Overview of the Issue

While you can send vtgate a SIGHUP to reload it's mysql_auth_server_static_file JSON credentials file, I noticed during some testing that a SIGHUP will actually kill vtgate entirely if mysql_auth_server_impl is set to none or if both mysql_auth_server_static_file and mysql_auth_server_static_string are empty.

Reproduction Steps

1. Launch the local example
2. Confirm vtgate is running w/ mysql_auth_server_impl none:

ps aux | grep [v]tgate
gary     17067  0.1  0.3 1940404 43372 pts/2   Rl   11:21   0:00 vtgate -topo_implementation etcd2 -topo_global_server_address localhost:2379 -topo_global_root /vitess/global -log_dir /home/gary/vtdataroot/tmp -log_queries_to_file /home/gary/vtdataroot/tmp/vtgate_querylog.txt -port 15001 -grpc_port 15991 -mysql_server_port 15306 -mysql_server_socket_path /tmp/mysql.sock -cell zone1 -cells_to_watch zone1 -tablet_types_to_wait MASTER,REPLICA -gateway_implementation discoverygateway -service_map grpc-vtgateservice -pid_file /home/gary/vtdataroot/tmp/vtgate.pid -mysql_auth_server_impl none

3. Send SIGHUP
   kill -1 $(pgrep vtgate)
4. Note that vtgate has been killed and is no longer running.

Binary version

Version: 945e105aa (Git branch 'master') built on Tue Jun 9 11:20:38 PDT 2020 by gary@xj13 using go1.14 linux/amd64

Log Fragments

I didn't notice anything relevant/useful in Vitess' logs.
strace of vtgate being killed by via SIGHUP:

--- SIGHUP {si_signo=SIGHUP, si_code=SI_USER, si_pid=13723, si_uid=1000} ---
rt_sigprocmask(SIG_UNBLOCK, [HUP], NULL, 8) = 0
getpid()                                = 17067
gettid()                                = 17067
tgkill(17067, 17067, SIGHUP)            = 0
--- SIGHUP {si_signo=SIGHUP, si_code=SI_TKILL, si_pid=17067, si_uid=1000} ---
rt_sigaction(SIGHUP, {sa_handler=SIG_DFL, sa_mask=~[], sa_flags=SA_RESTORER|SA_ONSTACK|SA_RESTART|SA_SIGINFO, sa_restorer=0x7f4222daa890}, NULL, 8) = 0
rt_sigprocmask(SIG_UNBLOCK, [HUP], NULL, 8) = 0
getpid()                                = 17067
gettid()                                = 17067
tgkill(17067, 17067, SIGHUP)            = 0
--- SIGHUP {si_signo=SIGHUP, si_code=SI_TKILL, si_pid=17067, si_uid=1000} ---
+++ killed by SIGHUP +++