Make ActiveSupport::Gzip.compress deterministic

`ActiveSupport::Gzip.compress` used to include the current timestamp in the gzipped output (the default behavior of Zlib), causing multiple calls with the same input data to output different results depending on when it was called. In many situations (such as caching), ensuring that the output is always the same for a given input is useful, so this helper now always sets the timestamp to `0` instead.

Author: Mr0grog

activesupport/CHANGELOG.md

@@ -1,3 +1,12 @@
+*   Make `ActiveSupport::Gzip.compress` deterministic based on input.
+
+    `ActiveSupport::Gzip.compress` used to include a timestamp in the output,
+    causing consecutive calls with the same input data to have different output
+    if called during different seconds. It now always sets the timestamp to `0`
+    so that the output is identical for any given input.
+
+    *Rob Brackett*
+
 *   Given an array of `Thread::Backtrace::Location` objects, the new method
     `ActiveSupport::BacktraceCleaner#clean_locations` returns an array with the
     clean ones:

activesupport/lib/active_support/gzip.rb

@@ -32,6 +32,7 @@ def self.decompress(source)
     def self.compress(source, level = Zlib::DEFAULT_COMPRESSION, strategy = Zlib::DEFAULT_STRATEGY)
       output = Stream.new
       gz = Zlib::GzipWriter.new(output, level, strategy)
+      gz.mtime = 0
       gz.write(source)
       gz.close
       output.string

activesupport/test/gzip_test.rb

@@ -42,4 +42,9 @@ def test_decompress_checks_crc
       ActiveSupport::Gzip.decompress(compressed)
     end
   end
+
+  def test_sets_mtime_to_zero
+    compressed = ActiveSupport::Gzip.compress("Hello World")
+    assert_equal Time.at(0), Zlib::GzipReader.new(StringIO.new(compressed)).mtime
+  end
 end