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Re: [HACKERS] Terminating a backend

From: Bruce Momjian <bruce(at)momjian(dot)us>
To: PostgreSQL-patches <pgsql-patches(at)postgresql(dot)org>
Subject: Re: [HACKERS] Terminating a backend
Date: 2008-04-12 15:32:00
Message-ID: 200804121532.m3CFW0Y23826@momjian.us (view raw or flat)
Thread:
Lists: pgsql-hackerspgsql-patches
Bruce Momjian wrote:
> Bruce Momjian wrote:
> > I have an idea for this TODO item:
> > 
> > 	* Allow administrators to safely terminate individual sessions either
> > 	  via an SQL function or SIGTERM
> > 	
> > 	  Lock table corruption following SIGTERM of an individual backend
> > 	  has been reported in 8.0.  A possible cause was fixed in 8.1, but
> > 	  it is unknown whether other problems exist.  This item mostly
> > 	  requires additional testing rather than of writing any new code.
> > 	
> > 	  http://archives.postgresql.org/pgsql-hackers/2006-08/msg00174.php
> > 
> > When we get the termination signal, why can't we just set a global
> > boolean, do a query cancel, and in the setjmp() code block check the
> > global and exit --- at that stage we know we have released all locks and
> > can exit cleanly.
> 
> I have implemented this idea with the attached patch.

One problem I have with my patch is that SIGTERM is currently used by
the postmaster to shut down backends.  Now because the postmaster knows
that all backend are terminating, it can accept a dirtier shutdown than
one where we are terminating just one backend and the rest are going to
keep running.  The new SIGTERM coding is going to exit a backend only in
a place where cancel is checked.

I need some thoughts in this area.

-- 
  Bruce Momjian  <bruce(at)momjian(dot)us>        http://momjian.us
  EnterpriseDB                             http://enterprisedb.com

  + If your life is a hard drive, Christ can be your backup. +

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