HashAggregate loses worker spill stats on rescan

Hi,

HashAgg seems to lose the worker's spill stats when Gather is rescanned.
In the example below, Worker 0 spills on the first scan, but EXPLAIN shows
one batch and no disk usage after the second:

CREATE TABLE hashagg_stats AS
SELECT value AS item, value % 10000 AS bucket
FROM generate_series(1, 1000000) AS value;
ALTER TABLE hashagg_stats SET (parallel_workers = 2);
ANALYZE hashagg_stats;

SET max_parallel_workers_per_gather = 2;
SET max_parallel_workers = 1;
SET parallel_leader_participation = off;
SET parallel_setup_cost = 0;
SET parallel_tuple_cost = 0;
SET min_parallel_table_scan_size = 0;
SET work_mem = '64kB';
SET hash_mem_multiplier = 2;
SET enable_sort = off;

EXPLAIN (ANALYZE, VERBOSE, COSTS OFF, TIMING OFF, SUMMARY OFF, BUFFERS OFF)
SELECT limits.maximum,
(SELECT count(*)
FROM (SELECT bucket, count(*) FROM hashagg_stats
WHERE item <= (SELECT limits.maximum)
GROUP BY bucket) AS grouped)
FROM (VALUES (1000000), (10)) AS limits(maximum);

AFAICS, ExecEndAgg() clobbers the shared slot on each worker exit, rather
than combining the stats from successive scans.

Patch attached to keep the memory/disk peaks and add the spill batches.
I think we want to count the initial batch only once, as in the
non-parallel case. Otherwise, wouldn't even a non-spilling rescan add
another batch?

Regards,
Ayush