MEDIUM: mux-h1: attempt to zero-copy Rx DATA transfers

When transferring large objects, most calls are made between a full
buffer and an empty buffer. In this case there is a large opportunity
for performing zero-copy calls, with a few exceptions : the input data
must fit into the output buffer, and the data need to be properly
aligned and formated to let the HTX header fit before and the HTX
block(s) fit after.

This patch does two things :

1) it makes sure that we prepare an empty input buffer before an recv()
   call so that it appears as holding an HTX block at the front, which is
   removed afterwards. This way the data received using recv() are placed
   exactly at the target position in the input buffer for a later cast to
   HTX.

2) when receiving data in h1_process_data(), if it appears that the input
   buffer can be cast to an HTX buffer and the target buffer is empty,
   then the buffers are swapped, an HTX block is prepended in front of the
   data area, and the HTX block is appended to reference this data block.

In practice, this ensures that in most cases when transferring large files,
calls to h1_rcv_buf() are made using zero copy and a little bit of buffer
preparation (~40 bytes to be written).

Doing this adds an extra 13% performance boost on top of previous patch,
resulting in a total of 50% speed up on large transfers.

src/mux_h1.c

@@ -959,7 +959,8 @@ static size_t h1_process_headers(struct h1s *h1s, struct h1m *h1m, struct htx *h
  * responsible to update the parser state <h1m>.
  */
 static size_t h1_process_data(struct h1s *h1s, struct h1m *h1m, struct htx *htx,
-			      struct buffer *buf, size_t *ofs, size_t max)
+                              struct buffer *buf, size_t *ofs, size_t max,
+                              struct buffer *htxbuf)
 {
 	uint32_t data_space = htx_free_data_space(htx);
 	size_t total = 0;
@@ -976,7 +977,34 @@ static size_t h1_process_data(struct h1s *h1s, struct h1m *h1m, struct htx *htx,
 			if (ret > b_contig_data(buf, *ofs))
 				ret = b_contig_data(buf, *ofs);
 			if (ret) {
-				if (!htx_add_data(htx, ist2(b_peek(buf, *ofs), ret)))
+				/* very often with large files we'll face the following
+				 * situation :
+				 *   - htx is empty and points to <htxbuf>
+				 *   - ret == buf->data
+				 *   - buf->head == sizeof(struct htx)
+				 *   => we can swap the buffers and place an htx header into
+				 *      the target buffer instead
+				 */
+				if (unlikely(htx_is_empty(htx) && ret == b_data(buf) &&
+					     !*ofs && b_head_ofs(buf) == sizeof(struct htx))) {
+					void *raw_area = buf->area;
+					void *htx_area = htxbuf->area;
+					struct htx_blk *blk;
+
+					buf->area = htx_area;
+					htxbuf->area = raw_area;
+					htx = (struct htx *)htxbuf->area;
+					htx->size = htxbuf->size - sizeof(*htx);
+					htx_reset(htx);
+					b_set_data(htxbuf, b_size(htxbuf));
+
+					blk = htx_add_blk(htx, HTX_BLK_DATA, ret);
+					blk->info += ret;
+					/* nothing else to do, the old buffer now contains an
+					 * empty pre-initialized HTX header
+					 */
+				}
+				else if (!htx_add_data(htx, ist2(b_peek(buf, *ofs), ret)))
 					goto end;
 				h1m->curr_len -= ret;
 				*ofs += ret;
@@ -1173,6 +1201,7 @@ static size_t h1_process_input(struct h1c *h1c, struct buffer *buf, int flags)
 	int errflag;
 
 	htx = htx_from_buf(buf);
+	b_set_data(buf, b_size(buf));
 	count = b_data(&h1c->ibuf);
 	max = htx_free_space(htx);
 	if (flags & CO_RFL_KEEP_RSV) {
@@ -1199,14 +1228,16 @@ static size_t h1_process_input(struct h1c *h1c, struct buffer *buf, int flags)
 				break;
 		}
 		else if (h1m->state <= H1_MSG_TRAILERS) {
-			ret = h1_process_data(h1s, h1m, htx, &h1c->ibuf, &total, count);
+			ret = h1_process_data(h1s, h1m, htx, &h1c->ibuf, &total, count, buf);
+			htx = htx_from_buf(buf);
 			if (!ret)
 				break;
 		}
 		else if (h1m->state == H1_MSG_DONE)
 			break;
 		else if (h1m->state == H1_MSG_TUNNEL) {
-			ret = h1_process_data(h1s, h1m, htx, &h1c->ibuf, &total, count);
+			ret = h1_process_data(h1s, h1m, htx, &h1c->ibuf, &total, count, buf);
+			htx = htx_from_buf(buf);
 			if (!ret)
 				break;
 		}
@@ -1493,8 +1524,21 @@ static int h1_recv(struct h1c *h1c)
 
 	max = buf_room_for_htx_data(&h1c->ibuf);
 	if (max) {
+		int aligned = 0;
 		h1c->flags &= ~H1C_F_IN_FULL;
+
+		if (!b_data(&h1c->ibuf)) {
+			/* try to pre-align the buffer like the rxbufs will be
+			 * to optimize memory copies.
+			 */
+			h1c->ibuf.data  = sizeof(struct htx);
+			aligned = 1;
+		}
 		ret = conn->xprt->rcv_buf(conn, &h1c->ibuf, max, 0);
+		if (aligned) {
+			h1c->ibuf.data -= sizeof(struct htx);
+			h1c->ibuf.head  = sizeof(struct htx);
+		}
 	}
 	if (ret > 0) {
 		rcvd = 1;