Re: libpq: PQgetCopyData() and allocation overhead

If you mean exposing PQExpBuffer to users of libpq-fe.h, I'd be very
seriously against that.  I realize that libpq exposes it at an ABI
level, but that doesn't mean we want non-Postgres code to use it.
I also don't see what it'd add for this particular use-case.

One thing I don't care for at all in the proposed API spec is the bit
about how the handler function can scribble on the passed buffer.
Let's not do that.  Declare it const char *, or maybe better const void *.

* Reduces risk of getting the two pointer arguments the wrong way around.

Rather than duplicating most of pqGetCopyData3, I'd suggest revising
it to take a callback, where the callback is either user-supplied
or is supplied by PQgetCopyData to emulate the existing behavior.
This would both avoid duplicate coding and provide a simple check that
you've made a usable callback API (in particular, one that can do
something sane for error cases).

Can do that, sure.  I'll also try benchmarking a variant that doesn't take a callback at all, but gives you the buffer pointer in addition to the size/status return.  I don't generally like callbacks.

Jeroen