Date: Wed, 1 Nov 2023 08:47:33 -0400
From: Bruce Momjian <bruce@momjian.us>
To: Tom Lane <tgl@sss.pgh.pa.us>
Cc: Yugo NAGATA <nagata@sraoss.co.jp>,
	Alvaro Herrera <alvherre@alvh.no-ip.org>,
	pgsql-hackers@postgresql.org
Subject: Re: Question about non-blocking mode in libpq
Message-ID: <ZUJI5ea7Ii0Um9am@momjian.us>
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On Tue, Oct 31, 2023 at 10:16:07PM -0400, Tom Lane wrote:
> Bruce Momjian <bruce@momjian.us> writes:
> > On Tue, Oct 31, 2023 at 09:11:06PM -0400, Tom Lane wrote:
> >> What I'm objecting to is removal of the bit about "if they need to be
> >> called again".  That provides a hint that retry is the appropriate
> >> response to a failure.  Admittedly, it's not 100% clear, but your
> >> version makes it 0% clear.
> 
> > I thought the original docs said you had to re-call on failure (it would
> > not block but it would fail if it could not be sent), while we are now
> > saying that it will be queued in the input buffer.
> 
> For these functions in nonblock mode, failure means "we didn't queue it".
> 
> > Is retry really something we need to mention now?  If out of memory is
> > our only failure case now ("unable to enlarge the buffer because OOM"),
> > is retry really a realistic option?
> 
> Well, ideally the application would do something to alleviate the
> OOM problem before retrying.  I don't know if we want to go so far
> as to discuss that.  I do object to giving the impression that
> failure is impossible, which I think your proposed wording does.
> 
> An orthogonal issue with your latest wording is that it's unclear
> whether *unsuccessful* calls to these functions will block.

Okay, I see your point now.  Here is an updated patch that addresses
both issues.

-- 
  Bruce Momjian  <bruce@momjian.us>        https://momjian.us
  EDB                                      https://enterprisedb.com

  Only you can decide what is important to you.

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diff --git a/doc/src/sgml/libpq.sgml b/doc/src/sgml/libpq.sgml
index 64b2910fee..ffff0e528d 100644
--- a/doc/src/sgml/libpq.sgml
+++ b/doc/src/sgml/libpq.sgml
@@ -5316,11 +5316,12 @@ int PQsetnonblocking(PGconn *conn, int arg);
       </para>
 
       <para>
-       In the nonblocking state, calls to
+       In the nonblocking state, successful calls to
        <xref linkend="libpq-PQsendQuery"/>, <xref linkend="libpq-PQputline"/>,
        <xref linkend="libpq-PQputnbytes"/>, <xref linkend="libpq-PQputCopyData"/>,
-       and <xref linkend="libpq-PQendcopy"/> will not block but instead return
-       an error if they need to be called again.
+       and <xref linkend="libpq-PQendcopy"/> will not block;  their changes
+       are stored in the local output buffer until they are flushed. 
+       Unsuccessful calls will return an error and must be retried.
       </para>
 
       <para>

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