Fix possible crash during WindowAgg evaluation

When short-circuiting WindowAgg node evaluation on the top-level
WindowAgg node using quals on monotonic window functions, because the
WindowAgg run condition can mean there's no need to evaluate subsequent
window function results in the same partition once the run condition
becomes false, it was possible that the executor would use stale results
from the previous invocation of the window function in some cases.

A fix for this was partially done by a5832722, but that commit only
fixed the issue for non-top-level WindowAgg nodes.  I mistakenly thought
that the top-level WindowAgg didn't have this issue, but Jayesh's example
case clearly shows that's incorrect.  At the time, I also thought that
this only affected 32-bit systems as all window functions which then
supported run conditions returned BIGINT, however, that's wrong as
ExecProject is still called and that could cause evaluation of any other
window function belonging to the same WindowAgg node, one of which may
return a byref type.

The only queries affected by this are WindowAggs with a "Run Condition"
which contains at least one window function with a byref result type,
such as lead() or lag() on a byref column.  The window clause must also
contain a PARTITION BY clause (without a PARTITION BY, execution of the
WindowAgg stops immediately when the run condition becomes false and
there's no risk of using the stale results).

Reported-by: Jayesh Dehankar
Discussion: https://postgr.es/m/193261e2c4d.3dd3cd7c1842.871636075166132237@zohocorp.com
Backpatch-through: 15, where WindowAgg run conditions were added

src/backend/executor/nodeWindowAgg.c

@@ -2296,6 +2296,23 @@ ExecWindowAgg(PlanState *pstate)
 				 */
 				if (winstate->use_pass_through)
 				{
+					/*
+					 * When switching into a pass-through mode, we'd better
+					 * NULLify the aggregate results as these are no longer
+					 * updated and NULLifying them avoids the old stale
+					 * results lingering.  Some of these might be byref types
+					 * so we can't have them pointing to free'd memory.  The
+					 * planner insisted that quals used in the runcondition
+					 * are strict, so the top-level WindowAgg will always
+					 * filter these NULLs out in the filter clause.
+					 */
+					numfuncs = winstate->numfuncs;
+					for (i = 0; i < numfuncs; i++)
+					{
+						econtext->ecxt_aggvalues[i] = (Datum) 0;
+						econtext->ecxt_aggnulls[i] = true;
+					}
+
 					/*
 					 * STRICT pass-through mode is required for the top window
 					 * when there is a PARTITION BY clause.  Otherwise we must
@@ -2310,24 +2327,6 @@ ExecWindowAgg(PlanState *pstate)
 					else
 					{
 						winstate->status = WINDOWAGG_PASSTHROUGH;
-
-						/*
-						 * If we're not the top-window, we'd better NULLify
-						 * the aggregate results.  In pass-through mode we no
-						 * longer update these and this avoids the old stale
-						 * results lingering.  Some of these might be byref
-						 * types so we can't have them pointing to free'd
-						 * memory.  The planner insisted that quals used in
-						 * the runcondition are strict, so the top-level
-						 * WindowAgg will filter these NULLs out in the filter
-						 * clause.
-						 */
-						numfuncs = winstate->numfuncs;
-						for (i = 0; i < numfuncs; i++)
-						{
-							econtext->ecxt_aggvalues[i] = (Datum) 0;
-							econtext->ecxt_aggnulls[i] = true;
-						}
 					}
 				}
 				else

src/test/regress/expected/window.out

@@ -4136,6 +4136,16 @@ WHERE c = 1;
    ->  Seq Scan on empsalary
 (3 rows)
 
+-- Try another case with a WindowFunc with a byref return type
+SELECT * FROM
+  (SELECT row_number() OVER (PARTITION BY salary) AS rn,
+          lead(depname) OVER (PARTITION BY salary) || ' Department' AS n_dep
+   FROM empsalary) emp
+WHERE rn < 1;
+ rn | n_dep 
+----+-------
+(0 rows)
+
 -- Some more complex cases with multiple window clauses
 EXPLAIN (COSTS OFF)
 SELECT * FROM

src/test/regress/sql/window.sql

@@ -1345,6 +1345,13 @@ SELECT * FROM
    FROM empsalary) emp
 WHERE c = 1;
 
+-- Try another case with a WindowFunc with a byref return type
+SELECT * FROM
+  (SELECT row_number() OVER (PARTITION BY salary) AS rn,
+          lead(depname) OVER (PARTITION BY salary) || ' Department' AS n_dep
+   FROM empsalary) emp
+WHERE rn < 1;
+
 -- Some more complex cases with multiple window clauses
 EXPLAIN (COSTS OFF)
 SELECT * FROM