[pgpool-hackers: 4047] Re: [pgpool-general: 7811] Re: Error: sorry, too many clients

[Tatsuo Ishii]

FIx committed.

https://git.postgresql.org/gitweb/?p=pgpool2.git;a=commit;h=6ed89298fb9a71e5554ee44d4abe0cfd0f2da0f0

> We have gotten multiple complains like below.
> 
> I still have not gotten good idea to fix the counter leak in any case,
> but I feel like I need to fix the case at least when
> reserved_connections is 0.
> 
> Varying no objection, I will commit the patch to all supported
> branches.
> --
> Tatsuo Ishii
> SRA OSS, Inc. Japan
> English: http://www.sraoss.co.jp/index_en.php
> Japanese:http://www.sraoss.co.jp
> 
>>>> In our environment:
>>>> reserved_connections=0
>>>> listen_backlog_multiplier=2
>>>> 
>>>> So, if I understand correctly, we have 2 different problematic cases:
>>>> 
>>>> (1) a client silently disappears (doesn't properly close the connection):
>>>> In this case, setting `child_idle_limit` to a non-zero value (say 15min)
>>>> could mitigate the problem, and eventually the child worker will again be
>>>> available to handle an incoming connection.
>>>>
>>>> (2) a server-side child process crashes: In this case, Pgpool still keeps a
>>>> dangling reference to this process, and if some client is routed there will
>>>> get a "sorry, too many clients" error.
>>> 
>>> Since the counter is shared by all process, any process could get a
>>> "sorry..." error.
>>> 
>>>> As for now, this problem cannot be
>>>> mitigated unless the (parent) Pgpool process is restarted.
>>> 
>>> Yes.
>>> 
>>>> One more question:
>>>> What can make a child process crash? (well, I mean, what are the most
>>>> common reasons for that).
>>> 
>>> Besides admin's mistake, probabbly the most case is OOM killer.
>>> 
>>>> Does this crash leave any trace (that I should search for) in Pgpool's
>>>> error log? Does parent Pgpool get notified on this (e.g. via waitpid or
>>>> SIGCHLD)?
>>> 
>>> Yes, Pgpool parent process should leave a log when catching SIGCHLD.
>>> If I kill a child process using kill -9, I got following log:
>>> 
>>> 2021-10-18 14:44:48.951: main pid 997555: LOG:  child process with pid: 997582 exits with status 9 by signal 9
>>> 2021-10-18 14:44:48.952: main pid 997555: LOG:  fork a new child process with pid: 997609
>> 
>> Actually if reserved_connections=0, then there's no need to manage the
>> connection counter. So I have created a patch to eliminate the
>> management part. You will never be troubled by the "sorry..."  error.
>> 
>> If you like, please try attached patch.
>> --
>> Tatsuo Ishii
>> SRA OSS, Inc. Japan
>> English: http://www.sraoss.co.jp/index_en.php
>> Japanese:http://www.sraoss.co.jp
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