Re: Logic problem in SerializeSnapshot()

On Tue, Mar 1, 2016 at 12:34 AM, Rushabh Lathia
<rushabh(dot)lathia(at)gmail(dot)com> wrote:
> During the testing of parallel query (with force_parallel_mode = regress),
> noticed random server crash with the below stack:
>
> #0 0x0000003fc84896d5 in memcpy () from /lib64/libc.so.6
> #1 0x0000000000a36867 in SerializeSnapshot (snapshot=0x1e49f40,
> start_address=0x7f391e9ec728 <Address 0x7f391e9ec728 out of bounds>) at
> snapmgr.c:1523
> #2 0x0000000000522a20 in InitializeParallelDSM (pcxt=0x1e49ce0) at
> parallel.c:330
> #3 0x00000000006dd256 in ExecInitParallelPlan (planstate=0x1f012b0,
> estate=0x1f00be8, nworkers=1) at execParallel.c:398
> #4 0x00000000006f8abb in ExecGather (node=0x1f00d00) at nodeGather.c:160
> #5 0x00000000006de42e in ExecProcNode (node=0x1f00d00) at
> execProcnode.c:516
> #6 0x00000000006da4fd in ExecutePlan (estate=0x1f00be8,
> planstate=0x1f00d00, use_parallel_mode=1 '\001', operation=CMD_SELECT,
> sendTuples=1 '\001', numberTuples=0,
> direction=ForwardScanDirection, dest=0x1e5e118) at execMain.c:1633
>
> So started looking into SerializeSnapshot() and with code reading I found
> that
> we ignore copying the SubXID array if it has overflowed, unless the snapshot
> was taken during recovey, and for this we mark the
> serialized_snapshot->subxcnt
> to 0. But later while copying the SubXID array we check then condition based
> on
> snapshot->subxcnt. We should check serialized_snapshot->subxcnt rather then
> snapshot->subxcnt.
>
> I tried hard to come up with individual test but somehow I was unable to
> create testcase.
>
> PFA patch to fix the issue.

Thanks, that looks right to me. Committed.

--
Robert Haas
EnterpriseDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company