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Re: Snapshot management, final

From: Simon Riggs <simon(at)2ndquadrant(dot)com>
To: Alvaro Herrera <alvherre(at)commandprompt(dot)com>
Cc: Pg Patches <pgsql-patches(at)postgresql(dot)org>
Subject: Re: Snapshot management, final
Date: 2008-04-22 21:58:38
Message-ID: 1208901518.4259.1272.camel@ebony.site (view raw or flat)
Thread:
Lists: pgsql-patches
On Tue, 2008-04-22 at 17:50 -0400, Alvaro Herrera wrote:
> Simon Riggs wrote:
> > On Tue, 2008-04-22 at 15:49 -0400, Alvaro Herrera wrote:
> > 
> > > - Three CopySnapshot call sites remain outside snapmgr.c: DoCopy() on
> > > copy.c, ExplainOnePlan() on explain.c and _SPI_execute_plan() on spi.c.
> > > They are there because they grab the current ActiveSnapshot, modify it,
> > > and then use the resulting snapshot.  There is no corresponding
> > > FreeSnapshot, because it's not needed.
> > 
> > Not needed? How can we be certain that the modified snapshot does not
> > outlive its original source?
> 
> It's not CopySnapshot that's not needed, but FreeSnapshot.  The point
> here is that the snapshot will be freed automatically as soon as it is
> PopActiveSnapshot'd out of existance.  CopySnapshot creates a new,
> separate copy of the passed snapshot, and each of them will be freed
> (separately) as soon as their refcounts reach zero.

OK, so it can;t be copied to a longer lived memory context?

-- 
  Simon Riggs
  2ndQuadrant  http://www.2ndQuadrant.com


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