Re: Practical impediment to supporting multiple SSL libraries

* Greg Stark (gsstark(at)mit(dot)edu) wrote:
> Martijn van Oosterhout <kleptog(at)svana(dot)org> writes:
> > Hmm, the simplest improvement I can think of is one where you
> > register a callback that libpq calls whenever it has received a new
> > tuple.
>
> That could be useful for applications but I think a driver really wants to
> retain control of the flow of control. To make use of a callback it would have
> to have an awkward dance of calling whatever function gives libpq license to
> call the callback, having the callback stuff the data in a temporary space,
> then checking for new data in the temporary space, and returning it to the
> user.

I doubt the callback would be called at some inopportune time...
Probably the callback would be passed into a libpq call which then
directly calls the callback and is done with it when it returns. The
libpq function would certainly need a parameter which is just passed to
the callback to allow the system to maintain state (such as how many
tuples the callback has processed so far) to avoid ugly global variables
but otherwise I don't really see that this is changing the flow of
control all that much...

I can see how having a callback would be useful though I think for a
good number of cases it's just going to be populating a memory region
with it and we could cover that common case by providing an API for
exactly that. The other issue with a callback is that libpq would have
to either call the callback for each value (not my preference) or have
some way to pass a whole variable-length tuple to the callback, which
would require libpq to allocate memory for the tuple (hopefully only
once and not per-tuple) and then build up whatever structure it's going
to give to the callback in memory (copy once) and then call the callback
which would be required to copy the tuple somewhere else (copy again).
Of course, all of this is after an initial copy from read() into the
read buffer, but I doubt that could be helped (and read()'ing small
enough amounts to make it happen wouldn't really improve things).

Thanks,

Stephen