Re: pg15b1: FailedAssertion("val > base", File: "...src/include/utils/relptr.h", Line: 67, PID: 30485)

From: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
To: Justin Pryzby <pryzby(at)telsasoft(dot)com>
Cc: pgsql-hackers(at)lists(dot)postgresql(dot)org, Thomas Munro <thomas(dot)munro(at)gmail(dot)com>
Subject: Re: pg15b1: FailedAssertion("val > base", File: "...src/include/utils/relptr.h", Line: 67, PID: 30485)
Date: 2022-05-19 21:16:03
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Justin Pryzby <pryzby(at)telsasoft(dot)com> writes:
> ./tmp_install/usr/local/pgsql/bin/postgres -D ./src/test/regress/tmp_check/data -c min_dynamic_shared_memory=1MB
> 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=fpm(at)entry=0x7f34b3ddd300,
base=base(at)entry=0x7f34b3ddd300 "") 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

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