Re: SegFault on 9.6.14

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?


regards

-- 
Tomas Vondra                  http://www.2ndQuadrant.com
PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services