Add a variable for appending a custom message to server read_only error

Summary:
Proposing a way to set a custom message in the `--read-only (super)` error.
Note: the variable can only be set when server is in read-only (super) state.

p.s. I am hesitant to add a mutex for the variable, so we will get a snapshot
of the current pointer of the variable (const char*) when printing the error.

Squash with https://reviews.facebook.net/D61383

Test Plan: Added tests.

Reviewers: santoshb

Reviewed By: santoshb

Subscribers: webscalesql-eng, ebergen

Differential Revision: https://reviews.facebook.net/D61713

mysql-test/r/bug58669.result

@@ -11,6 +11,7 @@ user1@localhost
 SHOW VARIABLES LIKE "read_only%";
 Variable_name	Value
 read_only	ON
+read_only_error_msg_extra	
 read_only_slave	ON
 INSERT INTO db1.t1 VALUES (1);
 ERROR HY000: The MySQL server is running with the --read-only option so it cannot execute this statement

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

@@ -872,6 +872,9 @@ The following options may be given as the first argument:
  --read-only         Make all non-temporary tables read-only, with the
  exception for replication (slave) threads and users with
  the SUPER privilege
+ --read-only-error-msg-extra[=name] 
+ Set this variable to print out extra error information,
+ which will be appended to read_only error messages.
  --read-only-slave   Blocks disabling read_only if the server is a slave. This
  is helpful in asserting that read_only is never disabled
  on a slave. Slave with read_only=0 may generate new GTID
@@ -1796,6 +1799,7 @@ range-alloc-block-size 4096
 rbr-idempotent-tables (No default value)
 read-buffer-size 131072
 read-only FALSE
+read-only-error-msg-extra 
 read-only-slave TRUE
 read-rnd-buffer-size 262144
 relay-log (No default value)

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

@@ -870,6 +870,9 @@ The following options may be given as the first argument:
  --read-only         Make all non-temporary tables read-only, with the
  exception for replication (slave) threads and users with
  the SUPER privilege
+ --read-only-error-msg-extra[=name] 
+ Set this variable to print out extra error information,
+ which will be appended to read_only error messages.
  --read-only-slave   Blocks disabling read_only if the server is a slave. This
  is helpful in asserting that read_only is never disabled
  on a slave. Slave with read_only=0 may generate new GTID
@@ -1793,6 +1796,7 @@ range-alloc-block-size 4096
 rbr-idempotent-tables (No default value)
 read-buffer-size 131072
 read-only FALSE
+read-only-error-msg-extra 
 read-only-slave TRUE
 read-rnd-buffer-size 262144
 relay-log (No default value)

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

@@ -121,6 +121,12 @@ insert into t1 values(1006);
 ERROR HY000: The MySQL server is running with the --read-only option so it cannot execute this statement. Current master_host: 127.0.0.1, master_port: MASTER_PORT
 insert into t2 values(2006);
 ERROR HY000: The MySQL server is running with the --read-only option so it cannot execute this statement. Current master_host: 127.0.0.1, master_port: MASTER_PORT
+set global read_only_error_msg_extra = "This is a custom message";
+insert into t1 values(1006);
+ERROR HY000: The MySQL server is running with the --read-only option so it cannot execute this statement. Current master_host: 127.0.0.1, master_port: MASTER_PORT. This is a custom message
+insert into t2 values(2006);
+ERROR HY000: The MySQL server is running with the --read-only option so it cannot execute this statement. Current master_host: 127.0.0.1, master_port: MASTER_PORT. This is a custom message
+set global read_only_error_msg_extra = default;
 drop user test;
 drop table t1;
 drop table t2;

mysql-test/suite/rpl/t/rpl_read_only.test

@@ -15,6 +15,7 @@ create user test;
 
 connect (master2,127.0.0.1,test,,test,$MASTER_MYPORT,);
 connect (slave2,127.0.0.1,test,,test,$SLAVE_MYPORT,);
+connect (slave2_root,127.0.0.1,root,,test,$SLAVE_MYPORT,);
 
 connection master1;
 
@@ -119,6 +120,18 @@ insert into t1 values(1006);
 --replace_result $MASTER_MYPORT MASTER_PORT
 --error ER_OPTION_PREVENTS_STATEMENT_EXTRA_INFO
 insert into t2 values(2006);
+# set extra info for the error message
+connection slave2_root;
+set global read_only_error_msg_extra = "This is a custom message";
+connection slave2;
+--replace_result $MASTER_MYPORT MASTER_PORT
+--error ER_OPTION_PREVENTS_STATEMENT_EXTRA_INFO
+insert into t1 values(1006);
+--replace_result $MASTER_MYPORT MASTER_PORT
+--error ER_OPTION_PREVENTS_STATEMENT_EXTRA_INFO
+insert into t2 values(2006);
+connection slave2_root;
+set global read_only_error_msg_extra = default;
 
 # Verify read_only cannot be disabled on slave
 connection slave;

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

@@ -0,0 +1,20 @@
+set @@global.read_only_error_msg_extra='Not in read_only';
+ERROR HY000: The system variable read_only_error_msg_extra can only be set in read-only (super) status.
+set global read_only = true;
+set @@global.read_only_error_msg_extra = default;
+select @@global.read_only_error_msg_extra;
+@@global.read_only_error_msg_extra
+
+set @saved_read_only_error_msg_extra = @@global.read_only_error_msg_extra;
+set @@global.read_only_error_msg_extra='This is a custom message';
+set global super_read_only = true;
+set @@global.read_only_error_msg_extra='This is another custom message';
+set @@global.read_only_error_msg_extra=1;
+ERROR 42000: Incorrect argument type to variable 'read_only_error_msg_extra'
+select @@session.read_only_error_msg_extra;
+ERROR HY000: Variable 'read_only_error_msg_extra' is a GLOBAL variable
+set @@session.read_only_error_msg_extra='This is a custom message';
+ERROR HY000: Variable 'read_only_error_msg_extra' is a GLOBAL variable and should be set with SET GLOBAL
+set global read_only_error_msg_extra = @saved_read_only_error_msg_extra;
+set global super_read_only = false;
+set global read_only = false;

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

@@ -0,0 +1,28 @@
+--source include/not_embedded.inc
+
+# Cannot set the value if server is not in read_only
+--error ER_VARIABLE_NOT_SETTABLE_WITHOUT_READ_ONLY
+set @@global.read_only_error_msg_extra='Not in read_only';
+
+set global read_only = true;
+set @@global.read_only_error_msg_extra = default;
+select @@global.read_only_error_msg_extra;
+set @saved_read_only_error_msg_extra = @@global.read_only_error_msg_extra;
+
+set @@global.read_only_error_msg_extra='This is a custom message';
+
+set global super_read_only = true;
+set @@global.read_only_error_msg_extra='This is another custom message';
+
+--error ER_WRONG_TYPE_FOR_VAR
+set @@global.read_only_error_msg_extra=1;
+
+--error ER_INCORRECT_GLOBAL_LOCAL_VAR
+select @@session.read_only_error_msg_extra;
+--error ER_GLOBAL_VARIABLE
+set @@session.read_only_error_msg_extra='This is a custom message';
+
+set global read_only_error_msg_extra = @saved_read_only_error_msg_extra;
+
+set global super_read_only = false;
+set global read_only = false;

sql/mysqld.cc

@@ -504,6 +504,7 @@ my_bool block_create_myisam = FALSE;
 my_bool block_create_memory = FALSE;
 my_bool block_create_no_primary_key = FALSE;
 my_bool read_only= 0, opt_readonly= 0;
+char* opt_read_only_error_msg_extra;
 my_bool super_read_only = 0, opt_super_readonly = 0;
 my_bool use_temp_pool, relay_log_purge;
 my_bool relay_log_recovery;

sql/mysqld.h

@@ -296,6 +296,7 @@ extern ulong slave_run_triggers_for_rbr;
 extern ulonglong slave_type_conversions_options;
 extern ulonglong admission_control_filter;
 extern my_bool read_only, opt_readonly, super_read_only, opt_super_readonly;
+extern char* opt_read_only_error_msg_extra;
 extern my_bool send_error_before_closing_timed_out_connection;
 extern my_bool allow_document_type;
 extern my_bool block_create_myisam;

sql/share/errmsg-utf8.txt

@@ -7229,6 +7229,9 @@ ER_CONNECTION_TIMEOUT
 
 ER_OPTION_PREVENTS_STATEMENT_EXTRA_INFO
   eng "The MySQL server is running with the %s option so it cannot execute this statement. %s"
+
+ER_VARIABLE_NOT_SETTABLE_WITHOUT_READ_ONLY
+  eng "The system variable %.200s can only be set in read-only (super) status."
 #
 #  End of 5.6 error messages.
 #

sql/sql_parse.cc

@@ -9920,6 +9920,12 @@ int get_active_master_info(std::string *str_ptr)
       *str_ptr += active_mi->host;
       *str_ptr += ", master_port: ";
       *str_ptr += std::to_string(active_mi->port);
+      const char *extra_str = opt_read_only_error_msg_extra;
+      if (extra_str && extra_str[0])
+      {
+        *str_ptr += ". ";
+        *str_ptr += extra_str;
+      }
       return ER_OPTION_PREVENTS_STATEMENT_EXTRA_INFO;
     }
 #endif

sql/sys_vars.cc

@@ -5505,3 +5505,23 @@ static Sys_var_uint Sys_select_into_file_fsync_timeout(
        "SELECT INTO OUTFILE",
        SESSION_VAR(select_into_file_fsync_timeout), CMD_LINE(OPT_ARG),
        VALID_RANGE(0, UINT_MAX), DEFAULT(0), BLOCK_SIZE(1));
+
+static bool check_read_only_error_msg_extra(
+    sys_var *self, THD *thd, set_var *var)
+{
+  if (!opt_readonly && !opt_super_readonly)
+  {
+    my_error(ER_VARIABLE_NOT_SETTABLE_WITHOUT_READ_ONLY,
+             MYF(0),
+             var->var->name.str);
+    return true;
+  }
+  return false;
+}
+static Sys_var_charptr Sys_read_only_error_msg_extra(
+       "read_only_error_msg_extra",
+       "Set this variable to print out extra error information, "
+       "which will be appended to read_only error messages.",
+       GLOBAL_VAR(opt_read_only_error_msg_extra), CMD_LINE(OPT_ARG),
+       IN_SYSTEM_CHARSET, DEFAULT(""), NO_MUTEX_GUARD, NOT_IN_BINLOG,
+       ON_CHECK(check_read_only_error_msg_extra));