Fix race condition in test_decoding "slot" test.

This test, just added in commit a924c327, sometimes fails because
the old backend hasn't finished dropping the temporary replication slot
when the new backend looks.  Borrow the previously-invented methodology
for waiting for the old process to disappear from pg_stat_activity.

Petr Jelinek

Discussion: https://postgr.es/m/62935e6f-4f1b-c433-e0fa-7f936a38b3e5@2ndquadrant.com

contrib/test_decoding/expected/slot.out

@@ -22,17 +22,26 @@ SELECT 'init' FROM pg_create_logical_replication_slot('regression_slot_p', 'test
  init
 (1 row)
 
--- reconnect to clean temp slots
-\c
+-- here we want to start a new session and wait till old one is gone
+select pg_backend_pid() as oldpid \gset
+\c -
+do 'declare c int = 0;
+begin
+  while (select count(*) from pg_stat_activity where pid = '
+    :'oldpid'
+  ') > 0 loop c := c + 1; perform pg_stat_clear_snapshot(); end loop;
+  raise log ''slot test looped % times'', c;
+end';
+-- should fail because the temporary slot was dropped automatically
+SELECT pg_drop_replication_slot('regression_slot_t');
+ERROR:  replication slot "regression_slot_t" does not exist
+-- permanent slot has survived
 SELECT pg_drop_replication_slot('regression_slot_p');
  pg_drop_replication_slot 
 --------------------------
  
 (1 row)
 
--- should fail because the temporary slot was dropped automatically
-SELECT pg_drop_replication_slot('regression_slot_t');
-ERROR:  replication slot "regression_slot_t" does not exist
 -- test switching between slots in a session
 SELECT 'init' FROM pg_create_logical_replication_slot('regression_slot1', 'test_decoding', true);
  ?column? 

contrib/test_decoding/sql/slot.sql

@@ -4,14 +4,22 @@ SELECT 'init' FROM pg_create_logical_replication_slot('regression_slot_t', 'test
 SELECT pg_drop_replication_slot('regression_slot_p');
 SELECT 'init' FROM pg_create_logical_replication_slot('regression_slot_p', 'test_decoding', false);
 
--- reconnect to clean temp slots
-\c
-
-SELECT pg_drop_replication_slot('regression_slot_p');
+-- here we want to start a new session and wait till old one is gone
+select pg_backend_pid() as oldpid \gset
+\c -
+do 'declare c int = 0;
+begin
+  while (select count(*) from pg_stat_activity where pid = '
+    :'oldpid'
+  ') > 0 loop c := c + 1; perform pg_stat_clear_snapshot(); end loop;
+  raise log ''slot test looped % times'', c;
+end';
 
 -- should fail because the temporary slot was dropped automatically
 SELECT pg_drop_replication_slot('regression_slot_t');
 
+-- permanent slot has survived
+SELECT pg_drop_replication_slot('regression_slot_p');
 
 -- test switching between slots in a session
 SELECT 'init' FROM pg_create_logical_replication_slot('regression_slot1', 'test_decoding', true);