BUG #17022: SQL causing engine crash

The following bug has been logged on the website:

Bug reference: 17022
Logged by: Robins Tharakan
Email address: tharakan(at)gmail(dot)com
PostgreSQL version: 13.3
Operating system: Amazon Linux 2
Description:

Hi,

SQLSmith is constantly crashing v13.3 with SQL(s) that appear linked to a
PostGIS (v3.0.3) bug - see Error Report below.

I decided to post this here (backtracking on an earlier thought), since #0 /
#1 are postgres functions and I wasn't really sure if the arguments to these
functions are sanitized. For e.g. whether pg_detoast_datum_slice() is
expected to check input bounds (count=40 in this case).

Sample SQL (I couldn't narrow this down further):
==========
$ psql
psql (13beta2, server 13.3)
Type "help" for help.

regression=>   select 1
from public.spatial_ref_sys as sample_0 tablesample bernoulli (6.8)
where case when ((cast(null as geometry) && cast(null as geometry))
        and (NULL is NULL))
       then case when sample_0.auth_srid is NULL then cast(null as geometry)
else cast(null as geometry) end
       else case when sample_0.auth_srid is NULL then cast(null as geometry)
else cast(null as geometry) end
       end
     &/& case when (pg_catalog.pg_current_snapshot() is not NULL)
      and (cast(null as timestamptz) >= (select backend_start from
pg_catalog.pg_stat_activity limit 1 offset 4)
          ) then public.geometry(
      cast(cast(null as geography) as geography)) else public.geometry(
      cast(cast(null as geography) as geography)) end;
server closed the connection unexpectedly
        This probably means the server terminated abnormally
        before or while processing the request.
The connection to the server was lost. Attempting reset: Failed.

Another Example:
==============
SELECT 1 FROM (SELECT NULL::int a LIMIT 0) AS ref_0
WHERE CASE
    WHEN CASE
        WHEN pg_catalog.jsonb_build_array() ? version()
          THEN cast(NULL AS point)
        ELSE cast(NULL AS point)
        END @(SELECT NULL::lseg LIMIT 0)
      THEN cast(NULL AS GEOGRAPHY)
    END && CASE
    WHEN (
        CASE
          WHEN ref_0.a IS NULL
          THEN cast(NULL AS float4)
          ELSE cast(NULL AS float4)
        END <= (
          SELECT NULL::real LIMIT 0
          )
        )
      THEN NULL::GEOGRAPHY
    END
limit 58;

Backtrace (common to most such crashes):
========
#0  detoast_attr_slice (attr=attr(at)entry=0x0,
sliceoffset=sliceoffset(at)entry=0, slicelength=slicelength(at)entry=40) at
detoast.c:226
#1  0x00000000008a3b55 in pg_detoast_datum_slice (datum=datum(at)entry=0x0,
first=first(at)entry=0, count=count(at)entry=40) at fmgr.c:1754
#2  0x000014e217f4f793 in gserialized_datum_get_gidx_p
(gsdatum=gsdatum(at)entry=0, gidx=gidx(at)entry=0x7fffb1ba7730) at
gserialized_gist.c:185
#3  0x000014e217f4faa9 in gserialized_datum_get_gbox_p (gsdatum=0,
gbox=gbox(at)entry=0x7fffb1ba77d0) at gserialized_gist.c:157
#4  0x000014e217ef971b in gserialized_sel_internal (root=<optimized out>,
args=<optimized out>, varRelid=<optimized out>, mode=0) at
gserialized_estimate.c:2253
#5  0x000014e217ef9799 in gserialized_gist_sel
(fcinfo=fcinfo(at)entry=0x7fffb1ba7850) at gserialized_estimate.c:2281
#6  0x00000000008a24f4 in DirectFunctionCall5Coll
(func=func(at)entry=0x14e217ef9780 <gserialized_gist_sel>,
collation=collation(at)entry=0, arg1=<optimized out>,
    arg2=<optimized out>, arg3=<optimized out>, arg4=<optimized out>,
arg5=0) at fmgr.c:908
#7  0x000014e217ef96a4 in gserialized_gist_sel_nd (fcinfo=<optimized out>)
at gserialized_estimate.c:2204
#8  0x00000000008a2c6e in FunctionCall4Coll (flinfo=0x7fffb1ba7970,
collation=<optimized out>, arg1=<optimized out>, arg2=<optimized out>,
arg3=<optimized out>,
    arg4=<optimized out>) at fmgr.c:1216
#9  0x00000000008a3247 in OidFunctionCall4Coll (functionId=<optimized out>,
collation=collation(at)entry=0, arg1=arg1(at)entry=49189016,
arg2=arg2(at)entry=1118028,
    arg3=arg3(at)entry=50164232, arg4=arg4(at)entry=0) at fmgr.c:1452
#10 0x000000000070b45d in restriction_selectivity
(root=root(at)entry=0x2ee9098, operatorid=1118028, args=0x2fd7208,
inputcollid=0, varRelid=varRelid(at)entry=0)
    at plancat.c:1785

Reference:
a) PostGIS Bug report - https://trac.osgeo.org/postgis/ticket/4919

-
Robins
AWS Database Engineer