Re: libpq COPY handling

Robert Haas <robertmhaas(at)gmail(dot)com> writes:
> On Thu, Apr 25, 2013 at 1:56 PM, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
>> However, the documentation in libpq.sgml is a bit bogus too, because it
>> counsels trying the PQputCopyEnd() call again, which will not work
>> (since we already changed the asyncStatus). We could make that say "a
>> zero result is informational, you might want to try PQflush() later".
>> The trouble with this, though, is that any existing callers that were
>> coded to the old spec would now be broken.

> Changing the meaning of a 0 return code seems like a bad idea.
> However, not ever returning 0 isn't great either: someone could be
> forgiven for writing code that calls PQputCopyData/End() until they
> get a 0 result, then waits for the socket to become write-OK before
> continuing.

Anybody who tried that would have already discovered that it doesn't
work, so that argument seems pretty hollow.

What I'm suggesting is that we fix the documentation to match what
the code actually does, ie 1 and -1 are the only return codes, but
in nonblock mode it may not actually have flushed all the data.
I do not see how that can break any code that works now.

An alternative possibility is to document the zero return case as
"can't happen now, but defined for possible future expansion", which
I rather suspect was the thought process last time this was looked at.
The trouble with that is that, if we ever did try to actually return
zero, we'd more than likely break some code that should have been
checking for the case and wasn't.

Anyway, in view of the lack of complaints from the field, I think
changing the behavior of this code would be much more likely to cause
problems than fix them.

regards, tom lane