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Nirbhay Choubey
committed
MDEV-7476: Allow SELECT to succeed even when node is not ready
Added a SESSION-only system variable "wsrep_dirty_reads" to allow SELECT queries to pass even when the node is not prepared to accept queries (wsrep_ready=OFF). Added a test case.
1 parent ab440b0 commit 0105bf3

12 files changed

+175
-5
lines changed

‎mysql-test/suite/sys_vars/r/sysvars_wsrep.result

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@@ -141,6 +141,20 @@ NUMERIC_BLOCK_SIZE NULL
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ENUM_VALUE_LIST OFF,ON
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READ_ONLY NO
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COMMAND_LINE_ARGUMENT OPTIONAL
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VARIABLE_NAME WSREP_DIRTY_READS
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SESSION_VALUE OFF
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GLOBAL_VALUE NULL
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GLOBAL_VALUE_ORIGIN COMPILE-TIME
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DEFAULT_VALUE OFF
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VARIABLE_SCOPE SESSION ONLY
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VARIABLE_TYPE BOOLEAN
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VARIABLE_COMMENT Do not reject SELECT queries even when the node is not ready.
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NUMERIC_MIN_VALUE NULL
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NUMERIC_MAX_VALUE NULL
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NUMERIC_BLOCK_SIZE NULL
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ENUM_VALUE_LIST OFF,ON
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READ_ONLY NO
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COMMAND_LINE_ARGUMENT NULL
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VARIABLE_NAME WSREP_DRUPAL_282555_WORKAROUND
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SESSION_VALUE NULL
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GLOBAL_VALUE OFF

‎mysql-test/suite/sys_vars/r/wsrep_dirty_reads_basic.result

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#
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# wsrep_dirty_reads
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#
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# save the initial value
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SET @wsrep_dirty_reads_session_saved = @@session.wsrep_dirty_reads;
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# default
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SELECT @@global.wsrep_dirty_reads;
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ERROR HY000: Variable 'wsrep_dirty_reads' is a SESSION variable
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SELECT @@session.wsrep_dirty_reads;
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@@session.wsrep_dirty_reads
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0
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# scope and valid values
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SET @@session.wsrep_dirty_reads=OFF;
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SELECT @@session.wsrep_dirty_reads;
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@@session.wsrep_dirty_reads
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0
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SET @@session.wsrep_dirty_reads=ON;
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SELECT @@session.wsrep_dirty_reads;
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@@session.wsrep_dirty_reads
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1
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SET @@session.wsrep_dirty_reads=default;
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SELECT @@session.wsrep_dirty_reads;
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@@session.wsrep_dirty_reads
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0
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# invalid values
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SET @@session.wsrep_dirty_reads=NULL;
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ERROR 42000: Variable 'wsrep_dirty_reads' can't be set to the value of 'NULL'
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SET @@session.wsrep_dirty_reads='junk';
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ERROR 42000: Variable 'wsrep_dirty_reads' can't be set to the value of 'junk'
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# restore the initial values
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SET @@session.wsrep_dirty_reads = @wsrep_dirty_reads_session_saved;
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# End of test

‎mysql-test/suite/sys_vars/t/wsrep_dirty_reads_basic.test

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@@ -0,0 +1,35 @@
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--source include/have_wsrep.inc
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--echo #
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--echo # wsrep_dirty_reads
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--echo #
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--echo # save the initial value
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SET @wsrep_dirty_reads_session_saved = @@session.wsrep_dirty_reads;
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--echo # default
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--error ER_INCORRECT_GLOBAL_LOCAL_VAR
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SELECT @@global.wsrep_dirty_reads;
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SELECT @@session.wsrep_dirty_reads;
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--echo
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--echo # scope and valid values
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SET @@session.wsrep_dirty_reads=OFF;
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SELECT @@session.wsrep_dirty_reads;
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SET @@session.wsrep_dirty_reads=ON;
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SELECT @@session.wsrep_dirty_reads;
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SET @@session.wsrep_dirty_reads=default;
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SELECT @@session.wsrep_dirty_reads;
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--echo
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--echo # invalid values
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--error ER_WRONG_VALUE_FOR_VAR
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SET @@session.wsrep_dirty_reads=NULL;
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--error ER_WRONG_VALUE_FOR_VAR
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SET @@session.wsrep_dirty_reads='junk';
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--echo
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--echo # restore the initial values
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SET @@session.wsrep_dirty_reads = @wsrep_dirty_reads_session_saved;
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--echo # End of test

‎mysql-test/suite/wsrep/r/galera_var_dirty_reads.result

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@@ -0,0 +1,25 @@
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CREATE TABLE t1(i INT) ENGINE=INNODB;
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INSERT INTO t1 VALUES(1);
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SELECT * FROM t1;
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i
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1
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SET @@global.wsrep_cluster_address = '';
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SET @@session.wsrep_dirty_reads=OFF;
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SET SESSION wsrep_sync_wait=0;
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SHOW STATUS LIKE 'wsrep_ready';
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Variable_name Value
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wsrep_ready OFF
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SHOW STATUS LIKE 'wsrep_cluster_status';
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Variable_name Value
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wsrep_cluster_status non-Primary
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SELECT * FROM t1;
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ERROR 08S01: WSREP has not yet prepared node for application use
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SET @@session.wsrep_dirty_reads=ON;
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SELECT * FROM t1;
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i
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1
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SELECT * FROM t1;
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i
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1
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DROP TABLE t1;
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# End of test

‎mysql-test/suite/wsrep/r/variables.result

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@@ -137,7 +137,6 @@ wsrep_last_committed #
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#
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# MDEV#6206: wsrep_slave_threads subtracts from max_connections
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#
140-
call mtr.add_suppression("safe_mutex: Found wrong usage of mutex 'LOCK_wsrep_slave_threads' and 'LOCK_global_system_variables'");
141140
SELECT @@global.wsrep_provider;
142141
@@global.wsrep_provider
143142
libgalera_smm.so

‎mysql-test/suite/wsrep/suite.pm

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@@ -31,6 +31,7 @@ push @::global_suppressions,
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qr(WSREP: last inactive check more than .* skipping check),
3232
qr(WSREP: Gap in state sequence. Need state transfer.),
3333
qr(WSREP: Failed to prepare for incremental state transfer: .*),
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qr(WSREP: Releasing seqno [0-9]* before [0-9]* was assigned.),
3435
);
3536

3637

‎mysql-test/suite/wsrep/t/galera_var_dirty_reads.test

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@@ -0,0 +1,46 @@
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#
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# Check the handling of @@wsrep_dirty_reads
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#
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--source include/galera_cluster.inc
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--source include/have_innodb.inc
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--connection node_2
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--let $wsrep_cluster_address_saved = `SELECT @@global.wsrep_cluster_address`
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11+
CREATE TABLE t1(i INT) ENGINE=INNODB;
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INSERT INTO t1 VALUES(1);
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SELECT * FROM t1;
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SET @@global.wsrep_cluster_address = '';
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SET @@session.wsrep_dirty_reads=OFF;
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# Set wsrep_sync_wait to avoid ER_LOCK_WAIT_TIMEOUT (MDEV-6832).
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SET SESSION wsrep_sync_wait=0;
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# Must return 'OFF'
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SHOW STATUS LIKE 'wsrep_ready';
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# Must return 'Non-primary'
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SHOW STATUS LIKE 'wsrep_cluster_status';
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--error ER_UNKNOWN_COM_ERROR
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SELECT * FROM t1;
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30+
SET @@session.wsrep_dirty_reads=ON;
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SELECT * FROM t1;
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--disable_query_log
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--eval SET @@global.wsrep_cluster_address = '$wsrep_cluster_address_saved'
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--enable_query_log
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--source include/wait_until_connected_again.inc
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--connection node_1
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SELECT * FROM t1;
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# Cleanup
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DROP TABLE t1;
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44+
--source include/galera_end.inc
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--echo # End of test
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‎mysql-test/suite/wsrep/t/variables.test

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@@ -52,7 +52,6 @@ SHOW STATUS LIKE 'wsrep_last_committed';
5252
--echo #
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--echo # MDEV#6206: wsrep_slave_threads subtracts from max_connections
5454
--echo #
55-
call mtr.add_suppression("safe_mutex: Found wrong usage of mutex 'LOCK_wsrep_slave_threads' and 'LOCK_global_system_variables'");
5655

5756
--replace_regex /.*libgalera_smm.*/libgalera_smm.so/
5857
SELECT @@global.wsrep_provider;

‎sql/sql_class.h

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@@ -647,6 +647,7 @@ typedef struct system_variables
647647

648648
my_bool wsrep_on;
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my_bool wsrep_causal_reads;
650+
my_bool wsrep_dirty_reads;
650651
uint wsrep_sync_wait;
651652
ulong wsrep_retry_autocommit;
652653
double long_query_time_double, max_statement_time_double;

‎sql/sql_parse.cc

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@@ -2607,11 +2607,13 @@ mysql_execute_command(THD *thd)
26072607
}
26082608

26092609
/*
2610-
* bail out if DB snapshot has not been installed. We however,
2611-
* allow SET and SHOW queries
2612-
*/
2610+
Bail out if DB snapshot has not been installed. We however,
2611+
allow SET and SHOW queries.
2612+
*/
26132613
if (lex->sql_command != SQLCOM_SET_OPTION &&
26142614
!wsrep_is_show_query(lex->sql_command) &&
2615+
!(thd->variables.wsrep_dirty_reads &&
2616+
lex->sql_command == SQLCOM_SELECT) &&
26152617
!wsrep_node_is_ready(thd))
26162618
goto error;
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}

‎sql/sys_vars.cc

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Original file line numberDiff line numberDiff line change
@@ -4840,6 +4840,12 @@ static Sys_var_mybool Sys_wsrep_slave_UK_checks(
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static Sys_var_mybool Sys_wsrep_restart_slave(
48414841
"wsrep_restart_slave", "Should MySQL slave be restarted automatically, when node joins back to cluster",
48424842
GLOBAL_VAR(wsrep_restart_slave), CMD_LINE(OPT_ARG), DEFAULT(FALSE));
4843+
4844+
static Sys_var_mybool Sys_wsrep_dirty_reads(
4845+
"wsrep_dirty_reads", "Do not reject SELECT queries even when the node "
4846+
"is not ready.", SESSION_ONLY(wsrep_dirty_reads), NO_CMD_LINE,
4847+
DEFAULT(FALSE), NO_MUTEX_GUARD, NOT_IN_BINLOG);
4848+
48434849
#endif /* WITH_WSREP */
48444850

48454851
static bool fix_host_cache_size(sys_var *, THD *, enum_var_type)

‎sql/wsrep_var.cc

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@@ -353,7 +353,14 @@ bool wsrep_cluster_address_update (sys_var *self, THD* thd, enum_var_type type)
353353
*/
354354
mysql_mutex_unlock(&LOCK_global_system_variables);
355355
wsrep_stop_replication(thd);
356+
357+
/*
358+
Unlock and lock LOCK_wsrep_slave_threads to maintain lock order & avoid
359+
any potential deadlock.
360+
*/
361+
mysql_mutex_unlock(&LOCK_wsrep_slave_threads);
356362
mysql_mutex_lock(&LOCK_global_system_variables);
363+
mysql_mutex_lock(&LOCK_wsrep_slave_threads);
357364

358365
if (wsrep_start_replication())
359366
{

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