One, it now returns the previous hook. That way people don't have to dig
around in libpq-int.h for that information anymore. It previously
returned void, so there should be no incompatibilities.

Second, you cannot set the callback to NULL anymore. (Of course you can
still call it with NULL just to get the current hook.) The way libpq uses
the callback pointer, having a NULL there wasn't very healthy.

Peter Eisentraut