Another try to fix the test case added by commit f5fc2f5b.

As per analysis, it appears that the 'drop slot' message from the previous
test and 'create slot' message of the new test are either missed or not
yet delivered to the stats collector due to which we will still see the
stats from the old slot. This can happen rarely which could be the reason
that we are seeing some failures in the buildfarm randomly. To avoid that
we are using a different slot name for the tests in
test_decoding/sql/stats.sql.

Reported-by: Tom Lane based on buildfarm reports
Author: Sawada Masahiko
Reviewed-by: Amit Kapila, Vignesh C
Discussion: https://postgr.es/m/20210319185247.ldebgpdaxsowiflw@alap3.anarazel.de

contrib/test_decoding/expected/stats.out

 -- predictability
 SET synchronous_commit = on;
-SELECT 'init' FROM pg_create_logical_replication_slot('regression_slot', 'test_decoding');
+SELECT 'init' FROM pg_create_logical_replication_slot('regression_slot_stats', 'test_decoding');
  ?column? 
 ----------
  init
@@ -23,7 +23,7 @@ BEGIN
                   ELSE (spill_txns > 0)
              END
       INTO updated
-      FROM pg_stat_replication_slots WHERE slot_name='regression_slot';
+      FROM pg_stat_replication_slots WHERE slot_name='regression_slot_stats';
 
     ELSE
 
@@ -32,7 +32,7 @@ BEGIN
                   ELSE (total_txns > 0)
              END
       INTO updated
-      FROM pg_stat_replication_slots WHERE slot_name='regression_slot';
+      FROM pg_stat_replication_slots WHERE slot_name='regression_slot_stats';
 
     END IF;
 
@@ -52,8 +52,9 @@ BEGIN
 END
 $$ LANGUAGE plpgsql;
 -- non-spilled xact
+SET logical_decoding_work_mem to '64MB';
 INSERT INTO stats_test values(1);
-SELECT count(*) FROM pg_logical_slot_get_changes('regression_slot', NULL, NULL, 'skip-empty-xacts', '1');
+SELECT count(*) FROM pg_logical_slot_get_changes('regression_slot_stats', NULL, NULL, 'skip-empty-xacts', '1');
  count 
 -------
      3
@@ -66,13 +67,14 @@ SELECT wait_for_decode_stats(false, false);
 (1 row)
 
 SELECT slot_name, spill_txns = 0 AS spill_txns, spill_count = 0 AS spill_count, total_txns > 0 AS total_txns, total_bytes > 0 AS total_bytes FROM pg_stat_replication_slots;
-    slot_name    | spill_txns | spill_count | total_txns | total_bytes 
------------------+------------+-------------+------------+-------------
- regression_slot | t          | t           | t          | t
+       slot_name       | spill_txns | spill_count | total_txns | total_bytes 
+-----------------------+------------+-------------+------------+-------------
+ regression_slot_stats | t          | t           | t          | t
 (1 row)
 
+RESET logical_decoding_work_mem;
 -- reset the slot stats, and wait for stats collector's total txn to reset
-SELECT pg_stat_reset_replication_slot('regression_slot');
+SELECT pg_stat_reset_replication_slot('regression_slot_stats');
  pg_stat_reset_replication_slot 
 --------------------------------
  
@@ -85,16 +87,16 @@ SELECT wait_for_decode_stats(true, false);
 (1 row)
 
 SELECT slot_name, spill_txns, spill_count, total_txns, total_bytes FROM pg_stat_replication_slots;
-    slot_name    | spill_txns | spill_count | total_txns | total_bytes 
------------------+------------+-------------+------------+-------------
- regression_slot |          0 |           0 |          0 |           0
+       slot_name       | spill_txns | spill_count | total_txns | total_bytes 
+-----------------------+------------+-------------+------------+-------------
+ regression_slot_stats |          0 |           0 |          0 |           0
 (1 row)
 
 -- 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, 'skip-empty-xacts', '1');
+SELECT count(*) FROM pg_logical_slot_peek_changes('regression_slot_stats', NULL, NULL, 'skip-empty-xacts', '1');
  count 
 -------
   5002
@@ -110,13 +112,13 @@ SELECT wait_for_decode_stats(false, true);
 (1 row)
 
 SELECT slot_name, spill_txns > 0 AS spill_txns, spill_count > 0 AS spill_count, total_txns > 0 AS total_txns, total_bytes > 0 AS total_bytes FROM pg_stat_replication_slots;
-    slot_name    | spill_txns | spill_count | total_txns | total_bytes 
------------------+------------+-------------+------------+-------------
- regression_slot | t          | t           | t          | t
+       slot_name       | spill_txns | spill_count | total_txns | total_bytes 
+-----------------------+------------+-------------+------------+-------------
+ regression_slot_stats | t          | t           | t          | t
 (1 row)
 
 -- reset the slot stats, and wait for stats collector to reset
-SELECT pg_stat_reset_replication_slot('regression_slot');
+SELECT pg_stat_reset_replication_slot('regression_slot_stats');
  pg_stat_reset_replication_slot 
 --------------------------------
  
@@ -129,13 +131,13 @@ SELECT wait_for_decode_stats(true, true);
 (1 row)
 
 SELECT slot_name, spill_txns, spill_count, total_txns, total_bytes FROM pg_stat_replication_slots;
-    slot_name    | spill_txns | spill_count | total_txns | total_bytes 
------------------+------------+-------------+------------+-------------
- regression_slot |          0 |           0 |          0 |           0
+       slot_name       | spill_txns | spill_count | total_txns | total_bytes 
+-----------------------+------------+-------------+------------+-------------
+ regression_slot_stats |          0 |           0 |          0 |           0
 (1 row)
 
 -- decode and check stats again.
-SELECT count(*) FROM pg_logical_slot_peek_changes('regression_slot', NULL, NULL, 'skip-empty-xacts', '1');
+SELECT count(*) FROM pg_logical_slot_peek_changes('regression_slot_stats', NULL, NULL, 'skip-empty-xacts', '1');
  count 
 -------
   5002
@@ -148,30 +150,30 @@ SELECT wait_for_decode_stats(false, true);
 (1 row)
 
 SELECT slot_name, spill_txns > 0 AS spill_txns, spill_count > 0 AS spill_count, total_txns > 0 AS total_txns, total_bytes > 0 AS total_bytes FROM pg_stat_replication_slots;
-    slot_name    | spill_txns | spill_count | total_txns | total_bytes 
------------------+------------+-------------+------------+-------------
- regression_slot | t          | t           | t          | t
+       slot_name       | spill_txns | spill_count | total_txns | total_bytes 
+-----------------------+------------+-------------+------------+-------------
+ regression_slot_stats | t          | t           | t          | t
 (1 row)
 
 -- Ensure stats can be repeatedly accessed using the same stats snapshot. See
 -- https://postgr.es/m/20210317230447.c7uc4g3vbs4wi32i%40alap3.anarazel.de
 BEGIN;
 SELECT slot_name FROM pg_stat_replication_slots;
-    slot_name    
------------------
- regression_slot
+       slot_name       
+-----------------------
+ regression_slot_stats
 (1 row)
 
 SELECT slot_name FROM pg_stat_replication_slots;
-    slot_name    
------------------
- regression_slot
+       slot_name       
+-----------------------
+ regression_slot_stats
 (1 row)
 
 COMMIT;
 DROP FUNCTION wait_for_decode_stats(bool, bool);
 DROP TABLE stats_test;
-SELECT pg_drop_replication_slot('regression_slot');
+SELECT pg_drop_replication_slot('regression_slot_stats');
  pg_drop_replication_slot 
 --------------------------
  

contrib/test_decoding/sql/stats.sql

 -- predictability
 SET synchronous_commit = on;
 
-SELECT 'init' FROM pg_create_logical_replication_slot('regression_slot', 'test_decoding');
+SELECT 'init' FROM pg_create_logical_replication_slot('regression_slot_stats', 'test_decoding');
 
 CREATE TABLE stats_test(data text);
 
@@ -21,7 +21,7 @@ BEGIN
                   ELSE (spill_txns > 0)
              END
       INTO updated
-      FROM pg_stat_replication_slots WHERE slot_name='regression_slot';
+      FROM pg_stat_replication_slots WHERE slot_name='regression_slot_stats';
 
     ELSE
 
@@ -30,7 +30,7 @@ BEGIN
                   ELSE (total_txns > 0)
              END
       INTO updated
-      FROM pg_stat_replication_slots WHERE slot_name='regression_slot';
+      FROM pg_stat_replication_slots WHERE slot_name='regression_slot_stats';
 
     END IF;
 
@@ -51,13 +51,15 @@ END
 $$ LANGUAGE plpgsql;
 
 -- non-spilled xact
+SET logical_decoding_work_mem to '64MB';
 INSERT INTO stats_test values(1);
-SELECT count(*) FROM pg_logical_slot_get_changes('regression_slot', NULL, NULL, 'skip-empty-xacts', '1');
+SELECT count(*) FROM pg_logical_slot_get_changes('regression_slot_stats', NULL, NULL, 'skip-empty-xacts', '1');
 SELECT wait_for_decode_stats(false, false);
 SELECT slot_name, spill_txns = 0 AS spill_txns, spill_count = 0 AS spill_count, total_txns > 0 AS total_txns, total_bytes > 0 AS total_bytes FROM pg_stat_replication_slots;
+RESET logical_decoding_work_mem;
 
 -- reset the slot stats, and wait for stats collector's total txn to reset
-SELECT pg_stat_reset_replication_slot('regression_slot');
+SELECT pg_stat_reset_replication_slot('regression_slot_stats');
 SELECT wait_for_decode_stats(true, false);
 SELECT slot_name, spill_txns, spill_count, total_txns, total_bytes FROM pg_stat_replication_slots;
 
@@ -65,7 +67,7 @@ SELECT slot_name, spill_txns, spill_count, total_txns, total_bytes FROM pg_stat_
 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, 'skip-empty-xacts', '1');
+SELECT count(*) FROM pg_logical_slot_peek_changes('regression_slot_stats', NULL, NULL, 'skip-empty-xacts', '1');
 
 -- Check stats, wait for the stats collector to update. We can't test the
 -- exact stats count as that can vary if any background transaction (say by
@@ -74,12 +76,12 @@ SELECT wait_for_decode_stats(false, true);
 SELECT slot_name, spill_txns > 0 AS spill_txns, spill_count > 0 AS spill_count, total_txns > 0 AS total_txns, total_bytes > 0 AS total_bytes 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 pg_stat_reset_replication_slot('regression_slot_stats');
 SELECT wait_for_decode_stats(true, true);
 SELECT slot_name, spill_txns, spill_count, total_txns, total_bytes FROM pg_stat_replication_slots;
 
 -- decode and check stats again.
-SELECT count(*) FROM pg_logical_slot_peek_changes('regression_slot', NULL, NULL, 'skip-empty-xacts', '1');
+SELECT count(*) FROM pg_logical_slot_peek_changes('regression_slot_stats', NULL, NULL, 'skip-empty-xacts', '1');
 SELECT wait_for_decode_stats(false, true);
 SELECT slot_name, spill_txns > 0 AS spill_txns, spill_count > 0 AS spill_count, total_txns > 0 AS total_txns, total_bytes > 0 AS total_bytes FROM pg_stat_replication_slots;
 
@@ -92,4 +94,4 @@ COMMIT;
 
 DROP FUNCTION wait_for_decode_stats(bool, bool);
 DROP TABLE stats_test;
-SELECT pg_drop_replication_slot('regression_slot');
+SELECT pg_drop_replication_slot('regression_slot_stats');