Do not raise Zlib::BufError for a repeated flush with no new input

ext/zlib/zlib.c

@@ -1195,6 +1195,13 @@ zstream_run_try(VALUE value_arg)
        }
     }
 
+    if (err == Z_BUF_ERROR && z->func == &deflate_funcs && z->stream.avail_in == 0) {
+	/* deflate() reports Z_BUF_ERROR for a flush when no input was consumed
+	 * since the previous flush of the same or higher rank. Nothing is lost,
+	 * so treat it as a no-op like zlib's own gzflush() does. */
+	err = Z_OK;
+    }
+
     if (flush != Z_FINISH && err == Z_BUF_ERROR && z->stream.avail_out > 0) {
         z->flags |= ZSTREAM_FLAG_IN_STREAM;
     }

test/zlib/test_zlib.rb

@@ -121,6 +121,16 @@ def test_flush
       assert_equal("foobarbazqux", Zlib::Inflate.inflate(s))
     end
 
+    def test_flush_twice
+      z = Zlib::Deflate.new
+      s = z.deflate("foo", Zlib::SYNC_FLUSH)
+      s << z.flush << z.flush
+      s << z.flush(Zlib::FULL_FLUSH) << z.flush(Zlib::SYNC_FLUSH)
+      s << z.deflate("", Zlib::SYNC_FLUSH)
+      s << z.finish
+      assert_equal("foo", Zlib::Inflate.inflate(s))
+    end
+
     def test_avail
       z = Zlib::Deflate.new
       assert_equal(0, z.avail_in)
@@ -1535,6 +1545,22 @@ def test_puts
       }
     end
 
+    def test_flush_twice
+      io = StringIO.new("".b)
+      Zlib::GzipWriter.wrap(io) do |gz|
+        gz.write("a")
+        gz.flush
+        gz.flush
+        gz.flush(Zlib::FULL_FLUSH)
+        gz.flush(Zlib::SYNC_FLUSH)
+        gz.sync = true
+        gz.write("")
+        gz.write("")
+        gz.write("b")
+      end
+      assert_equal("ab", Zlib.gunzip(io.string))
+    end
+
     def test_writer_wrap
       Tempfile.create("test_zlib_gzip_writer_wrap") {|t|
         t.binmode