MDEV-27978 fix wrong name in error when max_session_mem_used exceeded

Fixed typo in my_malloc_size_cb_func. There is no max-thread-mem-used
sys variable in MariaDB, only max-session-mem-used. The relevant entry
in sys_vars.cc is also fixed.

Added a fallback case in case we could allocate the 256 bytes for the
error message containing the exact setting.

mysql-test/r/error_simulation.result

@@ -128,3 +128,13 @@ SELECT f1(1);
 Got one of the listed errors
 DROP FUNCTION f1;
 SET debug_dbug= @saved_dbug;
+#
+# MDEV-27978 wrong option name in error when exceeding max_session_mem_used
+#
+SET SESSION max_session_mem_used = 8192;
+SELECT * FROM information_schema.processlist;
+ERROR HY000: The MariaDB server is running with the --max-session-mem-used=8192 option so it cannot execute this statement
+SET SESSION max_session_mem_used = DEFAULT;
+#
+# End of 10.2 tests
+#

mysql-test/r/truncate_notembedded.result

@@ -13,7 +13,7 @@ a
 UNLOCK TABLES;
 connection con1;
 TRUNCATE TABLE t1;
-ERROR HY000: The MariaDB server is running with the --max-thread-mem-used=45500 option so it cannot execute this statement
+ERROR HY000: The MariaDB server is running with the --max-session-mem-used=45500 option so it cannot execute this statement
 disconnect con1;
 connection default;
 DROP TABLE t1;

mysql-test/t/error_simulation.test

@@ -158,3 +158,16 @@ SET SESSION debug_dbug="+d,simulate_create_virtual_tmp_table_out_of_memory";
 SELECT f1(1);
 DROP FUNCTION f1;
 SET debug_dbug= @saved_dbug;
+
+--echo #
+--echo # MDEV-27978 wrong option name in error when exceeding max_session_mem_used
+--echo #
+SET SESSION max_session_mem_used = 8192;
+--error ER_OPTION_PREVENTS_STATEMENT
+SELECT * FROM information_schema.processlist;
+SET SESSION max_session_mem_used = DEFAULT;
+
+
+--echo #
+--echo # End of 10.2 tests
+--echo #

sql/mysqld.cc

@@ -4090,13 +4090,18 @@ static void my_malloc_size_cb_func(long long size, my_bool is_thread_specific)
       /* Ensure we don't get called here again */
       char buf[50], *buf2;
       thd->set_killed(KILL_QUERY);
-      my_snprintf(buf, sizeof(buf), "--max-thread-mem-used=%llu",
+      my_snprintf(buf, sizeof(buf), "--max-session-mem-used=%llu",
                   thd->variables.max_mem_used);
       if ((buf2= (char*) thd->alloc(256)))
       {
         my_snprintf(buf2, 256, ER_THD(thd, ER_OPTION_PREVENTS_STATEMENT), buf);
         thd->set_killed(KILL_QUERY, ER_OPTION_PREVENTS_STATEMENT, buf2);
       }
+      else
+      {
+        thd->set_killed(KILL_QUERY, ER_OPTION_PREVENTS_STATEMENT,
+                        "--max-session-mem-used");
+      }
     }
     DBUG_ASSERT((longlong) thd->status_var.local_memory_used >= 0 ||
                 !debug_assert_on_not_freed_memory);

sql/sys_vars.cc

@@ -5719,7 +5719,7 @@ static Sys_var_ulong Sys_log_tc_size(
        BLOCK_SIZE(my_getpagesize()));
 #endif
 
-static Sys_var_ulonglong Sys_max_thread_mem(
+static Sys_var_ulonglong Sys_max_session_mem_used(
        "max_session_mem_used", "Amount of memory a single user session "
        "is allowed to allocate. This limits the value of the "
        "session variable MEM_USED", SESSION_VAR(max_mem_used),