fix race between app and GC on ruby server shutdown

src/ruby/ext/grpc/rb_server.c

@@ -37,6 +37,7 @@
 #include "rb_server.h"
 
 #include <grpc/grpc.h>
+#include <grpc/support/atm.h>
 #include <grpc/grpc_security.h>
 #include <grpc/support/log.h>
 #include "rb_call.h"
@@ -59,11 +60,13 @@ typedef struct grpc_rb_server {
   /* The actual server */
   grpc_server *wrapped;
   grpc_completion_queue *queue;
+  gpr_atm shutdown_started;
 } grpc_rb_server;
 
 static void destroy_server(grpc_rb_server *server, gpr_timespec deadline) {
   grpc_event ev;
-  if (server->wrapped != NULL) {
+  // This can be started by app or implicitly by GC. Avoid a race between these.
+  if (gpr_atm_full_fetch_add(&server->shutdown_started, (gpr_atm)1) == 0) {
     grpc_server_shutdown_and_notify(server->wrapped, server->queue, NULL);
     ev = rb_completion_queue_pluck(server->queue, NULL, deadline, NULL);
     if (ev.type == GRPC_QUEUE_TIMEOUT) {
@@ -115,6 +118,7 @@ static const rb_data_type_t grpc_rb_server_data_type = {
 static VALUE grpc_rb_server_alloc(VALUE cls) {
   grpc_rb_server *wrapper = ALLOC(grpc_rb_server);
   wrapper->wrapped = NULL;
+  wrapper->shutdown_started = (gpr_atm)0;
   return TypedData_Wrap_Struct(cls, &grpc_rb_server_data_type, wrapper);
 }