CREATE USER u1;
 CREATE ROLE r1;
 CREATE ROLE r2;
 GRANT r1 to u1;
 GRANT r2 to r1;
 delimiter |;
 create procedure mysql.test_proc (OUT param1 INT)
 begin
   select COUNT(*) into param1 from mysql.roles_mapping;
 end|
 delimiter ;|
 GRANT EXECUTE ON PROCEDURE mysql.test_proc TO r1;
 GRANT EXECUTE ON PROCEDURE mysql.test_proc TO u1;
 CREATE FUNCTION mysql.test_func (param INT) RETURNS INT
   RETURN (SELECT COUNT(*) FROM mysql.user);
 GRANT EXECUTE ON FUNCTION mysql.test_func TO r1;
 GRANT EXECUTE ON FUNCTION mysql.test_func TO u1;
 DROP PROCEDURE mysql.test_proc;
 DROP FUNCTION mysql.test_func;
 DROP ROLE r1;
 DROP USER u1;

Server version: 10.1.3-MariaDB-wsrep-debug-log
key_buffer_size=1048576
read_buffer_size=131072
max_used_connections=1
max_threads=153
thread_count=1
It is possible that mysqld could use up to 
key_buffer_size + (read_buffer_size + sort_buffer_size)*max_threads = 63099 K  bytes of memory
Hope that's ok; if not, decrease some variables in the equation.

Thread pointer: 0x0x7f80cf212070
Attempting backtrace. You can use the following information to find out
where mysqld died. If you see no messages after this, something went
terribly wrong...
stack_bottom = 0x7f80d7dfcde8 thread_stack 0x48000
mysys/stacktrace.c:246(my_print_stacktrace)[0xfcf223]
sql/signal_handler.cc:155(handle_fatal_signal)[0x88e3cb]
/lib/x86_64-linux-gnu/libpthread.so.0(+0xfc90)[0x7f80d6fb4c90]
/lib/x86_64-linux-gnu/libc.so.6(gsignal+0x37)[0x7f80d63ebe37]
/lib/x86_64-linux-gnu/libc.so.6(abort+0x148)[0x7f80d63ed528]
/lib/x86_64-linux-gnu/libc.so.6(+0x2fce6)[0x7f80d63e4ce6]
/lib/x86_64-linux-gnu/libc.so.6(+0x2fd92)[0x7f80d63e4d92]
sql/sql_acl.cc:2611(add_role_user_mapping_action)[0x5cd9b6]
mysys/hash.c:761(my_hash_iterate)[0xfa6c1e]
sql/sql_acl.cc:2637(rebuild_role_grants)[0x5cda68]
sql/sql_acl.cc:9582(mysql_drop_user(THD*, List<st_lex_user>&, bool))[0x5e4047]
sql/sql_parse.cc:4543(mysql_execute_command(THD*))[0x664443]
sql/sql_parse.cc:7152(mysql_parse(THD*, char*, unsigned int, Parser_state*))[0x66c1d0]
sql/sql_parse.cc:1463(dispatch_command(enum_server_command, THD*, char*, unsigned int))[0x65b24f]
sql/sql_parse.cc:1090(do_command(THD*))[0x65a00c]
sql/sql_connect.cc:1347(do_handle_one_connection(THD*))[0x791e27]
sql/sql_connect.cc:1259(handle_one_connection)[0x791b70]
perfschema/pfs.cc:1862(pfs_spawn_thread)[0xc1aaff]
/lib/x86_64-linux-gnu/libpthread.so.0(+0x80a5)[0x7f80d6fad0a5]
/lib/x86_64-linux-gnu/libc.so.6(clone+0x6d)[0x7f80d64aecfd]

Trying to get some variables.
Some pointers may be invalid and cause the dump to abort.
Query (0x7f80ccce4088): DROP USER u1
Connection ID (thread ID): 3
Status: NOT_KILLED