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.

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

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

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

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

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

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

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

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

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

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

mysql-test/suite/wsrep/suite.pm

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

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

+#
+# Check the handling of @@wsrep_dirty_reads
+#
+
+--source include/galera_cluster.inc
+--source include/have_innodb.inc
+
+--connection node_2
+--let $wsrep_cluster_address_saved = `SELECT @@global.wsrep_cluster_address`
+
+CREATE TABLE t1(i INT) ENGINE=INNODB;
+INSERT INTO t1 VALUES(1);
+SELECT * FROM t1;
+
+SET @@global.wsrep_cluster_address = '';
+SET @@session.wsrep_dirty_reads=OFF;
+
+# Set wsrep_sync_wait to avoid ER_LOCK_WAIT_TIMEOUT (MDEV-6832).
+SET SESSION wsrep_sync_wait=0;
+
+# Must return 'OFF'
+SHOW STATUS LIKE 'wsrep_ready';
+
+# Must return 'Non-primary'
+SHOW STATUS LIKE 'wsrep_cluster_status';
+
+--error ER_UNKNOWN_COM_ERROR
+SELECT * FROM t1;
+
+SET @@session.wsrep_dirty_reads=ON;
+
+SELECT * FROM t1;
+
+--disable_query_log
+--eval SET @@global.wsrep_cluster_address = '$wsrep_cluster_address_saved'
+--enable_query_log
+--source include/wait_until_connected_again.inc
+
+--connection node_1
+SELECT * FROM t1;
+# Cleanup
+DROP TABLE t1;
+
+--source include/galera_end.inc
+--echo # End of test
+

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

@@ -52,7 +52,6 @@ SHOW STATUS LIKE 'wsrep_last_committed';
 --echo #
 --echo # MDEV#6206: wsrep_slave_threads subtracts from max_connections
 --echo #
-call mtr.add_suppression("safe_mutex: Found wrong usage of mutex 'LOCK_wsrep_slave_threads' and 'LOCK_global_system_variables'");
 
 --replace_regex /.*libgalera_smm.*/libgalera_smm.so/
 SELECT @@global.wsrep_provider;

sql/sql_class.h

@@ -647,6 +647,7 @@ typedef struct system_variables
 
   my_bool wsrep_on;
   my_bool wsrep_causal_reads;
+  my_bool wsrep_dirty_reads;
   uint wsrep_sync_wait;
   ulong wsrep_retry_autocommit;
   double long_query_time_double, max_statement_time_double;

sql/sql_parse.cc

@@ -2607,11 +2607,13 @@ mysql_execute_command(THD *thd)
     }
 
     /*
-     * bail out if DB snapshot has not been installed. We however,
-     * allow SET and SHOW queries
-     */
+      Bail out if DB snapshot has not been installed. We however,
+      allow SET and SHOW queries.
+    */
     if (lex->sql_command != SQLCOM_SET_OPTION &&
         !wsrep_is_show_query(lex->sql_command) &&
+        !(thd->variables.wsrep_dirty_reads &&
+          lex->sql_command == SQLCOM_SELECT) &&
         !wsrep_node_is_ready(thd))
       goto error;
   }

sql/sys_vars.cc

@@ -4840,6 +4840,12 @@ static Sys_var_mybool Sys_wsrep_slave_UK_checks(
 static Sys_var_mybool Sys_wsrep_restart_slave(
        "wsrep_restart_slave", "Should MySQL slave be restarted automatically, when node joins back to cluster",
        GLOBAL_VAR(wsrep_restart_slave), CMD_LINE(OPT_ARG), DEFAULT(FALSE));
+
+static Sys_var_mybool Sys_wsrep_dirty_reads(
+       "wsrep_dirty_reads", "Do not reject SELECT queries even when the node "
+       "is not ready.", SESSION_ONLY(wsrep_dirty_reads), NO_CMD_LINE,
+       DEFAULT(FALSE), NO_MUTEX_GUARD, NOT_IN_BINLOG);
+
 #endif /* WITH_WSREP */
 
 static bool fix_host_cache_size(sys_var *, THD *, enum_var_type)

sql/wsrep_var.cc

@@ -353,7 +353,14 @@ bool wsrep_cluster_address_update (sys_var *self, THD* thd, enum_var_type type)
   */
   mysql_mutex_unlock(&LOCK_global_system_variables);
   wsrep_stop_replication(thd);
+
+  /*
+    Unlock and lock LOCK_wsrep_slave_threads to maintain lock order & avoid
+    any potential deadlock.
+  */
+  mysql_mutex_unlock(&LOCK_wsrep_slave_threads);
   mysql_mutex_lock(&LOCK_global_system_variables);
+  mysql_mutex_lock(&LOCK_wsrep_slave_threads);
 
   if (wsrep_start_replication())
   {