Zlib::GzipReader#pos underflows after calling #ungetbyte or #ungetc at start of file [Bug #13616]

patched by Andrew Haines <andrew@haines.org.nz> [ruby-core:81488]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@59333 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

ext/zlib/zlib.c

@@ -3393,7 +3393,14 @@ static VALUE
 rb_gzfile_total_out(VALUE obj)
 {
     struct gzfile *gz = get_gzfile(obj);
-    return rb_uint2inum(gz->z.stream.total_out - ZSTREAM_BUF_FILLED(&gz->z));
+    uLong total_out = gz->z.stream.total_out;
+    long buf_filled = ZSTREAM_BUF_FILLED(&gz->z);
+
+    if (total_out >= (uLong)buf_filled) {
+        return rb_uint2inum(total_out - buf_filled);
+    } else {
+        return LONG2FIX(-(buf_filled - total_out));
+    }
 }
 
 /*

test/zlib/test_zlib.rb

@@ -663,6 +663,18 @@ if defined? Zlib
       }
     end
 
+    def test_ungetc_at_start_of_file
+      s = "".dup
+      w = Zlib::GzipWriter.new(StringIO.new(s))
+      w << "abc"
+      w.close
+      r = Zlib::GzipReader.new(StringIO.new(s))
+
+      r.ungetc ?!
+
+      assert_equal(-1, r.pos, "[ruby-core:81488][Bug #13616]")
+    end
+
     def test_open
       Tempfile.create("test_zlib_gzip_reader_open") {|t|
         t.close