Previously, if a getattr() for an open file could not get the size
information, the file position was returned as the size.  This is
incorrect if seek or ftruncate had previously been used on the stream.

Instead, track the size of the open file and return this value.  In
addition, use this size in preference to the size returned by
g_file_query_info() if a stream is open (this only happens if the stream
doesn't support g_file_query_info_on_write()) since the tracked size is
more likely to be correct due to implementing g_file_replace by writing
to a temporary file.  Tracking the size is only necessary for files
which are created or opened with O_TRUNC since files opened with
O_APPEND will be appended in-place.

https://bugzilla.gnome.org/show_bug.cgi?id=632296