[sqlsmith] Crash in gather_readnext

Hi,

on master as of a0ae54d, there's a 1 in 10e6 chance sqlsmith catches
gather_readnext reading beyond the gatherstate->readers array with
readers[gatherstate->readnext].  Sample backtrace below.

As readnext is never explicitly initialized, I think what happens is
that a rescan gets less workers than the initial scan, and the dangling
readnext points outside the array.  I'm no longer seeing these crashes
when explicitly initializing readnext to 0 like in the attached patch.

regards,
Andreas

Program terminated with signal SIGSEGV, Segmentation fault.
#0  shm_mq_receive (mqh=0x259, nbytesp=nbytesp@entry=0x7ffc55ce0580, datap=datap@entry=0x7ffc55ce0588,
nowait=nowait@entry=1'\001') at shm_mq.c:520 
520        shm_mq       *mq = mqh->mqh_queue;
(gdb) bt
#0  shm_mq_receive (mqh=0x259, nbytesp=nbytesp@entry=0x7ffc55ce0580, datap=datap@entry=0x7ffc55ce0588,
nowait=nowait@entry=1'\001') at shm_mq.c:520 
#1  0x000000000060b8b7 in TupleQueueReaderNext (reader=reader@entry=0x5446c10, nowait=nowait@entry=1 '\001',
done=done@entry=0x7ffc55ce065b"") at tqueue.c:692 
#2  0x00000000005f5e03 in gather_readnext (gatherstate=0x52a9918) at nodeGather.c:339
#3  gather_getnext (gatherstate=0x52a9918) at nodeGather.c:292
#4  ExecGather (node=node@entry=0x52a9918) at nodeGather.c:233
#5  0x00000000005e3b68 in ExecProcNode (node=0x52a9918) at execProcnode.c:515
#6  0x00000000005eb2f2 in ExecScanFetch (recheckMtd=0x605e40 <SubqueryRecheck>, accessMtd=0x605e50 <SubqueryNext>,
node=0x52a86c0)at execScan.c:95 
#7  ExecScan (node=node@entry=0x52a86c0, accessMtd=accessMtd@entry=0x605e50 <SubqueryNext>,
recheckMtd=recheckMtd@entry=0x605e40<SubqueryRecheck>) at execScan.c:180 
#8  0x0000000000605e6f in ExecSubqueryScan (node=node@entry=0x52a86c0) at nodeSubqueryscan.c:85
#9  0x00000000005e3c68 in ExecProcNode (node=node@entry=0x52a86c0) at execProcnode.c:445
#10 0x00000000006001d6 in ExecNestLoop (node=node@entry=0x52a7978) at nodeNestloop.c:123
#11 0x00000000005e3bf8 in ExecProcNode (node=node@entry=0x52a7978) at execProcnode.c:476
#12 0x00000000006001d6 in ExecNestLoop (node=node@entry=0x52a5120) at nodeNestloop.c:123
#13 0x00000000005e3bf8 in ExecProcNode (node=node@entry=0x52a5120) at execProcnode.c:476
#14 0x00000000006001d6 in ExecNestLoop (node=node@entry=0x52a3d50) at nodeNestloop.c:123
#15 0x00000000005e3bf8 in ExecProcNode (node=0x52a3d50) at execProcnode.c:476
#16 0x00000000006015e5 in ExecResult (node=node@entry=0x52a3140) at nodeResult.c:130
#17 0x00000000005e3d18 in ExecProcNode (node=node@entry=0x52a3140) at execProcnode.c:392
#18 0x00000000005fb360 in ExecLimit (node=node@entry=0x52a2e70) at nodeLimit.c:91
#19 0x00000000005e3af8 in ExecProcNode (node=node@entry=0x52a2e70) at execProcnode.c:531
#20 0x0000000000600299 in ExecNestLoop (node=node@entry=0x52a1a10) at nodeNestloop.c:174
#21 0x00000000005e3bf8 in ExecProcNode (node=node@entry=0x52a1a10) at execProcnode.c:476
#22 0x00000000006001d6 in ExecNestLoop (node=node@entry=0x52a16d0) at nodeNestloop.c:123
#23 0x00000000005e3bf8 in ExecProcNode (node=node@entry=0x52a16d0) at execProcnode.c:476
#24 0x00000000005dfdae in ExecutePlan (dest=0x50cbb00, direction=<optimized out>, numberTuples=0, sendTuples=<optimized
out>,operation=CMD_SELECT, use_parallel_mode=<optimized out>, planstate=0x52a16d0, estate=0x3610968) at execMain.c:1567 
#25 standard_ExecutorRun (queryDesc=0x36805b8, direction=<optimized out>, count=0) at execMain.c:338
#26 0x0000000000701a58 in PortalRunSelect (portal=portal@entry=0x529da38, forward=forward@entry=1 '\001', count=0,
count@entry=9223372036854775807,dest=dest@entry=0x50cbb00) at pquery.c:946 
#27 0x000000000070300e in PortalRun (portal=portal@entry=0x529da38, count=count@entry=9223372036854775807,
isTopLevel=isTopLevel@entry=1'\001', dest=dest@entry=0x50cbb00, altdest=altdest@entry=0x50cbb00,
completionTag=completionTag@entry=0x7ffc55ce0ed0"") at pquery.c:787 
#28 0x0000000000700869 in exec_simple_query (query_string=0x45d3028 "select ...") at postgres.c:1094
#29 PostgresMain (argc=<optimized out>, argv=argv@entry=0x23ce878, dbname=<optimized out>, username=<optimized out>) at
postgres.c:4069
#30 0x000000000046d9d9 in BackendRun (port=0x23d1ad0) at postmaster.c:4271
#31 BackendStartup (port=0x23d1ad0) at postmaster.c:3945
#32 ServerLoop () at postmaster.c:1701
#33 0x0000000000698ed9 in PostmasterMain (argc=argc@entry=4, argv=argv@entry=0x23a05c0) at postmaster.c:1309
#34 0x000000000046ebbd in main (argc=4, argv=0x23a05c0) at main.c:228

>From be80954688c406122b560161192cc1d2e64e3757 Mon Sep 17 00:00:00 2001
From: Andreas Seltenreich <seltenreich@gmx.de>
Date: Mon, 5 Dec 2016 20:46:28 +0100
Subject: [PATCH] Fix potential crash on ReScanGather.

Initialize gatherstate->nextreader to 0 in order to prevent a crash
when ReScanGather gets less workers than the original scan, leading to
nextreader pointing outside the readers[nworkers] array.
---
 src/backend/executor/nodeGather.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/src/backend/executor/nodeGather.c b/src/backend/executor/nodeGather.c
index 880ca62..2bdf223 100644
--- a/src/backend/executor/nodeGather.c
+++ b/src/backend/executor/nodeGather.c
@@ -173,6 +173,7 @@ ExecGather(GatherState *node)
             if (pcxt->nworkers_launched > 0)
             {
                 node->nreaders = 0;
+                node->nextreader = 0;
                 node->reader =
                     palloc(pcxt->nworkers_launched * sizeof(TupleQueueReader *));

@@ -335,6 +336,7 @@ gather_readnext(GatherState *gatherstate)
         CHECK_FOR_INTERRUPTS();

         /* Attempt to read a tuple, but don't block if none is available. */
+        Assert(gatherstate->nextreader < gatherstate->nreaders);
         reader = gatherstate->reader[gatherstate->nextreader];
         tup = TupleQueueReaderNext(reader, true, &readerdone);

--
2.10.2