Fix reload 100% CPU spinning after failed reload

After a failed reload, some UDB pointers were cleared that were still linked
in the UDB. This fix unlinks them in all the correct places.

This may be the fix to issue #417

server.c

@@ -2808,6 +2808,13 @@ server_main(struct nsd *nsd)
 					netio_remove_handler(netio, &reload_listener);
 					reload_listener.fd = -1;
 					reload_listener.event_types = NETIO_EVENT_NONE;
+					/* "load" failed a apply_xfr task.
+					 * "old-main" will become "main" and
+					 * MUST unlink last_task and xfrs2process.
+					 * see: UNLINKING last_task and xfrs2process
+					 */
+					udb_ptr_unlink(&last_task, nsd->task[nsd->mytask]);
+					udb_ptr_unlink(&xfrs2process, nsd->task[nsd->mytask]);
 					task_process_sync(nsd->task[nsd->mytask]);
 					/* inform xfrd reload attempt ended */
 					if(!write_socket(nsd->xfrd_listener->fd,
@@ -2912,6 +2919,13 @@ server_main(struct nsd *nsd)
 				netio_remove_handler(netio, &reload_listener);
 				reload_listener.fd = -1;
 				reload_listener.event_types = NETIO_EVENT_NONE;
+				/* "load" failed a apply_xfr task.
+				 * "old-main" will become "main" and
+				 * MUST unlink last_task and xfrs2process.
+				 * see: UNLINKING last_task and xfrs2process
+				 */
+				udb_ptr_unlink(&last_task, nsd->task[nsd->mytask]);
+				udb_ptr_unlink(&xfrs2process, nsd->task[nsd->mytask]);
 				task_process_sync(nsd->task[nsd->mytask]);
 				/* inform xfrd reload attempt ended */
 				if(!write_socket(nsd->xfrd_listener->fd,
@@ -2971,13 +2985,30 @@ server_main(struct nsd *nsd)
 			task_remap(nsd->task[nsd->mytask]);
 			udb_ptr_init(&xfrs2process, nsd->task[nsd->mytask]);
 			udb_ptr_init(&last_task   , nsd->task[nsd->mytask]);
+			/* UNLINKING last_task and xfrs2process
+			 * ------------------------------------
+			 * last_task and xfrs2processwill be unlinked when
+			 * the "load"process processed all tasks.
+			 *
+			 * If the "load" process fails, because of failed fork()
+			 * or due to failed task_process_apply_xfr(), then the
+			 * process that will take over the "main" role ("load"
+			 * on failed fork() and "old-main" on failed
+			 * process_apply_xfr() MUST unlink these udb_ptr's.
+			 */
 			reload_process_non_xfr_tasks(nsd, &xfrs2process
 			                                , &last_task);
 			/* Do actual reload */
 			reload_pid = fork();
 			switch (reload_pid) {
 			case -1:
 				log_msg(LOG_ERR, "fork failed: %s", strerror(errno));
+				/* fork() failed. "load" will become "main" and
+				 * MUST unlink last_task and xfrs2process.
+				 * see: UNLINKING last_task and xfrs2process
+				 */
+				udb_ptr_unlink(&last_task, nsd->task[nsd->mytask]);
+				udb_ptr_unlink(&xfrs2process, nsd->task[nsd->mytask]);
 				break;
 			default:
 				/* PARENT */
@@ -3029,13 +3060,6 @@ server_main(struct nsd *nsd)
 				log_set_process_role("main");
 #endif
 			}
-			/* xfrs2process and last_task need to be reset in case
-			 * "old-main" becomes "main" (due to an failed (exited)
-			 * xfr update). If needed xfrs2process gets unlinked by
-			 * "load", and last_task by the xfrd.
-			 */
-			memset(&xfrs2process, 0, sizeof(xfrs2process));
-			memset(&last_task, 0, sizeof(last_task));
 			break;
 		case NSD_QUIT_SYNC:
 			/* synchronisation of xfrd, parent and reload */