Make File.atomic_write copy the original permissions or use the directories default.

1 files changed, 46 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..4d3cf5423f
--- /dev/null
+++ b/activesupport/lib/active_support/core_ext/file/atomic.rb
@@ -0,0 +1,46 @@
+require 'tempfile'
+
+module ActiveSupport #:nodoc:
+  module CoreExtensions #:nodoc:
+    module File #:nodoc:
+      module Atomic
+        # 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 |f|
+        #     file.write("hello")
+        #   end
+        def atomic_write(file_name, temp_dir = Dir.tmpdir)
+          temp_file = Tempfile.new(basename(file_name), temp_dir)
+          yield temp_file
+          temp_file.close
+
+          begin
+            # Get original file permissions
+            old_stat = stat(file_name)
+          rescue Errno::ENOENT
+            # No old permissions, write a temp file to determine the defaults
+            check_name = ".permissions_check.#{Thread.current.object_id}.#{Process.pid}.#{rand(1000000)}"
+            new(check_name, "w")
+            old_stat = stat(check_name)
+            unlink(check_name)
+          end
+
+          # Overwrite original file with temp file
+          rename(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
+      end
+    end
+  end
+end