If used with fopen(3)/fdopen(3)-ed FILEs, stdio accurately uses non-cancellable internal versions of the functions, i.e. it seems to be fine with regard to cancellation. But if the funopen(3) and f{r,w}open(3) functions were used to open the FILE, and corresponding user functions create cancellation points (they typically have no other choice), then stdio code at least leaks FILE' lock.
The change installs cleanup handler which unlocks FILE. Some minimal restructuring of the code was required to make it use common return place to satisfy hand-rolled pthread_cleanup_pop() requirements.