From: Tom Lane <tgl@sss.pgh.pa.us>
To: Justin Pryzby <pryzby@telsasoft.com>
cc: pgsql-hackers@lists.postgresql.org, Thomas Munro <thomas.munro@gmail.com>
Subject: Re: pg15b1: FailedAssertion("val > base",
 File: "...src/include/utils/relptr.h", Line: 67, PID: 30485)
In-reply-to: <20220519193839.GT19626@telsasoft.com>
References: <20220519193839.GT19626@telsasoft.com>
Comments: In-reply-to Justin Pryzby <pryzby@telsasoft.com>
	message dated "Thu, 19 May 2022 14:38:39 -0500"
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Date: Thu, 19 May 2022 17:16:03 -0400
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Justin Pryzby <pryzby@telsasoft.com> writes:
> ./tmp_install/usr/local/pgsql/bin/postgres -D ./src/test/regress/tmp_che=
ck/data -c min_dynamic_shared_memory=3D1MB
> TRAP: FailedAssertion("val > base", File: "../../../../src/include/utils=
/relptr.h", Line: 67, PID: 21912)

Yeah, I see it too.

> It looks like this may be pre-existing problem exposed by
> commit e07d4ddc55fdcf82082950b3eb0cd8f728284c9d

Agreed.  Here I see

#5  FreePageManagerInitialize (fpm=3Dfpm@entry=3D0x7f34b3ddd300, =

    base=3Dbase@entry=3D0x7f34b3ddd300 "") at freepage.c:187
#6  0x000000000082423e in dsm_shmem_init () at dsm.c:473

so that where we do

	relptr_store(base, fpm->self, fpm);

the "relative" pointer value would have to be zero, making the case
indistinguishable from a NULL pointer.  We can either change the
caller so that these addresses aren't the same, or give up the
ability to store NULL in relptrs ... doesn't seem like a hard call.

One interesting question I didn't look into is why it takes a nondefault
value of min_dynamic_shared_memory to expose this bug.

			regards, tom lane