Lazily process new tasks (prefer current XFR tasks).

Throttled TCP master doesn't block current connections, so
it is not an issue.

fixes #1542

src/knot/server/tcp-handler.c

@@ -53,7 +53,7 @@ typedef struct tcp_worker_t {
 /*
  * Forward decls.
  */
-#define TCP_THROTTLE_LO 10 /*!< Minimum recovery time on errors. */
+#define TCP_THROTTLE_LO 5 /*!< Minimum recovery time on errors. */
 #define TCP_THROTTLE_HI 50 /*!< Maximum recovery time on errors. */
 
 /*! \brief Calculate TCP throttle time (random). */
@@ -290,7 +290,6 @@ static int tcp_accept(int fd)
 	/* Evaluate connection. */
 	if (incoming < 0) {
 		int en = errno;
-		/*! \todo Better solution so it doesn't block current connections (issue #1542). */
 		if (en != EINTR) {
 			log_server_error("Cannot accept connection "
 					 "(%d).\n", errno);

src/knot/server/xfr-handler.c

@@ -1500,16 +1500,12 @@ int xfr_worker(dthread_t *thread)
 		int rfd = evqueue_pollfd(w->q);
 		fdset_it_t it;
 		fdset_begin(w->fdset, &it);
+		int rfd_event = 0;
 		while(nfds > 0) {
 			
 			/* Check if it request. */
 			if (it.fd == rfd) {
-				dbg_xfr_verb("xfr: worker=%p processing request\n",
-				             w);
-				ret = xfr_process_request(w, buf, buflen);
-				if (ret == KNOTD_ENOTRUNNING) {
-					break;
-				}
+				rfd_event = 1; /* Delay new tasks after processing. */
 			} else {
 				/* Find data. */
 				data = xfr_handler_task(w, it.fd);
@@ -1536,6 +1532,12 @@ int xfr_worker(dthread_t *thread)
 			}
 		}
 		
+		/* Lazily process new tasks. */
+		if (rfd_event) {
+			dbg_xfr_verb("xfr: worker=%p processing request\n",  w);
+			ret = xfr_process_request(w, buf, buflen);
+		}
+		
 		/* Sweep inactive. */
 		timev_t now;
 		if (time_now(&now) == 0) {