Re: libpq COPY handling

<div class="moz-cite-prefix">On 27/04/13 02:48, Tom Lane wrote:<br /></div><blockquote
cite="mid:11420.1366987729@sss.pgh.pa.us"type="cite"><pre wrap="">Robert Haas <a class="moz-txt-link-rfc2396E"
href="mailto:robertmhaas@gmail.com"><robertmhaas@gmail.com></a>writes: 
</pre><blockquote type="cite"><pre wrap="">On Thu, Apr 25, 2013 at 1:56 PM, Tom Lane <a class="moz-txt-link-rfc2396E"
href="mailto:tgl@sss.pgh.pa.us"><tgl@sss.pgh.pa.us></a>wrote: 
</pre><blockquote type="cite"><pre wrap="">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.
</pre></blockquote></blockquote><pre wrap="">
</pre><blockquote type="cite"><pre wrap="">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.
</pre></blockquote><pre wrap="">
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


</pre></blockquote><font size="-1">The original usage of returns codes was a<font size="-1">s</font> an offset for the
OperatingSystem to jump to in a list of addresses to execute, after program co<font size="-1">mp<font
size="-1">letion.</font></font></font> 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. 
Thisdates back to the Mainframe days, pre UNIX, even!<br /><br /> Personally I would prefer zero to indicate an error,
onthe basis that is the default value for memory, but historical usage makes that impractical!  So we are stuck with
zerobeing interpreted as the return code indicating 'Normal' completion - bash, and other Linux shells, are designed on
thisassumption.<br /><br /> I first came across this return code convention when I was programming Mainframes in the
early1970's.<br /><br /><br /> Cheers,<br /> Gavin<br />