From: Tom Lane <tgl@sss.pgh.pa.us>
To: Robert Haas <robertmhaas@gmail.com>
cc: Alvaro Herrera <alvherre@2ndquadrant.com>,
        PostgreSQL Hackers <pgsql-hackers@lists.postgresql.org>
Subject: Re: hyrax vs. RelationBuildPartitionDesc
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Comments: In-reply-to Robert Haas <robertmhaas@gmail.com>
	message dated "Wed, 13 Mar 2019 12:51:05 -0400"
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Date: Wed, 13 Mar 2019 13:15:45 -0400
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Robert Haas <robertmhaas@gmail.com> writes:
> On Wed, Mar 13, 2019 at 12:42 PM Alvaro Herrera
> <alvherre@2ndquadrant.com> wrote:
>> I remember going over this code's memory allocation strategy a bit to
>> avoid the copy while not incurring potential leaks CacheMemoryContext;
>> as I recall, my idea was to use two contexts, one of which is temporary
>> and used for any potentially leaky callees, and destroyed at the end of
>> the function, and the other contains the good stuff and is reparented to
>> CacheMemoryContext at the end.  So if you have any accidental leaks,
>> they don't affect a long-lived context.  You have to be mindful of not
>> calling leaky code when you're using the permanent one.

> Well, that assumes that the functions which allocate the good stuff do
> not also leak, which seems a bit fragile.

I'm a bit confused as to why there's an issue here at all.  The usual
plan for computed-on-demand relcache sub-structures is that we compute
a working copy that we're going to return to the caller using the
caller's context (which is presumably statement-duration at most)
and then do the equivalent of copyObject to stash a long-lived copy
into the relcache context.  Is this case being done differently, and if
so why?  If it's being done the same, where are we leaking?

I recall having noticed someplace where I thought the relcache partition
support was simply failing to make provisions for cleaning up a cached
structure at relcache entry drop, but I didn't have time to pursue it
right then.  Let me see if I can reconstruct what I was worried about.

			regards, tom lane