Per-user session variable default values

Summary:
This diff introduces per-user session variable default values. This is for MyRocks and MySQL production debugging. A new global variable 'per-user-session-var-default-val' with type of string is introduced, it defines the default values of multiple session variables for multiple users, e.g.

--per-user-session-var-default-val="user1:user2:user3:tx_isolation=read-committed,gap_lock_raise_error=0,user4:user5:big_tables=1,auto_increment_increment=12345,long_query_time=123.45"

In this example, the default values of session variables tx_isolation and gap_lock_raise_error for user1, user2, and user3 are defined, the default values of session variables big_tables, auto_increment_increment, and long_query_time for user4 and user 5 are defined. When these users connect to server, the corresponding session variables will be set with these values so that these users don't have to execute SET command to set these session variables before executing queries. The cost of an extra round trip is saved.
(1) This global variable is dynamic so it can be set at runtime.
(2) Sys-vars have many types. The per-user session variables supported types are enum/bool/int/uint/double, which are the most common types for session variables. Other types like string(charptr)/proxy_user/external_user/key_cache/plugin/struct etc are *not* supported.
(3) The users won't be validated when per-user-session-var-default-val is set since users can be changed any time after the global is set
(4) The session variables will be validated when per-user-session-var-default-val is set. Invalid command line value will cause server fail to start. Invalid value in SET command will fail the command.

Test Plan:
New tests added:
  sys_vars.per_user_session_var_default_val_basic.test
  sys_vars.per_user_session_var_default_val_null.test
  sys_vars.per_user_session_var_default_val_empty.test
  sys_vars.per_user_session_var_default_val.test
  sys_vars.per_user_session_var_default_validation.test

Reviewers: santoshb, jkedgar

Reviewed By: jkedgar

Subscribers: webscalesql-eng

Differential Revision: https://reviews.facebook.net/D55791

include/my_getopt.h

@@ -107,6 +107,13 @@ extern my_bool my_getopt_print_errors;
 extern my_bool my_getopt_skip_unknown;
 extern my_error_reporter my_getopt_error_reporter;
 
+extern my_bool my_get_bool_argument(const struct my_option *opts,
+                                    const char *argument,
+                                    int *error);
+extern longlong getopt_ll(char *arg, const struct my_option *optp, int *err);
+extern ulonglong getopt_ull(char *, const struct my_option *, int *);
+extern double getopt_double(char *arg, const struct my_option *optp, int *err);
+
 extern int handle_options (int *argc, char ***argv, 
 			   const struct my_option *longopts, my_get_one_option);
 extern int handle_options_with_logging (int *argc, char ***argv,

mysql-test/r/mysqld--help-notwin-profiling.result

@@ -673,6 +673,8 @@ The following options may be given as the first argument:
  The rate of sampling replayed events on slave to
  determine the peak replication lag over some period.
  --peak-lag-time=#   The time frame peak lag is measured within, in seconds.
+ --per-user-session-var-default-val[=name] 
+ Per user session variable default value
  --performance-schema 
  Enable the performance schema.
  (Defaults to on; use --skip-performance-schema to disable.)
@@ -1684,6 +1686,7 @@ optimizer-trace-offset -1
 part-scan-max 10
 peak-lag-sample-rate 100
 peak-lag-time 60
+per-user-session-var-default-val (No default value)
 performance-schema TRUE
 performance-schema-accounts-size -1
 performance-schema-consumer-events-stages-current FALSE

mysql-test/r/mysqld--help-notwin.result

@@ -673,6 +673,8 @@ The following options may be given as the first argument:
  The rate of sampling replayed events on slave to
  determine the peak replication lag over some period.
  --peak-lag-time=#   The time frame peak lag is measured within, in seconds.
+ --per-user-session-var-default-val[=name] 
+ Per user session variable default value
  --performance-schema 
  Enable the performance schema.
  (Defaults to on; use --skip-performance-schema to disable.)
@@ -1682,6 +1684,7 @@ optimizer-trace-offset -1
 part-scan-max 10
 peak-lag-sample-rate 100
 peak-lag-time 60
+per-user-session-var-default-val (No default value)
 performance-schema TRUE
 performance-schema-accounts-size -1
 performance-schema-consumer-events-stages-current FALSE

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

@@ -0,0 +1,35 @@
+SET @original_val = @@global.per_user_session_var_default_val;
+SELECT @original_val;
+@original_val
+NULL
+SELECT @@GLOBAL.per_user_session_var_default_val = VARIABLE_VALUE
+FROM INFORMATION_SCHEMA.GLOBAL_VARIABLES
+WHERE VARIABLE_NAME='per_user_session_var_default_val';
+@@GLOBAL.per_user_session_var_default_val = VARIABLE_VALUE
+NULL
+SELECT @@SESSION.per_user_session_var_default_val;
+ERROR HY000: Variable 'per_user_session_var_default_val' is a GLOBAL variable
+SET SESSION per_user_session_var_default_val = "";
+ERROR HY000: Variable 'per_user_session_var_default_val' is a GLOBAL variable and should be set with SET GLOBAL
+SELECT @@GLOBAL.per_user_session_var_default_val;
+@@GLOBAL.per_user_session_var_default_val
+NULL
+## default value is NULL
+SET GLOBAL per_user_session_var_default_val = DEFAULT;
+SELECT @@GLOBAL.per_user_session_var_default_val;
+@@GLOBAL.per_user_session_var_default_val
+NULL
+## NULL is a valid value
+SET GLOBAL per_user_session_var_default_val = NULL;
+SELECT @@GLOBAL.per_user_session_var_default_val;
+@@GLOBAL.per_user_session_var_default_val
+NULL
+## "" is a valid value
+SET GLOBAL per_user_session_var_default_val = "";
+SELECT @@GLOBAL.per_user_session_var_default_val;
+@@GLOBAL.per_user_session_var_default_val
+
+SET @@global.per_user_session_var_default_val = @original_val;
+SELECT @@global.per_user_session_var_default_val;
+@@global.per_user_session_var_default_val
+NULL

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

@@ -0,0 +1,4 @@
+# empty string is a valid value
+SELECT @@GLOBAL.per_user_session_var_default_val;
+@@GLOBAL.per_user_session_var_default_val
+

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

@@ -0,0 +1,4 @@
+# default value is NULL ptr
+SELECT @@GLOBAL.per_user_session_var_default_val;
+@@GLOBAL.per_user_session_var_default_val
+NULL

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

@@ -0,0 +1,134 @@
+flush privileges;
+create user usr1@localhost;
+grant all privileges on *.* to admin1@localhost identified by password '' with grant option;
+flush user_resources;
+SET @original_val = @@global.per_user_session_var_default_val;
+SELECT @original_val;
+@original_val
+NULL
+## default value is NULL
+SET GLOBAL per_user_session_var_default_val = DEFAULT;
+SELECT @@GLOBAL.per_user_session_var_default_val;
+@@GLOBAL.per_user_session_var_default_val
+NULL
+## NULL is a valid value
+SET GLOBAL per_user_session_var_default_val = NULL;
+SELECT @@GLOBAL.per_user_session_var_default_val;
+@@GLOBAL.per_user_session_var_default_val
+NULL
+## "" is a valid value
+SET GLOBAL per_user_session_var_default_val = "";
+SELECT @@GLOBAL.per_user_session_var_default_val;
+@@GLOBAL.per_user_session_var_default_val
+
+##
+## Invalid values. SET command will fail
+##
+SET GLOBAL per_user_session_var_default_val = " ";
+ERROR 42000: Variable 'per_user_session_var_default_val' can't be set to the value of ' '
+SET GLOBAL per_user_session_var_default_val = " ";
+ERROR 42000: Variable 'per_user_session_var_default_val' can't be set to the value of ' '
+SET GLOBAL per_user_session_var_default_val = ":";
+ERROR 42000: Variable 'per_user_session_var_default_val' can't be set to the value of ':'
+SET GLOBAL per_user_session_var_default_val = "=";
+ERROR 42000: Variable 'per_user_session_var_default_val' can't be set to the value of '='
+SET GLOBAL per_user_session_var_default_val = ",";
+ERROR 42000: Variable 'per_user_session_var_default_val' can't be set to the value of ','
+SET GLOBAL per_user_session_var_default_val = ";";
+ERROR 42000: Variable 'per_user_session_var_default_val' can't be set to the value of ';'
+SET GLOBAL per_user_session_var_default_val = "\"";
+ERROR 42000: Variable 'per_user_session_var_default_val' can't be set to the value of '"'
+SET GLOBAL per_user_session_var_default_val = "'";
+ERROR 42000: Variable 'per_user_session_var_default_val' can't be set to the value of '''
+SET GLOBAL per_user_session_var_default_val = "\\";
+ERROR 42000: Variable 'per_user_session_var_default_val' can't be set to the value of '\'
+SET GLOBAL per_user_session_var_default_val = "/";
+ERROR 42000: Variable 'per_user_session_var_default_val' can't be set to the value of '/'
+SET GLOBAL per_user_session_var_default_val = "u";
+ERROR 42000: Variable 'per_user_session_var_default_val' can't be set to the value of 'u'
+SET GLOBAL per_user_session_var_default_val = "gap_lock_raise_error=0";
+ERROR 42000: Variable 'per_user_session_var_default_val' can't be set to the value of 'gap_lock_raise_error=0'
+SET GLOBAL per_user_session_var_default_val = "gap_lock_raise_error=0,u2:gap_lock_raise_error=1";
+ERROR 42000: Variable 'per_user_session_var_default_val' can't be set to the value of 'gap_lock_raise_error=0,u2:gap_lock_raise_error=1'
+SET GLOBAL per_user_session_var_default_val = " u";
+ERROR 42000: Variable 'per_user_session_var_default_val' can't be set to the value of ' u'
+SET GLOBAL per_user_session_var_default_val = "u ";
+ERROR 42000: Variable 'per_user_session_var_default_val' can't be set to the value of 'u '
+SET GLOBAL per_user_session_var_default_val = "u :";
+ERROR 42000: Variable 'per_user_session_var_default_val' can't be set to the value of 'u :'
+SET GLOBAL per_user_session_var_default_val = "u:gap_lock_raise_error";
+ERROR 42000: Variable 'per_user_session_var_default_val' can't be set to the value of 'u:gap_lock_raise_error'
+SET GLOBAL per_user_session_var_default_val = "u: ";
+ERROR 42000: Variable 'per_user_session_var_default_val' can't be set to the value of 'u: '
+SET GLOBAL per_user_session_var_default_val = " u:gap_lock_raise_error";
+ERROR 42000: Variable 'per_user_session_var_default_val' can't be set to the value of ' u:gap_lock_raise_error'
+SET GLOBAL per_user_session_var_default_val = "u:v:";
+ERROR 42000: Variable 'per_user_session_var_default_val' can't be set to the value of 'u:v:'
+SET GLOBAL per_user_session_var_default_val = "u:gap_lock_raise_error=";
+ERROR 42000: Variable 'per_user_session_var_default_val' can't be set to the value of 'u:gap_lock_raise_error='
+SET GLOBAL per_user_session_var_default_val = "u:gap_lock_raise_error=0:";
+ERROR 42000: Variable 'per_user_session_var_default_val' can't be set to the value of 'u:gap_lock_raise_error=0:'
+SET GLOBAL per_user_session_var_default_val = "u:gap_lock_raise_error=0=";
+ERROR 42000: Variable 'per_user_session_var_default_val' can't be set to the value of 'u:gap_lock_raise_error=0='
+SET GLOBAL per_user_session_var_default_val = "u:gap_lock_raise_error=0=1";
+ERROR 42000: Variable 'per_user_session_var_default_val' can't be set to the value of 'u:gap_lock_raise_error=0=1'
+SET GLOBAL per_user_session_var_default_val = "u:gap_lock_raise_error=0,u:gap_lock_write_log=0";
+ERROR 42000: Variable 'per_user_session_var_default_val' can't be set to the value of 'u:gap_lock_raise_error=0,u:gap_lock_write_log=0'
+SET GLOBAL per_user_session_var_default_val = "u:= 0";
+ERROR 42000: Variable 'per_user_session_var_default_val' can't be set to the value of 'u:= 0'
+SET GLOBAL per_user_session_var_default_val = "u:gap_lock_raise_error= 0";
+ERROR 42000: Variable 'per_user_session_var_default_val' can't be set to the value of 'u:gap_lock_raise_error= 0'
+SET GLOBAL per_user_session_var_default_val = "u:gap_lock_raise_error= ";
+ERROR 42000: Variable 'per_user_session_var_default_val' can't be set to the value of 'u:gap_lock_raise_error= '
+SET GLOBAL per_user_session_var_default_val = "u:gap_lock_raise_error=0,";
+ERROR 42000: Variable 'per_user_session_var_default_val' can't be set to the value of 'u:gap_lock_raise_error=0,'
+SET GLOBAL per_user_session_var_default_val = "u:gap_lock_raise_error=0;";
+ERROR 42000: Variable 'per_user_session_var_default_val' can't be set to the value of 'u:gap_lock_raise_error=0;'
+SET GLOBAL per_user_session_var_default_val = "u1:u2:gap_lock_raise_error=0,gap_lock_write_log=0,,u3:big_tables=1";
+ERROR 42000: Variable 'per_user_session_var_default_val' can't be set to the value of 'u1:u2:gap_lock_raise_error=0,gap_lock_write_log=0,,u3:big_tables=1'
+SET GLOBAL per_user_session_var_default_val = "u1::u2:gap_lock_raise_error=0,gap_lock_write_log=0,u3:big_tables=1";
+ERROR 42000: Variable 'per_user_session_var_default_val' can't be set to the value of 'u1::u2:gap_lock_raise_error=0,gap_lock_write_log=0,u3:big_tables=1'
+SET GLOBAL per_user_session_var_default_val = "u1:gap_lock_raise_error=0,gap_lock_write_log=0,u3:big_tables=1 ";
+ERROR 42000: Variable 'per_user_session_var_default_val' can't be set to the value of 'u1:gap_lock_raise_error=0,gap_lock_write_log=0,u3:big_tables=1 '
+SET GLOBAL per_user_session_var_default_val = "u1:gap_lock_raise_error=0,gap_lock_write_log=0,u3:big_tables=1,";
+ERROR 42000: Variable 'per_user_session_var_default_val' can't be set to the value of 'u1:gap_lock_raise_error=0,gap_lock_write_log=0,u3:big_tables=1,'
+SET GLOBAL per_user_session_var_default_val = "u1:gap_lock_raise_error=0,gap_lock_write_log=0,u3:big_tables=1'";
+ERROR 42000: Variable 'per_user_session_var_default_val' can't be set to the value of 'u1:gap_lock_raise_error=0,gap_lock_write_log=0,u3:big_tables=1''
+SET GLOBAL per_user_session_var_default_val = "u1:gap_lock_raise_error=0,gap_lock_write_log=0,u3:big_tables='1'";
+ERROR 42000: Variable 'per_user_session_var_default_val' can't be set to the value of 'u1:gap_lock_raise_error=0,gap_lock_write_log=0,u3:big_tables='1''
+SET GLOBAL per_user_session_var_default_val = "u1:gap_lock_raise_error=0,gap_lock_write_log=0,u3:big_tables=\"1\"";
+ERROR 42000: Variable 'per_user_session_var_default_val' can't be set to the value of 'u1:gap_lock_raise_error=0,gap_lock_write_log=0,u3:big_tables="1"'
+SET GLOBAL per_user_session_var_default_val = "u1:time_zone=\'+10:00\'";
+ERROR 42000: Variable 'per_user_session_var_default_val' can't be set to the value of 'u1:time_zone='+10:00''
+## Syntax error
+SET GLOBAL per_user_session_var_default_val = "u1:optimizer_switch=index_merge=on";
+ERROR 42000: Variable 'per_user_session_var_default_val' can't be set to the value of 'u1:optimizer_switch=index_merge=on'
+## Unsupported type 'flagset'
+SET GLOBAL per_user_session_var_default_val = "u1:optimizer_switch=xyz";
+ERROR 42000: Variable 'per_user_session_var_default_val' can't be set to the value of 'u1:optimizer_switch=xyz'
+##
+## root privilege is required to set session variable binglog_format
+## root privilege will be granted to usr1 temporarily to get this done
+##
+SET GLOBAL per_user_session_var_default_val = "usr1:binlog_format=MIXED";
+SELECT @@GLOBAL.binlog_format, @@SESSION.binlog_format;
+@@GLOBAL.binlog_format	@@SESSION.binlog_format
+STATEMENT	MIXED
+SET GLOBAL per_user_session_var_default_val = "usr1:binlog_format=ROW";
+SELECT @@GLOBAL.binlog_format, @@SESSION.binlog_format;
+@@GLOBAL.binlog_format	@@SESSION.binlog_format
+STATEMENT	ROW
+SET GLOBAL per_user_session_var_default_val = "admin1:binlog_format=MIXED";
+SELECT @@GLOBAL.binlog_format, @@SESSION.binlog_format;
+@@GLOBAL.binlog_format	@@SESSION.binlog_format
+STATEMENT	MIXED
+SET GLOBAL per_user_session_var_default_val = "admin1:binlog_format=ROW";
+SELECT @@GLOBAL.binlog_format, @@SESSION.binlog_format;
+@@GLOBAL.binlog_format	@@SESSION.binlog_format
+STATEMENT	ROW
+drop user usr1@localhost;
+drop user admin1@localhost;
+SET @@global.per_user_session_var_default_val = @original_val;
+SELECT @@global.per_user_session_var_default_val;
+@@global.per_user_session_var_default_val
+NULL

mysql-test/suite/sys_vars/t/per_user_session_var_conn_create.inc

@@ -0,0 +1,9 @@
+--echo ## create connection batch $i
+let $j=1;
+while ($j<=5) {
+  connect (con_tao_$i$j,localhost,tao$j,,);
+  connect (con_tao_ssl_$i$j,localhost,tao$j,,,,,SSL);
+  inc $j;
+}
+connect (con_root_$i,localhost,root,,);
+connect (con_root_ssl_$i,localhost,root,,,,,SSL);

mysql-test/suite/sys_vars/t/per_user_session_var_default_val-master.opt

@@ -0,0 +1 @@
+--per-user-session-var-default-val="tao2:tao3:root:tx_isolation=read-committed,gap_lock_raise_error=0,gap_lock_write_log=0,big_tables=1,tao1:bulk_insert_buffer_size=16M,gap_lock_raise_error=0,gap_lock_write_log=0,auto_increment_increment=12345,long_query_time=123.45"

mysql-test/suite/sys_vars/t/per_user_session_var_default_val-slave.opt

@@ -0,0 +1 @@
+--per-user-session-var-default-val="tao2:tao3:root:tx_isolation=read-committed,gap_lock_raise_error=0,gap_lock_write_log=0,big_tables=1,tao1:bulk_insert_buffer_size=16M,gap_lock_raise_error=0,gap_lock_write_log=0,auto_increment_increment=12345,long_query_time=123.45"

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

@@ -0,0 +1,121 @@
+--source include/load_sysvars.inc
+
+# Wait for all the connections from previous tests except
+# the default one to be completedly disconnected
+let $count_sessions= 1;
+--source include/wait_until_count_sessions.inc
+
+flush privileges;
+
+--source include/master-slave.inc
+connection master;
+
+let $original_val = query_get_value(select @@GLOBAL.per_user_session_var_default_val as val, val, 1);
+--echo $original_val
+SELECT @@GLOBAL.per_user_session_var_default_val;
+
+--echo ## create non-super users tao1, tao2, and tao3
+create user tao1@localhost;
+create user tao2@localhost;
+create user tao3@localhost;
+
+--echo ## create super users tao4 and tao5
+grant all privileges on *.* to tao4@localhost identified by password '' with grant option;
+grant all privileges on *.* to tao5@localhost identified by password '' with grant option;
+
+flush user_resources;
+
+let $i=1;
+--echo ## ($i) Initial non-empty value
+--source per_user_session_var_conn_create.inc
+
+inc $i;
+--echo ## ($i) DEFAULT value
+SET GLOBAL per_user_session_var_default_val = DEFAULT;
+SELECT @@GLOBAL.per_user_session_var_default_val;
+--source per_user_session_var_conn_create.inc
+
+inc $i;
+--echo ## ($i) Non-empty value
+SET GLOBAL per_user_session_var_default_val="tao1:tao2:tao3:tao4:tx_isolation=serializable,big_tables=1,auto_increment_increment=12345,long_query_time=123.45,bulk_insert_buffer_size=1G,tao5:root:binlog_format=row";
+SELECT @@GLOBAL.per_user_session_var_default_val;
+--source per_user_session_var_conn_create.inc
+
+inc $i;
+--echo ## ($i) NULL value
+SET GLOBAL per_user_session_var_default_val = NULL;
+SELECT @@GLOBAL.per_user_session_var_default_val;
+--source per_user_session_var_conn_create.inc
+
+inc $i;
+--echo ## ($i) Non-empty value
+SET GLOBAL per_user_session_var_default_val="tao1:tx_isolation=READ-UNCOMMITTED,tao2:tao3:gap_lock_raise_error=FALSE,root:gap_lock_write_log=off,tao4:big_tables=1,auto_increment_increment=12345,long_query_time=123.45,tao5:binlog_format=MIXED,bulk_insert_buffer_size=32M";
+SELECT @@GLOBAL.per_user_session_var_default_val;
+--source per_user_session_var_conn_create.inc
+
+inc $i;
+--echo ## ($i) Empty value
+SET GLOBAL per_user_session_var_default_val = "";
+SELECT @@GLOBAL.per_user_session_var_default_val;
+--source per_user_session_var_conn_create.inc
+
+inc $i;
+--echo ## ($i) Restored initial non-empty value
+eval SET @@global.per_user_session_var_default_val = "$original_val";
+SELECT @@global.per_user_session_var_default_val;
+--source per_user_session_var_conn_create.inc
+
+--echo ##
+--echo ## Session variables of all the connnections
+--echo ##
+
+let $k=1;
+while ($k<=$i) {
+  --echo ##
+  --echo ## ($k) Connection batch $k
+  --echo ##
+
+  let $j=1;
+  while ($j<=5) {
+    --echo ##
+    --echo ## Connection con_tao_$k$j for user tao$j
+    --echo ##
+    connection con_tao_$k$j;
+    --source per_user_session_var_print.inc
+    disconnect con_tao_$k$j;
+
+    --echo ##
+    --echo ## SSL connection con_tao_ssl_$k$j for user tao$j
+    --echo ##
+    connection con_tao_ssl_$k$j;
+    --source per_user_session_var_print.inc
+    disconnect con_tao_ssl_$k$j;
+
+    inc $j;
+  }
+  --echo ##
+  --echo ## Connection con_root_$k for user root
+  --echo ##
+  connection con_root_$k;
+  --source per_user_session_var_print.inc
+  disconnect con_root_$k;
+
+  --echo ##
+  --echo ## SSL connection con_root_ssl_$k for user root
+  --echo ##
+  connection con_root_ssl_$k;
+  --source per_user_session_var_print.inc
+  disconnect con_root_ssl_$k;
+
+  inc $k;
+}
+
+connection default;
+
+drop user tao1@localhost;
+drop user tao2@localhost;
+drop user tao3@localhost;
+drop user tao4@localhost;
+drop user tao5@localhost;
+
+--source include/rpl_end.inc

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

@@ -0,0 +1,31 @@
+--source include/load_sysvars.inc
+
+SET @original_val = @@global.per_user_session_var_default_val;
+SELECT @original_val;
+
+SELECT @@GLOBAL.per_user_session_var_default_val = VARIABLE_VALUE
+FROM INFORMATION_SCHEMA.GLOBAL_VARIABLES
+WHERE VARIABLE_NAME='per_user_session_var_default_val';
+
+--error ER_INCORRECT_GLOBAL_LOCAL_VAR
+SELECT @@SESSION.per_user_session_var_default_val;
+
+--error ER_GLOBAL_VARIABLE
+SET SESSION per_user_session_var_default_val = "";
+
+SELECT @@GLOBAL.per_user_session_var_default_val;
+
+--echo ## default value is NULL
+SET GLOBAL per_user_session_var_default_val = DEFAULT;
+SELECT @@GLOBAL.per_user_session_var_default_val;
+
+--echo ## NULL is a valid value
+SET GLOBAL per_user_session_var_default_val = NULL;
+SELECT @@GLOBAL.per_user_session_var_default_val;
+
+--echo ## "" is a valid value
+SET GLOBAL per_user_session_var_default_val = "";
+SELECT @@GLOBAL.per_user_session_var_default_val;
+
+SET @@global.per_user_session_var_default_val = @original_val;
+SELECT @@global.per_user_session_var_default_val;

mysql-test/suite/sys_vars/t/per_user_session_var_default_val_empty-master.opt

@@ -0,0 +1 @@
+--per-user-session-var-default-val=""