gdata_entry_get_id should not free current value if not changed
It is not expected that if you call
gdata_entry_get_id for the second time, the firstly returned value became invalid. This needs to be documented, or
self->priv->id should not be freed if not changed. This is a problem in several places of gvfsbackendgoogle.c, where we have to call
g_strdup as a workaround to prevent segfaults...