BUG: http: disable TCP delayed ACKs when forwarding content-length data

Commits 5c6209 and 072930 were aimed at avoiding undesirable PUSH flags
when forwarding chunked data, but had the undesired effect of causing
data advertised by content-length to be affected by the delayed ACK too.
This can happen when the data to be forwarded are small enough to fit into
a single send() call, otherwise the BF_EXPECT_MORE flag would be removed.

Content-length data don't need the BF_EXPECT_MORE flag since the low-level
forwarder already knows it can safely rely on bf->to_forward to set the
appropriate TCP flags.

Note that the issue is only observed in requests at the moment, though the
later introduction of server-side keep-alive could trigger the issue on the
response path too.

Special thanks to Randy Shults for reporting this issue with a lot of
details helping to reproduce it.

The fix must be backported to 1.4.
(cherry picked from commit 869fc1edc2acfa8ff88de2f4e638ad48dca5d246)

src/proto_http.c

@@ -4550,9 +4550,13 @@ int http_request_forward_body(struct session *s, struct buffer *req, int an_bit)
 	/* We know that more data are expected, but we couldn't send more that
 	 * what we did. So we always set the BF_EXPECT_MORE flag so that the
 	 * system knows it must not set a PUSH on this first part. Interactive
-	 * modes are already handled by the stream sock layer.
+	 * modes are already handled by the stream sock layer. We must not do
+	 * this in content-length mode because it could present the MSG_MORE
+	 * flag with the last block of forwarded data, which would cause an
+	 * additional delay to be observed by the receiver.
 	 */
-	req->flags |= BF_EXPECT_MORE;
+	if (txn->flags & TX_REQ_TE_CHNK)
+		req->flags |= BF_EXPECT_MORE;
 
 	http_silent_debug(__LINE__, s);
 	return 0;
@@ -5573,9 +5577,13 @@ int http_response_forward_body(struct session *s, struct buffer *res, int an_bit
 	/* We know that more data are expected, but we couldn't send more that
 	 * what we did. So we always set the BF_EXPECT_MORE flag so that the
 	 * system knows it must not set a PUSH on this first part. Interactive
-	 * modes are already handled by the stream sock layer.
+	 * modes are already handled by the stream sock layer. We must not do
+	 * this in content-length mode because it could present the MSG_MORE
+	 * flag with the last block of forwarded data, which would cause an
+	 * additional delay to be observed by the receiver.
 	 */
-	res->flags |= BF_EXPECT_MORE;
+	if (txn->flags & TX_RES_TE_CHNK)
+		res->flags |= BF_EXPECT_MORE;
 
 	/* the session handler will take care of timeouts and errors */
 	http_silent_debug(__LINE__, s);