Investigate possible bug in Monitor

[renecannao]

This happened while keeping proxysql stopped in gdb for long time.
Maybe caused by some timeout of FD becoming invalidated

[warn] Epoll ADD(4) on fd 49 failed.  Old events were 0; read change was 0 (none); write change was 1 (add): Bad file descriptor
[warn] Epoll ADD(4) on fd 50 failed.  Old events were 0; read change was 0 (none); write change was 1 (add): Invalid argument
[warn] Epoll ADD(4) on fd 53 failed.  Old events were 0; read change was 0 (none); write change was 1 (add): Invalid argument
proxysql: MySQL_Monitor.cpp:273: MySQL_Monitor_State_Data::~MySQL_Monitor_State_Data(): Assertion `mysql==__null' failed.

Program received signal SIGABRT, Aborted.
[Switching to Thread 0x7ffff1bff700 (LWP 31821)]
0x00007ffff65bcf79 in __GI_raise (sig=sig@entry=6) at ../nptl/sysdeps/unix/sysv/linux/raise.c:56
56      ../nptl/sysdeps/unix/sysv/linux/raise.c: No such file or directory.
(gdb) bt
#0  0x00007ffff65bcf79 in __GI_raise (sig=sig@entry=6) at ../nptl/sysdeps/unix/sysv/linux/raise.c:56
#1  0x00007ffff65c0388 in __GI_abort () at abort.c:89
#2  0x00007ffff65b5e36 in __assert_fail_base (fmt=0x7ffff6707718 "%s%s%s:%u: %s%sAssertion `%s' failed.\n%n", assertion=assertion@entry=0x635421 "mysql==__null",
    file=file@entry=0x63540f "MySQL_Monitor.cpp", line=line@entry=273,
    function=function@entry=0x636a20 <MySQL_Monitor_State_Data::~MySQL_Monitor_State_Data()::__PRETTY_FUNCTION__> "MySQL_Monitor_State_Data::~MySQL_Monitor_State_Data()") at assert.c:92
#3  0x00007ffff65b5ee2 in __GI___assert_fail (assertion=0x635421 "mysql==__null", file=0x63540f "MySQL_Monitor.cpp", line=273,
    function=0x636a20 <MySQL_Monitor_State_Data::~MySQL_Monitor_State_Data()::__PRETTY_FUNCTION__> "MySQL_Monitor_State_Data::~MySQL_Monitor_State_Data()")
    at assert.c:101
#4  0x00000000004eebef in MySQL_Monitor_State_Data::~MySQL_Monitor_State_Data (this=0x7ffff1c44b40, __in_chrg=<optimized out>) at MySQL_Monitor.cpp:273
#5  0x00000000004eb15f in MySQL_Monitor::monitor_connect (this=0x7ffff6035f80) at MySQL_Monitor.cpp:838
#6  0x0000000000485f95 in std::_Mem_fn<void* (MySQL_Monitor::*)()>::operator()<, void>(MySQL_Monitor*) const (this=0x7ffff283a038, __object=0x7ffff6035f80)
    at /usr/include/c++/4.8/functional:601
#7  0x0000000000485ee5 in std::_Bind_simple<std::_Mem_fn<void* (MySQL_Monitor::*)()> (MySQL_Monitor*)>::_M_invoke<0ul>(std::_Index_tuple<0ul>) (this=0x7ffff283a030)
    at /usr/include/c++/4.8/functional:1732
#8  0x0000000000485ded in std::_Bind_simple<std::_Mem_fn<void* (MySQL_Monitor::*)()> (MySQL_Monitor*)>::operator()() (this=0x7ffff283a030)
    at /usr/include/c++/4.8/functional:1720
#9  0x0000000000485d86 in std::thread::_Impl<std::_Bind_simple<std::_Mem_fn<void* (MySQL_Monitor::*)()> (MySQL_Monitor*)> >::_M_run() (this=0x7ffff283a018)
    at /usr/include/c++/4.8/thread:115
#10 0x00007ffff6ef6390 in ?? () from /usr/lib/x86_64-linux-gnu/libstdc++.so.6
#11 0x00007ffff7bc4182 in start_thread (arg=0x7ffff1bff700) at pthread_create.c:312
#12 0x00007ffff668130d in clone () at ../sysdeps/unix/sysv/linux/x86_64/clone.S:111

[renecannao]

For reference:

(gdb) p *this
$47 = {task_id = MON_CONNECT, tv_out = {tv_sec = 1463573002, tv_usec = 890912}, t1 = 1463573002890912, t2 = 0, ST = 1, hostname = 0x7ffff1db5190 "127.0.0.1",
  port = 21891, ev_mysql = 0x7ffff1c44960, mysql = 0x7ffff1ce5200, base = 0x7ffff1c12000, result = 0x0, ret = 0x0, interr = 0, mysql_error_msg = 0x0, row = 0x0,
  repl_lag = 4056322443, hostgroup_id = 32767}
(gdb) p *ev_mysql
$48 = {ev_active_next = {tqe_next = 0xffffffff, tqe_prev = 0x0}, ev_next = {tqe_next = 0x200000000, tqe_prev = 0x0}, ev_timeout_pos = {ev_next_with_common_timeout = {
      tqe_next = 0xffffffff, tqe_prev = 0x0}, min_heap_idx = -1}, ev_fd = 53, ev_base = 0x7ffff1c12000, _ev = {ev_io = {ev_io_next = {tqe_next = 0x0,
        tqe_prev = 0x2f50435400000000}, ev_timeout = {tv_sec = 11600075979489280, tv_usec = 0}}, ev_signal = {ev_signal_next = {tqe_next = 0x0,
        tqe_prev = 0x2f50435400000000}, ev_ncalls = 0, ev_pncalls = 0x0}}, ev_events = 4, ev_res = 0, ev_flags = 128, ev_pri = 0 '\000', ev_closure = 0 '\000',
  ev_timeout = {tv_sec = 0, tv_usec = 0}, ev_callback = 0x4ea18e <state_machine_handler(int, short, void*)>, ev_arg = 0x7ffff1c44b40}
(gdb) p *mysql
$49 = {net = {vio = 0x7ffff1c44320, buff = 0x0, buff_end = 0x0, write_pos = 0x0, read_pos = 0x0, fd = 0, remain_in_buf = 0, length = 0, buf_length = 0, where_b = 0,
    max_packet = 0, max_packet_size = 0, pkt_nr = 0, compress_pkt_nr = 0, write_timeout = 0, read_timeout = 0, retry_count = 0, fcntl = 0, return_status = 0x0,
    reading_or_writing = 0 '\000', save_char = 0 '\000', unused_1 = 0 '\000', unused_2 = 0 '\000', compress = 0 '\000', unused_3 = 0 '\000', unused_4 = 0x0,
    last_errno = 0, error = 0 '\000', unused_5 = 0 '\000', unused_6 = 0 '\000', last_error = '\000' <repeats 511 times>, sqlstate = "00000", extension = 0x0},
  unused_0 = 0x0, host = 0x0, user = 0x0, passwd = 0x0, unix_socket = 0x0, server_version = 0x0, host_info = 0x0, info = 0x0, db = 0x0,
  charset = 0x8afa08 <compiled_charsets+360>, fields = 0x0, field_alloc = {free = 0x0, used = 0x0, pre_alloc = 0x0, min_malloc = 0, block_size = 0, block_num = 0,
    first_block_usage = 0, error_handler = 0x0}, affected_rows = 0, insert_id = 0, extra_info = 0, thread_id = 0, packet_length = 0, port = 0, client_flag = 0,
  server_capabilities = 0, protocol_version = 0, field_count = 0, server_status = 2, server_language = 0, warning_count = 0, options = {connect_timeout = 0,
    read_timeout = 0, write_timeout = 0, port = 0, protocol = 0, client_flag = 128, host = 0x0, user = 0x0, password = 0x0, unix_socket = 0x0, db = 0x0,
    init_command = 0x0, my_cnf_file = 0x0, my_cnf_group = 0x0, charset_dir = 0x0, charset_name = 0x0, ssl_key = 0x0, ssl_cert = 0x0, ssl_ca = 0x0, ssl_capath = 0x0,
    ssl_cipher = 0x0, shared_memory_base_name = 0x0, max_allowed_packet = 0, use_ssl = 0 '\000', compress = 0 '\000', named_pipe = 0 '\000', unused_1 = 0 '\000',
    unused_2 = 0 '\000', unused_3 = 0 '\000', unused_4 = 0 '\000', methods_to_use = MYSQL_OPT_CONNECT_TIMEOUT, client_ip = 0x0, secure_auth = 0 '\000',
    report_data_truncation = 0 '\000', local_infile_init = 0x0, local_infile_read = 0x0, local_infile_end = 0x0, local_infile_error = 0x0, local_infile_userdata = 0x0,
    extension = 0x7ffff1c44780}, status = MYSQL_STATUS_READY, free_me = 1 '\001', reconnect = 0 '\000', scramble_buff = '\000' <repeats 20 times>, unused_1 = 0 '\000',
  unused_2 = 0x0, unused_3 = 0x0, unused_4 = 0x0, unused_5 = 0x0, stmts = 0x0, methods = 0x8b6ae0 <MARIADB_DEFAULT_METHODS>, thd = 0x0, unbuffered_fetch_owner = 0x0,
  info_buffer = 0x0, extension = 0x0}

[renecannao]

Monitor was completely rewritten