1 files changed, 70 insertions, 0 deletions
diff --git a/activesupport/lib/active_support/core_ext/file/atomic.rb b/activesupport/lib/active_support/core_ext/file/atomic.rb
new file mode 100644
index 0000000000..9deceb1bb4
--- /dev/null
+++ b/activesupport/lib/active_support/core_ext/file/atomic.rb
@@ -0,0 +1,70 @@
+# frozen_string_literal: true
+
+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)
+      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
+
+  # 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