Call eq_func correctly in time_bucket_gapfill

The equality comparison function is called using
`DirectFunctionCall2Coll`, which do not set the `fcinfo->flinfo` when
calling the PostgreSQL function.  Since `array_eq` uses
`fcinfo->flinfo->fn_extra` for caching, and `flinfo` is null, this
causes a crash.

Fix this issue by using `FunctionCall2Coll` instead, which sets
`fcinfo->flinfo` before calling the PostgreSQL function.

Fixes #5981

.unreleased/bugfix_5991

@@ -0,0 +1,2 @@
+Fixes: #5991 Call eq_func correctly in time_bucket_gapfill
+Thanks: @willsbit for reporting a crash in time_bucket_gapfill

tsl/src/nodes/gapfill/gapfill_exec.c

@@ -1040,10 +1040,12 @@ gapfill_state_is_new_group(GapFillState *state, TupleTableSlot *slot)
 				continue;
 			if (isnull != column.group->isnull)
 				return true;
-			if (!DatumGetBool(DirectFunctionCall2Coll(column.group->eq_func.fn_addr,
+			/* We need to use FunctionCall2Coll here since equality comparison
-													  column.group->collation,
+			 * functions can try to access flinfo (see arrayfuncs.c). */
-													  value,
+			if (!DatumGetBool(FunctionCall2Coll(&column.group->eq_func,
-													  column.group->value)))
+												column.group->collation,
+												value,
+												column.group->value)))
 				return true;
 		}
 	}

tsl/test/shared/expected/gapfill.out

@@ -3324,3 +3324,34 @@ SELECT time_bucket_gapfill('2 month'::interval, ts, 'Europe/Berlin', '2000-01-01
 
 RESET timezone;
 DROP INDEX gapfill_plan_test_indx;
+-- Test gapfill with arrays (#5981)
+SELECT time_bucket_gapfill(5, ts, 1, 100) as ts, int_arr, locf(last(value, ts))
+FROM (
+    SELECT ARRAY[1,2,3,4]::int[] as int_arr, x as ts, x+500000 as value
+    FROM generate_series(1, 10, 100) as x
+    ) t
+GROUP BY 1, 2
+ ts |  int_arr  |  locf  
+----+-----------+--------
+  0 | {1,2,3,4} | 500001
+  5 | {1,2,3,4} | 500001
+ 10 | {1,2,3,4} | 500001
+ 15 | {1,2,3,4} | 500001
+ 20 | {1,2,3,4} | 500001
+ 25 | {1,2,3,4} | 500001
+ 30 | {1,2,3,4} | 500001
+ 35 | {1,2,3,4} | 500001
+ 40 | {1,2,3,4} | 500001
+ 45 | {1,2,3,4} | 500001
+ 50 | {1,2,3,4} | 500001
+ 55 | {1,2,3,4} | 500001
+ 60 | {1,2,3,4} | 500001
+ 65 | {1,2,3,4} | 500001
+ 70 | {1,2,3,4} | 500001
+ 75 | {1,2,3,4} | 500001
+ 80 | {1,2,3,4} | 500001
+ 85 | {1,2,3,4} | 500001
+ 90 | {1,2,3,4} | 500001
+ 95 | {1,2,3,4} | 500001
+(20 rows)
+

tsl/test/shared/sql/gapfill.sql

@@ -1506,3 +1506,10 @@ RESET timezone;
 
 DROP INDEX gapfill_plan_test_indx;
 
+-- Test gapfill with arrays (#5981)
+SELECT time_bucket_gapfill(5, ts, 1, 100) as ts, int_arr, locf(last(value, ts))
+FROM (
+    SELECT ARRAY[1,2,3,4]::int[] as int_arr, x as ts, x+500000 as value
+    FROM generate_series(1, 10, 100) as x
+    ) t
+GROUP BY 1, 2