zlib (gzfile_write_raw): do not resize string after .write call

Apparently, a component of Rails implements a buffering .write
method which keeps the String buffer around and makes it unsafe
for us to clear it after calling .write.

This caused Rack::Deflater to give empty results when enabled.

Fortunately, per r61631 / a55abcc0ca,
this misguided optimization was only worth a small (0.5MB) savings
and we still benefit from the majority of the memory savings in
that change.

Thanks to zunda for the bug report.

[ruby-core:90133] [Bug #15356]

Fixes: r61631 (commit a55abcc0ca)
       ("zlib: reduce garbage on gzip writes (deflate)")

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@66268 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

ext/zlib/zlib.c

@@ -2364,7 +2364,6 @@ gzfile_write_raw(struct gzfile *gz)
 	str = zstream_detach_buffer(&gz->z);
 	OBJ_TAINT(str);  /* for safe */
 	rb_funcall(gz->io, id_write, 1, str);
-	rb_str_resize(str, 0);
 	if ((gz->z.flags & GZFILE_FLAG_SYNC)
 	    && rb_respond_to(gz->io, id_flush))
 	    rb_funcall(gz->io, id_flush, 0);

test/zlib/test_zlib.rb

@@ -1081,6 +1081,23 @@ if defined? Zlib
         assert_nothing_raised { w.close }
       }
     end
+
+    def test_zlib_writer_buffered_write
+      bug15356 = '[ruby-core:90346] [Bug #15356]'.freeze
+      fixes = 'r61631 (commit a55abcc0ca6f628fc05304f81e5a044d65ab4a68)'.freeze
+      ary = []
+      def ary.write(*args)
+        self.concat(args)
+      end
+      gz = Zlib::GzipWriter.new(ary)
+      gz.write(bug15356)
+      gz.write("\n")
+      gz.write(fixes)
+      gz.close
+      assert_not_predicate ary, :empty?
+      exp = [ bug15356, fixes ]
+      assert_equal exp, Zlib.gunzip(ary.join('')).split("\n")
+    end
   end
 
   class TestZlib < Test::Unit::TestCase