daemon: drop RECVMMSG_BATCH

The support hasn't landed in libuv over all the years,
and we've been still reserving memory for it in advance.
Also comment on the singleton buffer usage.

daemon/engine.h

@@ -34,7 +34,7 @@
 # endif
 #endif
 #ifndef RECVMMSG_BATCH
-#define RECVMMSG_BATCH 4
+#define RECVMMSG_BATCH 1
 #endif
 #ifndef QUERY_RATE_THRESHOLD
 #define QUERY_RATE_THRESHOLD (2 * MP_FREELIST_SIZE) /**< Nr of parallel queries considered as high rate */

daemon/io.c

@@ -36,11 +36,12 @@
 
 static void check_bufsize(uv_handle_t* handle)
 {
+	return; /* TODO: resurrect after https://github.com/libuv/libuv/issues/419 */
 	/* We want to buffer at least N waves in advance.
 	 * This is magic presuming we can pull in a whole recvmmsg width in one wave.
 	 * Linux will double this the bufsize wanted.
 	 */
-	const int bufsize_want = RECVMMSG_BATCH * 65535 * 2;
+	const int bufsize_want = 2 * sizeof( ((struct worker_ctx *)NULL)->wire_buf ) ;
 	negotiate_bufsize(uv_recv_buffer_size, handle, bufsize_want);
 	negotiate_bufsize(uv_send_buffer_size, handle, bufsize_want);
 }

daemon/session.c

@@ -270,6 +270,14 @@ int session_set_handle(struct session *session, uv_handle_t *handle)
 		session->wire_buf = wire_buf;
 		session->wire_buf_size = KNOT_WIRE_MAX_PKTSIZE;
 	} else if (handle->type == UV_UDP) {
+		/* We use the singleton buffer from worker for all UDP (!)
+		 * libuv documentation doesn't really guarantee this is OK,
+		 * but the implementation for unix systems does not hold
+		 * the buffer (both UDP and TCP) - always makes a NON-blocking
+		 * syscall that fills the buffer and immediately calls
+		 * the callback, whatever the result of the operation.
+		 * We still need to keep in mind to only touch the buffer
+		 * in this callback... */
 		assert(handle->loop->data);
 		struct worker_ctx *worker = handle->loop->data;
 		session->wire_buf = worker->wire_buf;

daemon/worker.h

@@ -127,11 +127,8 @@ struct worker_ctx {
 	struct sockaddr_in out_addr4;
 	struct sockaddr_in6 out_addr6;
 
-#if __linux__
 	uint8_t wire_buf[RECVMMSG_BATCH * KNOT_WIRE_MAX_PKTSIZE];
-#else
-	uint8_t wire_buf[KNOT_WIRE_MAX_PKTSIZE];
-#endif
+
 	struct {
 		size_t concurrent;
 		size_t rconcurrent;