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

activesupport/lib/active_support/core_ext/file.rb

-require 'tempfile'
+require 'active_support/core_ext/file/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 File.atomic_write(file_name, temp_dir = Dir.tmpdir)
-  temp_file = Tempfile.new(File.basename(file_name), temp_dir)
-  yield temp_file
-  temp_file.close
-  File.rename(temp_file.path, file_name)
-end
\ No newline at end of file
+class File #:nodoc:
+  extend ActiveSupport::CoreExtensions::File::Atomic
+end

activesupport/lib/active_support/core_ext/file/atomic.rb

+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

activesupport/test/core_ext/file_test.rb

 require 'abstract_unit'
 
 class AtomicWriteTest < Test::Unit::TestCase
-  
   def test_atomic_write_without_errors
-    contents  = "Atomic Text"
+    contents = "Atomic Text"
     File.atomic_write(file_name, Dir.pwd) do |file|
       file.write(contents)
       assert !File.exist?(file_name)
@@ -13,7 +12,7 @@ def test_atomic_write_without_errors
   ensure
     File.unlink(file_name) rescue nil
   end
-  
+
   def test_atomic_write_doesnt_write_when_block_raises
     File.atomic_write(file_name) do |file|
       file.write("testing")
@@ -22,8 +21,47 @@ def test_atomic_write_doesnt_write_when_block_raises
   rescue
     assert !File.exist?(file_name)
   end
-  
-  def file_name
-    "atomic.file"
+
+  def test_atomic_write_preserves_file_permissions
+    contents = "Atomic Text"
+    File.open(file_name, "w", 0755) do |file|
+      file.write(contents)
+      assert File.exist?(file_name)
+    end
+    assert File.exist?(file_name)
+    assert_equal "100755", file_mode
+    assert_equal contents, File.read(file_name)
+
+    File.atomic_write(file_name, Dir.pwd) do |file|
+      file.write(contents)
+      assert File.exist?(file_name)
+    end
+    assert File.exist?(file_name)
+    assert_equal "100755", file_mode
+    assert_equal contents, File.read(file_name)
+  ensure
+    File.unlink(file_name) rescue nil
+  end
+
+  def test_atomic_write_preserves_default_file_permissions
+    contents = "Atomic Text"
+    File.atomic_write(file_name, Dir.pwd) do |file|
+      file.write(contents)
+      assert !File.exist?(file_name)
+    end
+    assert File.exist?(file_name)
+    assert_equal "100644", file_mode
+    assert_equal contents, File.read(file_name)
+  ensure
+    File.unlink(file_name) rescue nil
   end
+
+  private
+    def file_name
+      "atomic.file"
+    end
+
+    def file_mode
+      sprintf("%o", File.stat(file_name).mode)
+    end
 end