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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
#
# This method needs to create a temporary file. By default it will create it
# in the same directory as the destination file. If you don't like this
# behavior you can provide a different directory but it must be on the
# same physical filesystem as the file you're trying to write.
#
# File.atomic_write('/data/something.important', '/data/tmp') do |file|
# file.write('hello')
# end
def self.atomic_write(file_name, temp_dir = dirname(file_name))
require "tempfile" unless defined?(Tempfile)
Tempfile.open(".#{basename(file_name)}", temp_dir) do |temp_file|
temp_file.binmode
return_val = yield temp_file
temp_file.close
old_stat = if exist?(file_name)
# Get original file permissions
stat(file_name)
elsif temp_dir != dirname(file_name)
# If not possible, probe which are the default permissions in the
# destination directory.
probe_stat_in(dirname(file_name))
end
if old_stat
# Set correct permissions on new file
begin
chown(old_stat.uid, old_stat.gid, temp_file.path)
# This operation will affect filesystem ACL's
chmod(old_stat.mode, temp_file.path)
rescue Errno::EPERM, Errno::EACCES
# Changing file ownership failed, moving on.
end
end
# Overwrite original file with temp file
rename(temp_file.path, file_name)
return_val
end
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