Re: libpq COPY handling

On 27/04/13 02:48, Tom Lane wrote:
> 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
>
>
The original usage of returns codes was as an offset for the Operating
System to jump to in a list of addresses to execute, after program
completion. Zero offset was to the first address for normal completion,
then 4 for the next address... Addresses were stored in 4 bytes. Hence
an historical tendency to define return codes in multiple of 4. This
dates back to the Mainframe days, pre UNIX, even!

Personally I would prefer zero to indicate an error, on the basis that
is the default value for memory, but historical usage makes that
impractical! So we are stuck with zero being interpreted as the return
code indicating 'Normal' completion - bash, and other Linux shells, are
designed on this assumption.

I first came across this return code convention when I was programming
Mainframes in the early 1970's.

Cheers,
Gavin