Class: File

Inherits:
Object show all
Defined in:
lib/active_support/core_ext/file/atomic.rb

Class Method Summary collapse

Class Method Details

.atomic_write(file_name, temp_dir = dirname(file_name)) ⇒ Object

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


21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
# File 'lib/active_support/core_ext/file/atomic.rb', line 21

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)
    else
      # 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

.probe_stat_in(dir) ⇒ Object

Private utility method.



56
57
58
59
60
61
62
63
64
65
66
67
68
69
# File 'lib/active_support/core_ext/file/atomic.rb', line 56

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