Ensure that we don't flush too often when deflating.

When fixing rubyzip#322 we initially flushed on every write, but this is bad for
compression perfomance (size-wise). So this commit ensures we only flush
when we're running under JRuby.

Fixes rubyzip#322

Author: hainesr

lib/zip.rb

@@ -60,6 +60,10 @@ module Zip
     restore_times:       true
   }.freeze # :nodoc:
 
+  # :nodoc:
+  # Remove this when JRuby#3962 is fixed.
+  ZLIB_FLUSHING_STRATEGY = defined?(JRUBY_VERSION) ? Zlib::SYNC_FLUSH : Zlib::NO_FLUSH
+
   def reset! # :nodoc:
     @_ran_once = false
     @unicode_names = false

lib/zip/deflater.rb

@@ -15,7 +15,9 @@ def <<(data)
       val   = data.to_s
       @crc  = Zlib.crc32(val, @crc)
       @size += val.bytesize
-      buffer = @zlib_deflater.deflate(data, Zlib::SYNC_FLUSH)
+
+      # When JRuby#3962 is fixed, we can remove the flushing parameter here.
+      buffer = @zlib_deflater.deflate(data, Zip::ZLIB_FLUSHING_STRATEGY)
       return @output_stream if buffer.empty?
 
       @output_stream << @encrypter.encrypt(buffer)

test/deflater_test.rb

@@ -14,6 +14,15 @@ def teardown
     Zip.reset!
   end
 
+  # Remove this test when JRuby#3962 is fixed.
+  def test_deflate_strategy
+    if defined?(JRUBY_VERSION)
+      assert_equal(Zlib::SYNC_FLUSH, Zip::ZLIB_FLUSHING_STRATEGY)
+    else
+      assert_equal(Zlib::NO_FLUSH, Zip::ZLIB_FLUSHING_STRATEGY)
+    end
+  end
+
   def test_output_operator
     txt = load_file('test/data/file2.txt')
     deflate(txt, DEFLATER_TEST_FILE)