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require 'fileutils'
class File
# Write to a file atomically. Useful for situations where you don't
# want other processes or threads to see half-written files.
#
# File.atomic_write('important.file') do |file|
# file.write('hello')
# end
#
# If your temp directory is not on the same filesystem as the file you're
# trying to write, you can provide a different temporary directory.
#
# File.atomic_write('/data/something.important', '/data/tmp') do |file|
# file.write('hello')
# end
def self.atomic_write(file_name, temp_dir = Dir.tmpdir)
require 'tempfile' unless defined?(Tempfile)
require 'fileutils' unless defined?(FileUtils)
temp_file = Tempfile.new(basename(file_name), temp_dir)
temp_file.binmode
yield temp_file
temp_file.close
begin
# Get original file permissions
old_stat = stat(file_name)
rescue Errno::ENOENT
# If not possible, probe which are the default permissions in the
# destination directory.
old_stat = probe_stat_in(dirname(file_name))
end
# Overwrite original file with temp file
FileUtils.mv(temp_file.path, file_name)
# Set correct permissions on new file
chown(old_stat.uid, old_stat.gid, file_name)
chmod(old_stat.mode, file_name)
end
# Private utility method.
def self.probe_stat_in(dir) #:nodoc:
basename = [
'.permissions_check',
Thread.current.object_id,
Process.pid,
rand(1000000)
].join('.')
file_name = join(dir, basename)
FileUtils.touch(file_name)
stat(file_name)
ensure
FileUtils.rm_f(file_name) if file_name
end
end
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