Re: SegFault on 9.6.14

Tomas Vondra <tomas.vondra@2ndquadrant.com> writes:

> On Mon, Jul 15, 2019 at 06:48:05PM -0500, Jerry Sievers wrote:
>
>>Greetings Hackers.
>>
>>We have a reproduceable case of $subject that issues a backtrace such as
>>seen below.
>>
>>The query that I'd prefer to sanitize before sending is <30 lines of at
>>a glance, not terribly complex logic.
>>
>>It nonetheless dies hard after a few seconds of running and as expected,
>>results in an automatic all-backend restart.
>>
>>Please advise on how to proceed.  Thanks!
>>
>>bt
>>#0  initscan (scan=scan@entry=0x55d7a7daa0b0, key=0x0, keep_startblock=keep_startblock@entry=1 '\001')
>>    at /build/postgresql-9.6-5O8OLM/postgresql-9.6-9.6.14/build/../src/backend/access/heap/heapam.c:233
>>#1  0x000055d7a72fa8d0 in heap_rescan (scan=0x55d7a7daa0b0, key=key@entry=0x0) at /build/postgresql-9.6-5O8OLM/postgresql-9.6-9.6.14/build/../src/backend/access/heap/heapam.c:1529
>>#2  0x000055d7a7451fef in ExecReScanSeqScan (node=node@entry=0x55d7a7d85100) at /build/postgresql-9.6-5O8OLM/postgresql-9.6-9.6.14/build/../src/backend/executor/nodeSeqscan.c:280
>>#3  0x000055d7a742d36e in ExecReScan (node=0x55d7a7d85100) at /build/postgresql-9.6-5O8OLM/postgresql-9.6-9.6.14/build/../src/backend/executor/execAmi.c:158
>>#4  0x000055d7a7445d38 in ExecReScanGather (node=node@entry=0x55d7a7d84d30) at /build/postgresql-9.6-5O8OLM/postgresql-9.6-9.6.14/build/../src/backend/executor/nodeGather.c:475
>>#5  0x000055d7a742d255 in ExecReScan (node=0x55d7a7d84d30) at /build/postgresql-9.6-5O8OLM/postgresql-9.6-9.6.14/build/../src/backend/executor/execAmi.c:166
>>#6  0x000055d7a7448673 in ExecReScanHashJoin (node=node@entry=0x55d7a7d84110) at /build/postgresql-9.6-5O8OLM/postgresql-9.6-9.6.14/build/../src/backend/executor/nodeHashjoin.c:1019
>>#7  0x000055d7a742d29e in ExecReScan (node=node@entry=0x55d7a7d84110) at /build/postgresql-9.6-5O8OLM/postgresql-9.6-9.6.14/build/../src/backend/executor/execAmi.c:226
>><about 30 lines omitted>
>>
>
> Hmmm, that means it's crashing here:
>
>    if (scan->rs_parallel != NULL)
>        scan->rs_nblocks = scan->rs_parallel->phs_nblocks;     <--- here
>    else
>        scan->rs_nblocks = RelationGetNumberOfBlocks(scan->rs_rd);
>
> But clearly, scan is valid (otherwise it'd crash on the if condition),
> and scan->rs_parallel must me non-NULL. Which probably means the pointer
> is (no longer) valid.
>
> Could it be that the rs_parallel DSM disappears on rescan, or something
> like that?

No clue but something I just tried was to disable parallelism by setting
max_parallel_workers_per_gather to 0 and however the query has not
finished after a few minutes, there is no crash.

Please advise.

Thx

>
>
> regards

-- 
Jerry Sievers
Postgres DBA/Development Consulting
e: postgres.consulting@comcast.net