Add File.atomic_write to prevent users from seeing half written files. Useful for situations like Page Caching and the like. [Koz]

git-svn-id: http://svn-commit.rubyonrails.org/rails/trunk@6262 5ecf4fe2-1ee6-0310-87b1-e25e094e27de

activesupport/CHANGELOG

@@ -1,5 +1,7 @@
 *SVN*
 
+* Add File.atomic_write,  allows you to write large files in an atomic manner, preventing users from seeing half written files.  [Koz]
+
 * Allow users to provide custom formatters to Logger. [aeden]
 
 * Hash#to_query CGI-escapes its keys.  [Jeremy Kemper]

activesupport/lib/active_support/core_ext/file.rb

@@ -0,0 +1,21 @@
+require 'tempfile'
+
+# 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.imporant", "/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

activesupport/test/core_ext/file_test.rb

@@ -0,0 +1,29 @@
+require File.dirname(__FILE__) + '/../abstract_unit'
+
+class AtomicWriteTest < Test::Unit::TestCase
+  
+  def test_atomic_write_without_errors
+    contents  = "Atomic Text"
+    File.atomic_write(file_name) do |file|
+      file.write(contents)
+      assert !File.exists?(file_name)
+    end
+    assert File.exists?(file_name)
+    assert_equal contents, File.read(file_name)
+  ensure
+    File.unlink(file_name)
+  end
+  
+  def test_atomic_write_doesnt_write_when_block_raises
+    File.atomic_write(file_name) do |file|
+      file.write("testing")
+      raise "something bad"
+    end
+  rescue
+    assert !File.exists?(file_name)
+  end
+  
+  def file_name
+    "atomic.file"
+  end
+end