is maintained.
# Note: has effect only if the filesystem has atomic file rename
# (as do POSIX platforms Linux, MacOS, FreeBSD and others).
#
# - +false+ (the default): changes are posted by rewinding the open file
# and writing the updated data.
# File integrity is maintained if the filesystem raises
# no unexpected I/O error;
# if such an error occurs during a write to the store,
# the file may become corrupted.
#
attr_accessor :ultra_safe
# Returns a new \PStore object.
#
# Argument +file+ is the path to the file in which objects are to be stored;
# if the file exists, it should be one that was written by \PStore.
#
# path = 't.store'
# store = PStore.new(path)
#
# A \PStore object is
# {reentrant}[https://en.wikipedia.org/wiki/Reentrancy_(computing)].
# If argument +thread_safe+ is given as +true+,
# the object is also thread-safe (at the cost of a small performance penalty):
#
# store = PStore.new(path, true)
#
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
# Returns the value for the given +key+ if the key exists.
# +nil+ otherwise;
# if not +nil+, the returned value is an object or a hierarchy of objects:
#
# example_store do |store|
# store.transaction do
# store[:foo] # => 0
# store[:nope] # => nil
# end
# end
#
# Returns +nil+ if there is no such key.
#
# See also {Hierarchical Values}[rdoc-ref:PStore@Hierarchical+Values].
#
# Raises an exception if called outside a transaction block.
def [](key)
in_transaction
@table[key]
end
# Like #[], except that it accepts a default value for the store.
# If the +key+ does not exist:
#
# - Raises an exception if +default+ is +PStore::Error+.
# - Returns the value of +default+ otherwise:
#
# example_store do |store|
# store.transaction do
# store.fetch(:nope, nil) # => nil
# store.fetch(:nope) # Raises an exception.
# end
# end
#
# Raises an exception if called outside a transaction block.
def fetch(key, default=PStore::Error)
in_transaction
unless @table.key? key
if default == PStore::Error
raise PStore::Error, format("undefined key `%s'", key)
else
return default
end
end
@table[key]
end
# Creates or replaces the value for the given +key+:
#
# example_store do |store|
# temp.transaction do
# temp[:bat] = 3
# end
# end
#
# See also {Hierarchical Values}[rdoc-ref:PStore@Hierarchical+Values].
#
# Raises an exception if called outside a transaction block.
def []=(key, value)
in_transaction_wr
@table[key] = value
end
# Removes and returns the value at +key+ if it exists:
#
# example_store do |store|
# store.transaction do
# store[:bat] = 3
# store.delete(:bat)
# end
# end
#
# Returns +nil+ if there is no such key.
#
# Raises an exception if called outside a transaction block.
def delete(key)
in_transaction_wr
@table.delete key
end
# Returns an array of the existing keys:
#
# example_store do |store|
# store.transaction do
# store.keys # => [:foo, :bar, :baz]
# end
# end
#
# Raises an exception if called outside a transaction block.
def keys
in_transaction
@table.keys
end
alias roots keys
# Returns +true+ if +key+ exists, +false+ otherwise:
#
# example_store do |store|
# store.transaction do
# store.key?(:foo) # => true
# end
# end
#
# Raises an exception if called outside a transaction block.
def key?(key)
in_transaction
@table.key? key
end
alias root? key?
# Returns the string file path used to create the store:
#
# store.path # => "flat.store"
#
def path
@filename
end
# Exits the current transaction block, committing any changes
# specified in the
# {transaction block}[rdoc-ref:PStore@The+Transaction+Block].
#
# Raises an exception if called outside a transaction block.
def commit
in_transaction
@abort = false
throw :pstore_abort_transaction
end
# Exits the current transaction block, discarding any changes
# specified in the
# {transaction block}[rdoc-ref:PStore@The+Transaction+Block].
#
# Raises an exception if called outside a transaction block.
def abort
in_transaction
@abort = true
throw :pstore_abort_transaction
end
# Opens a transaction block for the store.
# See {Transactions}[rdoc-ref:PStore@Transactions].
#
# With argument +read_only+ as +false+, the block may both read from
# and write to the store.
#
# With argument +read_only+ as +true+, the block may not include calls
# to #transaction, #[]=, or #delete.
#
# Raises an exception if called within a transaction block.
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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