Merge pull request rails#55382 from Mr0grog/deterministic-gzip-compression

Make `ActiveSupport::Gzip.compress` deterministic

Author: byroot

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