Message-ID: <10906444-d995-0a34-bb22-faf6d9f07f92@enterprisedb.com>
Date: Mon, 27 Dec 2021 13:44:29 +0100
MIME-Version: 1.0
User-Agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10.15; rv:91.0)
 Gecko/20100101 Thunderbird/91.4.0
Subject: Re: psql - add SHOW_ALL_RESULTS option
Content-Language: en-US
To: Fabien COELHO <coelho@cri.ensmp.fr>
Cc: "pgsql-hackers@lists.postgresql.org" <pgsql-hackers@lists.postgresql.org>
References: <69C0B369-570C-4524-8EE4-BCCACECB6BEE@amazon.com>
 <20210412193301.GA11204@alvherre.pgsql> <YHeVHH0wVcRwdogL@paquier.xyz>
 <3738387.1618451507@sss.pgh.pa.us>
 <alpine.DEB.2.22.394.2104151336060.191178@pseudo>
 <451a2111-3757-c511-9413-8f8af531a167@enterprisedb.com>
 <alpine.DEB.2.22.394.2106121130400.4105466@pseudo>
 <CALDaNm0+pzGgp-M1mAoUVYRtjadbWYuB96J8RaV9iH3+HA8FTA@mail.gmail.com>
 <alpine.DEB.2.22.394.2107151742300.1705285@pseudo>
 <18c78a01-4a34-9dd4-f78b-6860f1420c8e@enterprisedb.com>
 <alpine.DEB.2.22.394.2107221655020.25741@pseudo>
 <6954328d-96f2-77f7-735f-7ce493a40949@enterprisedb.com>
 <alpine.DEB.2.22.394.2109251058000.3216312@pseudo>
 <alpine.DEB.2.22.394.2110021622530.2817726@pseudo>
 <2570e2ae-fa0f-aac9-f72f-bb59a9983a20@enterprisedb.com>
 <alpine.DEB.2.22.394.2112230703530.2668598@pseudo>
From: Peter Eisentraut <peter.eisentraut@enterprisedb.com>
In-Reply-To: <alpine.DEB.2.22.394.2112230703530.2668598@pseudo>
Content-Type: text/plain; charset=UTF-8; format=flowed
Content-Transfer-Encoding: 8bit
Archived-At: 
 <https://www.postgresql.org/message-id/10906444-d995-0a34-bb22-faf6d9f07f92%40enterprisedb.com>
Precedence: bulk

On 23.12.21 12:40, Fabien COELHO wrote:
>> In [0], it was reported that certain replication commands result in 
>> infinite loops because of faulty error handling.  This still happens.  
>> I wrote a test for it, attached here.  (I threw in a few more basic 
>> tests, just to have some more coverage that was lacking, and to have a 
>> file to put the new test in.)
> 
> Hmmm… For some unclear reason on errors on a PGRES_COPY_* state 
> PQgetResult keeps on returning an empty result. PQexec manually ignores 
> it, so I did the same with a comment, but for me the real bug is somehow 
> in PQgetResult behavior…
> 
>> In [1], it was reported that server crashes produce duplicate error 
>> messages. This also still happens.  I didn't write a test for it, 
>> maybe you have an idea.  (Obviously, we could check whether the error 
>> message is literally there twice in the output, but that doesn't seem 
>> very general.)  But it's easy to test manually: just have psql 
>> connect, shut down the server, then run a query.
> 
> This is also a feature/bug of libpq which happens to be hidden by 
> PQexec: when one command crashes PQgetResult actually returns *2* 
> results. First one with the FATAL message, second one when libpq figures 
> out that the connection was lost with the second message appended to the 
> first. PQexec just happen to silently ignore the first result. I added a 
> manual reset of the error message when first shown so that it is not 
> shown twice. It is unclear to me whether the reset should be somewhere 
> in libpq instead. I added a voluntary crash at the end of the psql test.

I agree that these two behaviors in libpq are dubious, especially the 
second one.  I want to spend some time analyzing this more and see if 
fixes in libpq might be appropriate.