Add tests for logical replication spilled stats.

Commit 98681675 added a mechanism to track statistics corresponding to
the spilling of changes from ReorderBuffer but didn't add any tests.

Author: Amit Kapila and Sawada Masahiko
Discussion: https://postgr.es/m/CA+fd4k5_pPAYRTDrO2PbtTOe0eHQpBvuqmCr8ic39uTNmR49Eg@mail.gmail.com

contrib/test_decoding/Makefile

@@ -5,7 +5,7 @@ PGFILEDESC = "test_decoding - example of a logical decoding output plugin"
 
 REGRESS = ddl xact rewrite toast permissions decoding_in_xact \
 	decoding_into_rel binary prepared replorigin time messages \
-	spill slot truncate stream
+	spill slot truncate stream stats
 ISOLATION = mxact delayed_startup ondisk_startup concurrent_ddl_dml \
 	oldest_xmin snapshot_transfer subxact_without_top concurrent_stream
 

contrib/test_decoding/expected/stats.out

+-- predictability
+SET synchronous_commit = on;
+SELECT 'init' FROM pg_create_logical_replication_slot('regression_slot', 'test_decoding');
+ ?column? 
+----------
+ init
+(1 row)
+
+CREATE TABLE stats_test(data text);
+-- function to wait for counters to advance
+CREATE FUNCTION wait_for_decode_stats(check_reset bool) RETURNS void AS $$
+DECLARE
+  start_time timestamptz := clock_timestamp();
+  updated bool;
+BEGIN
+  -- we don't want to wait forever; loop will exit after 30 seconds
+  FOR i IN 1 .. 300 LOOP
+
+    -- check to see if all updates have been reset/updated
+    SELECT CASE WHEN check_reset THEN (spill_txns = 0)
+                ELSE (spill_txns > 0)
+           END
+    INTO updated
+    FROM pg_stat_replication_slots WHERE name='regression_slot';
+
+    exit WHEN updated;
+
+    -- wait a little
+    perform pg_sleep_for('100 milliseconds');
+
+    -- reset stats snapshot so we can test again
+    perform pg_stat_clear_snapshot();
+
+  END LOOP;
+
+  -- report time waited in postmaster log (where it won't change test output)
+  RAISE LOG 'wait_for_decode_stats delayed % seconds',
+    extract(epoch from clock_timestamp() - start_time);
+END
+$$ LANGUAGE plpgsql;
+-- spilling the xact
+BEGIN;
+INSERT INTO stats_test SELECT 'serialize-topbig--1:'||g.i FROM generate_series(1, 5000) g(i);
+COMMIT;
+SELECT count(*) FROM pg_logical_slot_peek_changes('regression_slot', NULL,NULL);
+ count 
+-------
+  5006
+(1 row)
+
+-- check stats, wait for stats collector to update.
+SELECT wait_for_decode_stats(false);
+ wait_for_decode_stats 
+-----------------------
+ 
+(1 row)
+
+SELECT name, spill_txns, spill_count FROM pg_stat_replication_slots;
+      name       | spill_txns | spill_count 
+-----------------+------------+-------------
+ regression_slot |          1 |          12
+(1 row)
+
+-- reset the slot stats, and wait for stats collector to reset
+SELECT pg_stat_reset_replication_slot('regression_slot');
+ pg_stat_reset_replication_slot 
+--------------------------------
+ 
+(1 row)
+
+SELECT wait_for_decode_stats(true);
+ wait_for_decode_stats 
+-----------------------
+ 
+(1 row)
+
+SELECT name, spill_txns, spill_count FROM pg_stat_replication_slots;
+      name       | spill_txns | spill_count 
+-----------------+------------+-------------
+ regression_slot |          0 |           0
+(1 row)
+
+-- decode and check stats again.
+SELECT count(*) FROM pg_logical_slot_peek_changes('regression_slot', NULL,NULL);
+ count 
+-------
+  5006
+(1 row)
+
+SELECT wait_for_decode_stats(false);
+ wait_for_decode_stats 
+-----------------------
+ 
+(1 row)
+
+SELECT name, spill_txns, spill_count FROM pg_stat_replication_slots;
+      name       | spill_txns | spill_count 
+-----------------+------------+-------------
+ regression_slot |          1 |          12
+(1 row)
+
+DROP FUNCTION wait_for_decode_stats(bool);
+DROP TABLE stats_test;
+SELECT pg_drop_replication_slot('regression_slot');
+ pg_drop_replication_slot 
+--------------------------
+ 
+(1 row)
+

contrib/test_decoding/sql/stats.sql

+-- predictability
+SET synchronous_commit = on;
+
+SELECT 'init' FROM pg_create_logical_replication_slot('regression_slot', 'test_decoding');
+
+CREATE TABLE stats_test(data text);
+
+-- function to wait for counters to advance
+CREATE FUNCTION wait_for_decode_stats(check_reset bool) RETURNS void AS $$
+DECLARE
+  start_time timestamptz := clock_timestamp();
+  updated bool;
+BEGIN
+  -- we don't want to wait forever; loop will exit after 30 seconds
+  FOR i IN 1 .. 300 LOOP
+
+    -- check to see if all updates have been reset/updated
+    SELECT CASE WHEN check_reset THEN (spill_txns = 0)
+                ELSE (spill_txns > 0)
+           END
+    INTO updated
+    FROM pg_stat_replication_slots WHERE name='regression_slot';
+
+    exit WHEN updated;
+
+    -- wait a little
+    perform pg_sleep_for('100 milliseconds');
+
+    -- reset stats snapshot so we can test again
+    perform pg_stat_clear_snapshot();
+
+  END LOOP;
+
+  -- report time waited in postmaster log (where it won't change test output)
+  RAISE LOG 'wait_for_decode_stats delayed % seconds',
+    extract(epoch from clock_timestamp() - start_time);
+END
+$$ LANGUAGE plpgsql;
+
+-- spilling the xact
+BEGIN;
+INSERT INTO stats_test SELECT 'serialize-topbig--1:'||g.i FROM generate_series(1, 5000) g(i);
+COMMIT;
+SELECT count(*) FROM pg_logical_slot_peek_changes('regression_slot', NULL,NULL);
+
+-- check stats, wait for stats collector to update.
+SELECT wait_for_decode_stats(false);
+SELECT name, spill_txns, spill_count FROM pg_stat_replication_slots;
+
+-- reset the slot stats, and wait for stats collector to reset
+SELECT pg_stat_reset_replication_slot('regression_slot');
+SELECT wait_for_decode_stats(true);
+SELECT name, spill_txns, spill_count FROM pg_stat_replication_slots;
+
+-- decode and check stats again.
+SELECT count(*) FROM pg_logical_slot_peek_changes('regression_slot', NULL,NULL);
+SELECT wait_for_decode_stats(false);
+SELECT name, spill_txns, spill_count FROM pg_stat_replication_slots;
+
+DROP FUNCTION wait_for_decode_stats(bool);
+DROP TABLE stats_test;
+SELECT pg_drop_replication_slot('regression_slot');