Add isc__nm_senddns()

The new internal function works in the same way as isc_nm_send()
except that it sends a DNS message size ahead of the DNS message
data (the format used in DNS over TCP).

The intention is to provide a fast path for sending DNS messages over
streams protocols - that is, without allocating any intermediate
memory buffers.

lib/isc/netmgr/netmgr-int.h

@@ -1337,6 +1337,14 @@ isc__nm_async_tcpstop(isc__networker_t *worker, isc__netievent_t *ev0);
  * stoplisten, send, read, pause, close).
  */
 
+void
+isc__nm_tcp_senddns(isc_nmhandle_t *handle, const isc_region_t *region,
+		    isc_nm_cb_t cb, void *cbarg);
+/*%<
+ * The same as 'isc__nm_tcp_send()', but with data length sent
+ * ahead of data (two bytes (16 bit) in big-endian format).
+ */
+
 void
 isc__nm_async_tlsclose(isc__networker_t *worker, isc__netievent_t *ev0);
 
@@ -1993,3 +2001,11 @@ isc__nmhandle_get_selected_alpn(isc_nmhandle_t *handle,
  * not negotiated of the underlying protocol of the connection
  * represented via the given handle does not support ALPN.
  */
+
+void
+isc__nm_senddns(isc_nmhandle_t *handle, isc_region_t *region, isc_nm_cb_t cb,
+		void *cbarg);
+/*%<
+ * The same as 'isc_nm_send()', but with data length sent
+ * ahead of data (two bytes (16 bit) in big-endian format).
+ */

lib/isc/netmgr/netmgr.c

@@ -1873,6 +1873,20 @@ isc_nm_send(isc_nmhandle_t *handle, isc_region_t *region, isc_nm_cb_t cb,
 	}
 }
 
+void
+isc__nm_senddns(isc_nmhandle_t *handle, isc_region_t *region, isc_nm_cb_t cb,
+		void *cbarg) {
+	REQUIRE(VALID_NMHANDLE(handle));
+
+	switch (handle->sock->type) {
+	case isc_nm_tcpsocket:
+		isc__nm_tcp_senddns(handle, region, cb, cbarg);
+		break;
+	default:
+		UNREACHABLE();
+	}
+}
+
 void
 isc_nm_read(isc_nmhandle_t *handle, isc_nm_recv_cb_t cb, void *cbarg) {
 	REQUIRE(VALID_NMHANDLE(handle));

lib/isc/netmgr/tcp.c

@@ -996,9 +996,9 @@ failure:
 	return (result);
 }
 
-void
-isc__nm_tcp_send(isc_nmhandle_t *handle, const isc_region_t *region,
-		 isc_nm_cb_t cb, void *cbarg) {
+static void
+tcp_send(isc_nmhandle_t *handle, const isc_region_t *region, isc_nm_cb_t cb,
+	 void *cbarg, const bool dnsmsg) {
 	REQUIRE(VALID_NMHANDLE(handle));
 	REQUIRE(VALID_NMSOCK(handle->sock));
 
@@ -1011,6 +1011,9 @@ isc__nm_tcp_send(isc_nmhandle_t *handle, const isc_region_t *region,
 	REQUIRE(sock->tid == isc_tid());
 
 	uvreq = isc__nm_uvreq_get(sock->worker, sock);
+	if (dnsmsg) {
+		*(uint16_t *)uvreq->tcplen = htons(region->length);
+	}
 	uvreq->uvbuf.base = (char *)region->base;
 	uvreq->uvbuf.len = region->length;
 
@@ -1034,6 +1037,18 @@ isc__nm_tcp_send(isc_nmhandle_t *handle, const isc_region_t *region,
 	return;
 }
 
+void
+isc__nm_tcp_send(isc_nmhandle_t *handle, const isc_region_t *region,
+		 isc_nm_cb_t cb, void *cbarg) {
+	tcp_send(handle, region, cb, cbarg, false);
+}
+
+void
+isc__nm_tcp_senddns(isc_nmhandle_t *handle, const isc_region_t *region,
+		    isc_nm_cb_t cb, void *cbarg) {
+	tcp_send(handle, region, cb, cbarg, true);
+}
+
 static void
 tcp_send_cb(uv_write_t *req, int status) {
 	isc__nm_uvreq_t *uvreq = (isc__nm_uvreq_t *)req->data;
@@ -1065,27 +1080,59 @@ tcp_send_direct(isc_nmsocket_t *sock, isc__nm_uvreq_t *req) {
 	REQUIRE(sock->type == isc_nm_tcpsocket);
 
 	int r;
+	uv_buf_t bufs[2] = { { 0 }, { 0 } }; /* ugly, but required for old GCC
+						versions */
+	size_t nbufs = 1;
 
 	if (isc__nmsocket_closing(sock)) {
 		return (ISC_R_CANCELED);
 	}
 
-	uv_buf_t uvbuf = { .base = req->uvbuf.base, .len = req->uvbuf.len };
+	/* Check if we are not trying to send a DNS message */
+	if (*(uint16_t *)req->tcplen == 0) {
+		bufs[0].base = req->uvbuf.base;
+		bufs[0].len = req->uvbuf.len;
 
-	r = uv_try_write(&sock->uv_handle.stream, &uvbuf, 1);
+		r = uv_try_write(&sock->uv_handle.stream, bufs, nbufs);
 
-	if (r == (int)(uvbuf.len)) {
+		if (r == (int)(bufs[0].len)) {
 			/* Wrote everything */
 			isc__nm_sendcb(sock, req, ISC_R_SUCCESS, true);
 			return (ISC_R_SUCCESS);
 		} else if (r > 0) {
-		uvbuf.base += (size_t)r;
-		uvbuf.len -= (size_t)r;
+			bufs[0].base += (size_t)r;
+			bufs[0].len -= (size_t)r;
 		} else if (!(r == UV_ENOSYS || r == UV_EAGAIN)) {
 			return (isc_uverr2result(r));
 		}
+	} else {
+		nbufs = 2;
+		bufs[0].base = req->tcplen;
+		bufs[0].len = 2;
+		bufs[1].base = req->uvbuf.base;
+		bufs[1].len = req->uvbuf.len;
 
-	r = uv_write(&req->uv_req.write, &sock->uv_handle.stream, &uvbuf, 1,
+		r = uv_try_write(&sock->uv_handle.stream, bufs, nbufs);
+
+		if (r == (int)(bufs[0].len + bufs[1].len)) {
+			/* Wrote everything */
+			isc__nm_sendcb(sock, req, ISC_R_SUCCESS, true);
+			return (ISC_R_SUCCESS);
+		} else if (r == 1) {
+			/* Partial write of DNSMSG length */
+			bufs[0].base = req->tcplen + 1;
+			bufs[0].len = 1;
+		} else if (r > 0) {
+			/* Partial write of DNSMSG */
+			nbufs = 1;
+			bufs[0].base = req->uvbuf.base + (r - 2);
+			bufs[0].len = req->uvbuf.len - (r - 2);
+		} else if (!(r == UV_ENOSYS || r == UV_EAGAIN)) {
+			return (isc_uverr2result(r));
+		}
+	}
+
+	r = uv_write(&req->uv_req.write, &sock->uv_handle.stream, bufs, nbufs,
 		     tcp_send_cb);
 	if (r < 0) {
 		return (isc_uverr2result(r));