MDEV-38849 slave_connections_needed_for_purge prevents independent machine from purging binary logs

`@@slave_connections_needed_for_purge`’s default of `1` ensures binary
log availability on replication masters, but is not a sensible default
suitable for all scenarios, especially for long-term slave servers and
standalone (not in a replication setup) servers.
The outcome was that standalone server users were confused why automatic
binlog purging does not work.

This commit changes this default to `0`, which is suitable for both
standalone and (when backed by prompt failure recovery)
replication setups.
`0` also more closely matches the behaviour before MDEV-31404,
which added this variable, out of the box.

This commit also adds a one-time replication warning when registering a
slave, but `@@slave_connections_needed_for_purge` is left unchanged.
Rather than enforcing a defence with an unsensible default, this
reminder will bring awareness of the risk of automatic binlog purging.

This commit also cleans up Galera and MTR workarounds to the
introduction of the `@@slave_connections_needed_for_purge=1` default.

Author: ParadoxV5

mysql-test/include/mtr_warnings.sql

@@ -148,6 +148,7 @@ INSERT INTO global_suppressions VALUES
  ("Slave: .*Duplicate entry"),
 
  ("Statement may not be safe to log in statement format"),
+ ("slave_connections_needed_for_purge"),
 
  /* innodb foreign key tests that fail in ALTER or RENAME produce this */
  ("InnoDB: Error: in ALTER TABLE `test`.`t[123]`"),

mysql-test/main/mysqld--help.result

@@ -1321,7 +1321,6 @@ The following specify which files/extra groups are read (specified before remain
  Minimum number of connected slaves required for automatic
  binary log purge with max_binlog_total_size,
  binlog_expire_logs_seconds or binlog_expire_logs_days.
- Default is 0 when Galera is enabled and 1 otherwise.
  --slave-ddl-exec-mode=name 
  How replication events should be executed. Legal values
  are STRICT and IDEMPOTENT (default). In IDEMPOTENT mode,
@@ -1955,7 +1954,7 @@ skip-networking FALSE
 skip-show-database FALSE
 skip-slave-start FALSE
 slave-compressed-protocol FALSE
-slave-connections-needed-for-purge 1
+slave-connections-needed-for-purge 0
 slave-ddl-exec-mode IDEMPOTENT
 slave-domain-parallel-threads 0
 slave-exec-mode STRICT

mysql-test/suite/binlog/my.cnf

@@ -1,11 +1,5 @@
 connection node_2;
 connection node_1;
-select @@slave_connections_needed_for_purge;
-@@slave_connections_needed_for_purge
-0
-select VARIABLE_NAME, GLOBAL_VALUE, GLOBAL_VALUE_ORIGIN from information_schema.system_variables where variable_name="slave_connections_needed_for_purge";
-VARIABLE_NAME	GLOBAL_VALUE	GLOBAL_VALUE_ORIGIN
-SLAVE_CONNECTIONS_NEEDED_FOR_PURGE	0	AUTO
 USE test;
 CREATE TABLE t1(c1 INT PRIMARY KEY) ENGINE=INNODB;
 INSERT INTO t1 VALUES (1), (2), (3), (4), (5);

mysql-test/suite/binlog_encryption/binlog_index-master.opt

@@ -1,9 +1,6 @@
 --source include/galera_cluster.inc
 --source include/have_innodb.inc
 
-select @@slave_connections_needed_for_purge;
-select VARIABLE_NAME, GLOBAL_VALUE, GLOBAL_VALUE_ORIGIN from information_schema.system_variables where variable_name="slave_connections_needed_for_purge";
-
 USE test;
 CREATE TABLE t1(c1 INT PRIMARY KEY) ENGINE=INNODB;
 INSERT INTO t1 VALUES (1), (2), (3), (4), (5);

mysql-test/suite/galera/r/basic.result

@@ -6,5 +6,3 @@
 # E.g. after !include ../my.cnf, in your `test.cnf`, specify your configuration
 # in option group e.g [mysqld.x], so that number `x` corresponds to the number
 # in the rpl server topology.
-[mariadbd]
-slave_connections_needed_for_purge=0

mysql-test/suite/galera/t/basic.test

@@ -0,0 +1,25 @@
+include/master-slave.inc
+[connection master]
+NOT FOUND /slave_connections_needed_for_purge/ in mysqld.1.err
+connection slave;
+START SLAVE IO_THREAD;
+include/wait_for_slave_io_to_start.inc
+FOUND 1 /slave_connections_needed_for_purge/ in mysqld.1.err
+include/stop_slave_io.inc
+START SLAVE IO_THREAD;
+include/wait_for_slave_io_to_start.inc
+include/stop_slave_io.inc
+include/rpl_restart_server.inc [server_number=1 parameters: --slave-connections-needed-for-purge 0]
+connection slave;
+START SLAVE IO_THREAD;
+include/wait_for_slave_io_to_start.inc
+include/stop_slave_io.inc
+include/rpl_restart_server.inc [server_number=1]
+connection master;
+SET @@GLOBAL.slave_connections_needed_for_purge= DEFAULT;
+connection slave;
+START SLAVE IO_THREAD;
+include/wait_for_slave_io_to_start.inc
+connection master;
+FOUND 1 /slave_connections_needed_for_purge/ in mysqld.1.err
+include/rpl_end.inc

mysql-test/suite/rpl/my.cnf

@@ -0,0 +1,52 @@
+# MDEV-38849: Test the warning that the _default_ for
+# `@@slave_connections_needed_for_purge` does not match the replication setup.
+# (Do _not_ warn if `@@slave_connections_needed_for_purge` is manually set.)
+
+--source include/have_binlog_format_mixed.inc # no actual binlog content
+--let $rpl_skip_start_slave= 1
+--source include/master-slave.inc
+
+
+--let SEARCH_FILE= `SELECT @@log_error`
+--let SEARCH_PATTERN= slave_connections_needed_for_purge
+# Not connected: should not warn
+--source include/search_pattern_in_file.inc
+
+
+--connection slave
+START SLAVE IO_THREAD;
+--source include/wait_for_slave_io_to_start.inc
+# Connected when left as default: should warn
+--source include/search_pattern_in_file.inc
+
+
+--source include/stop_slave_io.inc
+START SLAVE IO_THREAD;
+--source include/wait_for_slave_io_to_start.inc
+# Reconnected: should not warn a second time
+
+--source include/stop_slave_io.inc
+  --let $rpl_server_parameters= --slave-connections-needed-for-purge 0
+  --let $rpl_server_number= 1
+  --source include/rpl_restart_server.inc
+  --connection slave
+START SLAVE IO_THREAD;
+--source include/wait_for_slave_io_to_start.inc
+# Explicitly set to 0 in server options: should not warn
+
+--source include/stop_slave_io.inc
+  --let $rpl_server_parameters=
+  --source include/rpl_restart_server.inc
+  --connection master
+    SET @@GLOBAL.slave_connections_needed_for_purge= DEFAULT;
+  --connection slave
+START SLAVE IO_THREAD;
+--source include/wait_for_slave_io_to_start.inc
+# Explicitly set to DEFAULT in system variables: should _not_ warn
+
+--connection master
+--source include/search_pattern_in_file.inc
+
+
+--let $rpl_only_running_threads= 1
+--source include/rpl_end.inc

mysql-test/suite/rpl/r/warn_slaves_not_needed_for_purge.result

@@ -3465,7 +3465,7 @@ COMMAND_LINE_ARGUMENT	OPTIONAL
 VARIABLE_NAME	SLAVE_CONNECTIONS_NEEDED_FOR_PURGE
 VARIABLE_SCOPE	GLOBAL
 VARIABLE_TYPE	INT UNSIGNED
-VARIABLE_COMMENT	Minimum number of connected slaves required for automatic binary log purge with max_binlog_total_size, binlog_expire_logs_seconds or binlog_expire_logs_days. Default is 0 when Galera is enabled and 1 otherwise.
+VARIABLE_COMMENT	Minimum number of connected slaves required for automatic binary log purge with max_binlog_total_size, binlog_expire_logs_seconds or binlog_expire_logs_days.
 NUMERIC_MIN_VALUE	0
 NUMERIC_MAX_VALUE	4294967295
 NUMERIC_BLOCK_SIZE	1