Preserve permissions when using Pathname#atomic_write

Library/Homebrew/extend/pathname.rb

@@ -96,13 +96,35 @@ class Pathname
 
   # NOTE always overwrites
   def atomic_write content
-    require 'tempfile'
-    tf = Tempfile.new(self.basename.to_s)
+    require "tempfile"
+    tf = Tempfile.new(basename.to_s)
     tf.write(content)
     tf.close
-    FileUtils.mv tf.path, self.to_s
+
+    begin
+      old_stat = stat
+    rescue Errno::ENOENT
+      old_stat = default_stat
+    end
+
+    FileUtils.mv tf.path, self
+
+    begin
+      chown(old_stat.uid, old_stat.gid)
+      chmod(old_stat.mode)
+    rescue Errno::EPERM
+    end
   end
 
+  def default_stat
+    sentinel = parent.join(".brew.#{Process.pid}.#{rand(Time.now.to_i)}")
+    sentinel.open("w") { }
+    sentinel.stat
+  ensure
+    sentinel.unlink
+  end
+  private :default_stat
+
   def cp dst
     if file?
       FileUtils.cp to_s, dst

Library/Homebrew/test/test_pathname.rb

@@ -58,6 +58,19 @@ class PathnameExtensionTests < Test::Unit::TestCase
     assert_equal 'CONTENT', File.read(@file)
   end
 
+  def test_atomic_write_preserves_permissions
+    File.open(@file, "w", 0100777) { }
+    @file.atomic_write("CONTENT")
+    assert_equal 0100777 & ~File.umask, @file.stat.mode
+  end
+
+  def test_atomic_write_preserves_default_permissions
+    @file.atomic_write("CONTENT")
+    sentinel = @file.parent.join("sentinel")
+    touch sentinel
+    assert_equal sentinel.stat.mode, @file.stat.mode
+  end
+
   def test_cp
     touch @file
     mkdir_p @dir