MariaDB · ParadoxV5 · Aug 18, 2026
diff --git a/mysql-test/include/mtr_warnings.sql b/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]`"),

diff --git a/mysql-test/main/mysqld--help.result b/mysql-test/main/mysqld--help.result
@@ -1320,7 +1320,6 @@ 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,
@@ -1954,7 +1953,7 @@ 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

diff --git a/mysql-test/suite/binlog/my.cnf b/mysql-test/suite/binlog/my.cnf
diff --git a/mysql-test/suite/binlog_encryption/binlog_index-master.opt b/mysql-test/suite/binlog_encryption/binlog_index-master.opt
diff --git a/mysql-test/suite/galera/r/basic.result b/mysql-test/suite/galera/r/basic.result
@@ -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);

diff --git a/mysql-test/suite/galera/t/basic.test b/mysql-test/suite/galera/t/basic.test
@@ -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);

diff --git a/mysql-test/suite/rpl/my.cnf b/mysql-test/suite/rpl/my.cnf
@@ -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
diff --git a/mysql-test/suite/rpl/r/warn_slaves_not_needed_for_purge.result b/mysql-test/suite/rpl/r/warn_slaves_not_needed_for_purge.result
@@ -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
diff --git a/mysql-test/suite/rpl/t/warn_slaves_not_needed_for_purge.test b/mysql-test/suite/rpl/t/warn_slaves_not_needed_for_purge.test
@@ -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
diff --git a/mysql-test/suite/sys_vars/r/sysvars_server_embedded.result b/mysql-test/suite/sys_vars/r/sysvars_server_embedded.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

diff --git a/mysql-test/suite/sys_vars/r/sysvars_server_notembedded.result b/mysql-test/suite/sys_vars/r/sysvars_server_notembedded.result
@@ -4035,7 +4035,7 @@ 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

diff --git a/sql/mysqld.cc b/sql/mysqld.cc
@@ -5956,16 +5956,6 @@ int mysqld_main(int argc, char **argv)
       SYSVAR_AUTOSIZE(global_system_variables.binlog_format, BINLOG_FORMAT_ROW);
     }
     binlog_format_used= 1;
-    if (IS_SYSVAR_AUTOSIZE(&internal_slave_connections_needed_for_purge))
-    {
-      slave_connections_needed_for_purge=
-        internal_slave_connections_needed_for_purge= 0;
-      SYSVAR_AUTOSIZE(internal_slave_connections_needed_for_purge, 0);
-      sql_print_information(
-      "slave_connections_needed_for_purge changed to 0 because "
-      "of Galera. Change it to 1 or higher if this Galera node "
-      "is also Master in a normal replication setup");
-    }
   }
 #endif /* WITH_WSREP */
 
@@ -8353,6 +8343,11 @@ mysqld_get_one_option(const struct my_option *opt, const char *argument,
   }
 
 #ifdef HAVE_REPLICATION
+  case OPT_SLAVE_CONNECTIONS_NEEDED_FOR_PURGE:
+    warn_slaves_not_needed_for_purge.store(false,
+      std::memory_order_relaxed); // no other threads to sync with
+    break;
+
   case (int)OPT_REPLICATE_IGNORE_DB:
   {
     cur_rpl_filter->add_ignore_db(argument);

diff --git a/sql/mysqld.h b/sql/mysqld.h
@@ -828,6 +828,7 @@ enum options_mysqld
   OPT_COSTS_ROWID_COPY_COST,
   OPT_EXPIRE_LOGS_DAYS,
   OPT_BINLOG_EXPIRE_LOGS_SECONDS,
+  OPT_SLAVE_CONNECTIONS_NEEDED_FOR_PURGE,
   OPT_CONSOLE,
   OPT_DEBUG_SYNC_TIMEOUT,
   OPT_REMOVED_OPTION,

diff --git a/sql/repl_failsafe.cc b/sql/repl_failsafe.cc
@@ -50,6 +50,7 @@ struct Slave_info
 };
 
 
+std::atomic<bool> warn_slaves_not_needed_for_purge= {true};
 Atomic_counter<uint32_t> binlog_dump_thread_count;
 ulong rpl_status=RPL_NULL;
 mysql_mutex_t LOCK_rpl_status;
@@ -120,6 +121,13 @@ int THD::register_slave(uchar *packet, size_t packet_length)
   Slave_info *si;
   uchar *p= packet, *p_end= packet + packet_length;
   const char *errmsg= "Wrong parameters to function register_slave";
+  if (variables.log_warnings >= 1 && // Verbosity Level "Binlog/Replication"
+      warn_slaves_not_needed_for_purge.exchange(false,
+        // no need to sync with `@@slave_connections_needed_for_purge` itself
+        std::memory_order_relaxed))
+    sql_print_warning(
+      "`@@slave_connections_needed_for_purge` defaults to 0, which means "
+      "this master does not retain older logs even if slaves need them.");
 
   if (check_access(this, PRIV_COM_REGISTER_SLAVE, any_db.str, NULL,NULL,0,0))
     return 1;

diff --git a/sql/sql_repl.h b/sql/sql_repl.h
@@ -23,6 +23,15 @@
 
 struct slave_connection_state;
 
+/**
+  Whether to log a warning once about an unfit
+  @ref slave_connections_needed_for_purge upon @ref COM_REGISTER_SLAVE
+  * Starts as `true` (do warn), and becomes `false`
+    (don't warn) when warning for the first time
+  * Also becomes `false` when the user intetionally sets
+    `@@slave_connections_needed_for_purge`
+*/
+extern std::atomic<bool> warn_slaves_not_needed_for_purge;
 extern my_bool opt_show_slave_auth_info;
 extern char *master_host, *master_info_file;
 

diff --git a/sql/sys_vars.cc b/sql/sys_vars.cc
@@ -1326,13 +1326,23 @@ Sys_slave_connections_needed_for_purge(
       "slave_connections_needed_for_purge",
       "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.",
+      "or binlog_expire_logs_days.",
        GLOBAL_VAR(internal_slave_connections_needed_for_purge),
-       CMD_LINE(REQUIRED_ARG),
-       VALID_RANGE(0, UINT_MAX), DEFAULT(1), BLOCK_SIZE(1),
-       NO_MUTEX_GUARD, NOT_IN_BINLOG, ON_CHECK(0),
-       ON_UPDATE(update_binlog_space_limit));
+       CMD_LINE(REQUIRED_ARG, OPT_SLAVE_CONNECTIONS_NEEDED_FOR_PURGE),
+       VALID_RANGE(0, UINT_MAX), DEFAULT(0), BLOCK_SIZE(1),
+       NO_MUTEX_GUARD, NOT_IN_BINLOG,
+#ifdef HAVE_REPLICATION
+        ON_CHECK([](sys_var *, THD *, set_var *)
+        {
+          warn_slaves_not_needed_for_purge.store(false,
+            // no need to sync with the variable itself
+            std::memory_order_relaxed);
+          return false;
+        })
+#else
+        ON_CHECK(0)
+#endif
+       , ON_UPDATE(update_binlog_space_limit));
 
 
 static Sys_var_mybool Sys_flush(