MDEV-14784: Slave crashes in show_status_array upon running a trigger with

select from I_S

Problem:
========
When applier thread tries to access 'variable_name' of
INFORMATION_SCHEMA.SESSION_VARIABLES table through triggers, it results in an
abnormal exit of slave server.

Analysis:
========
At the time of replication of stored routines and triggers, their associated
security context will be sent by the master. The applier thread on the slave
server will use this information to set the required security context for the
execution of stored routines and triggers. This is achieved as follows.

->The stored routine object has a member named 'm_security_ctx' which holds the
  security context received from master.
->The applier thread's security_ctx is stored into a 'backup' object.

->Set the applier thread's security_ctx to 'm_security_ctx'.

->Upon the completion of stored routine execution restore the original security
  context of applier thread from the backup.

During the above process the 'm_security_ctx' object is not initialized
properly. Hence the 'external_user' of 'm_security_ctx' has invalid value for
this variable and accessing this variable results in abnormal exit of server.

Fix:
===
Invoke the Security_context::init() call from the constructor of stored routine
so that 'm_security_ctx' gets initialized properly.

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

+include/master-slave.inc
+[connection master]
+CREATE USER test_user@localhost;
+SET PASSWORD FOR test_user@localhost = password('PWD');
+GRANT ALL ON *.* TO test_user@localhost WITH GRANT OPTION;
+connect conn_test,localhost,test_user,PWD,test,$MASTER_MYPORT,$MASTER_MYSOCK;
+connection conn_test;
+CREATE TABLE t1 (f1 INT);
+CREATE TABLE t2 (f2 VARCHAR(64));
+CREATE TRIGGER tr_before BEFORE INSERT ON t1 FOR EACH ROW INSERT INTO t2 SELECT variable_name FROM INFORMATION_SCHEMA.SESSION_VARIABLES;
+CREATE DEFINER='root'@'localhost' TRIGGER tr_after AFTER INSERT ON t1 FOR EACH ROW INSERT INTO t2 SELECT variable_name FROM INFORMATION_SCHEMA.SESSION_VARIABLES;
+INSERT INTO t1 VALUES (1);
+DROP USER 'test_user'@'localhost';
+DROP TABLE t1, t2;
+include/rpl_end.inc

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

+# ==== Purpose ====
+#
+# Test verifies that when applier thread tries to access 'variable_name' of
+# INFORMATION_SCHEMA.SESSION_VARIABLES table through triggers it successfully
+# retrieves all the session variables.
+#
+# ==== Implementation ====
+#
+# Steps:
+#    0 - Create two tables t1 and t2.
+#    1 - Create a trigger such that it reads the names of all session variables
+#        from INFORMATION_SCHEMA.SESSION_VARIABLES table and populates one of the
+#        tables.
+#    2 - Do a DML on master and wait for it to be replicated and ensure that
+#        slave is in sync with master and it is up and running.
+#
+# ==== References ====
+#
+# MDEV-14784: Slave crashes in show_status_array upon running a trigger with
+# select from I_S
+
+--source include/master-slave.inc
+--source include/have_binlog_format_mixed.inc
+--enable_connect_log
+CREATE USER test_user@localhost;
+SET PASSWORD FOR test_user@localhost = password('PWD');
+GRANT ALL ON *.* TO test_user@localhost WITH GRANT OPTION;
+connect(conn_test,localhost,test_user,PWD,test,$MASTER_MYPORT,$MASTER_MYSOCK);
+
+--connection conn_test
+CREATE TABLE t1 (f1 INT);
+CREATE TABLE t2 (f2 VARCHAR(64));
+CREATE TRIGGER tr_before BEFORE INSERT ON t1 FOR EACH ROW INSERT INTO t2 SELECT variable_name FROM INFORMATION_SCHEMA.SESSION_VARIABLES;
+CREATE DEFINER='root'@'localhost' TRIGGER tr_after AFTER INSERT ON t1 FOR EACH ROW INSERT INTO t2 SELECT variable_name FROM INFORMATION_SCHEMA.SESSION_VARIABLES;
+
+INSERT INTO t1 VALUES (1);
+--disable_connect_log
+# Cleanup
+--connection master
+DROP USER 'test_user'@'localhost';
+DROP TABLE t1, t2;
+--source include/rpl_end.inc

sql/sp_head.cc

@@ -563,6 +563,7 @@ sp_head::sp_head()
 
   DBUG_ENTER("sp_head::sp_head");
 
+  m_security_ctx.init();
   m_backpatch.empty();
   m_cont_backpatch.empty();
   m_lex.empty();