diff --git a/include/proto/channel.h b/include/proto/channel.h
index 44cb89012..49a68b26b 100644
--- a/include/proto/channel.h
+++ b/include/proto/channel.h
@@ -102,10 +102,28 @@ static inline unsigned int channel_is_empty(struct channel *c)
 	return !(c->buf->o | (long)c->pipe);
 }
 
-/* Returns non-zero if the buffer input is considered full. The reserved space
- * is taken into account if ->to_forward indicates that an end of transfer is
- * close to happen. The test is optimized to avoid as many operations as
- * possible for the fast case and to be used as an "if" condition.
+/* Returns non-zero if the buffer input has all of its reserve available. This
+ * is used to decide when a request or response may be parsed when some data
+ * from a previous exchange might still be present.
+ */
+static inline int channel_reserved(const struct channel *chn)
+{
+	int rem = chn->buf->size;
+
+	rem -= chn->buf->o;
+	rem -= chn->buf->i;
+	rem -= global.tune.maxrewrite;
+	return rem >= 0;
+}
+
+/* Returns non-zero if the buffer input is considered full. This is used to
+ * decide when to stop reading into a buffer when we want to ensure that we
+ * leave the reserve untouched after all pending outgoing data are forwarded.
+ * The reserved space is taken into account if ->to_forward indicates that an
+ * end of transfer is close to happen. Note that both ->buf->o and ->to_forward
+ * are considered as available since they're supposed to leave the buffer. The
+ * test is optimized to avoid as many operations as possible for the fast case
+ * and to be used as an "if" condition.
  */
 static inline int channel_full(const struct channel *chn)
 {
diff --git a/src/proto_http.c b/src/proto_http.c
index d1bf51ead..d4e93e592 100644
--- a/src/proto_http.c
+++ b/src/proto_http.c
@@ -2219,11 +2219,8 @@ int http_wait_for_request(struct session *s, struct channel *req, int an_bit)
 	 * data later, which is much more complicated.
 	 */
 	if (buffer_not_empty(req->buf) && msg->msg_state < HTTP_MSG_ERROR) {
-		if ((txn->flags & TX_NOT_FIRST) &&
-		    unlikely(channel_full(req) ||
-			     bi_end(req->buf) < b_ptr(req->buf, msg->next) ||
-			     bi_end(req->buf) > req->buf->data + req->buf->size - global.tune.maxrewrite)) {
-			if (req->buf->o) {
+		if (txn->flags & TX_NOT_FIRST) {
+			if (unlikely(!channel_reserved(req))) {
 				if (req->flags & (CF_SHUTW|CF_SHUTW_NOW|CF_WRITE_ERROR|CF_WRITE_TIMEOUT))
 					goto failed_keep_alive;
 				/* some data has still not left the buffer, wake us once that's done */
@@ -2231,9 +2228,9 @@ int http_wait_for_request(struct session *s, struct channel *req, int an_bit)
 				req->flags |= CF_READ_DONTWAIT; /* try to get back here ASAP */
 				return 0;
 			}
-			if (bi_end(req->buf) < b_ptr(req->buf, msg->next) ||
-			    bi_end(req->buf) > req->buf->data + req->buf->size - global.tune.maxrewrite)
-				buffer_slow_realign(msg->chn->buf);
+			if (unlikely(bi_end(req->buf) < b_ptr(req->buf, msg->next) ||
+			             bi_end(req->buf) > req->buf->data + req->buf->size - global.tune.maxrewrite))
+				buffer_slow_realign(req->buf);
 		}
 
 		/* Note that we have the same problem with the response ; we
@@ -2244,7 +2241,7 @@ int http_wait_for_request(struct session *s, struct channel *req, int an_bit)
 		 * keep-alive requests.
 		 */
 		if ((txn->flags & TX_NOT_FIRST) &&
-		    unlikely(channel_full(s->rep) ||
+		    unlikely(!channel_reserved(s->rep) ||
 			     bi_end(s->rep->buf) < b_ptr(s->rep->buf, txn->rsp.next) ||
 			     bi_end(s->rep->buf) > s->rep->buf->data + s->rep->buf->size - global.tune.maxrewrite)) {
 			if (s->rep->buf->o) {
@@ -5004,21 +5001,19 @@ int http_wait_for_response(struct session *s, struct channel *rep, int an_bit)
 	 * data later, which is much more complicated.
 	 */
 	if (buffer_not_empty(rep->buf) && msg->msg_state < HTTP_MSG_ERROR) {
-		if (unlikely(channel_full(rep) ||
-			     bi_end(rep->buf) < b_ptr(rep->buf, msg->next) ||
-			     bi_end(rep->buf) > rep->buf->data + rep->buf->size - global.tune.maxrewrite)) {
-			if (rep->buf->o) {
-				/* some data has still not left the buffer, wake us once that's done */
-				if (rep->flags & (CF_SHUTW|CF_SHUTW_NOW|CF_WRITE_ERROR|CF_WRITE_TIMEOUT))
-					goto abort_response;
-				channel_dont_close(rep);
-				rep->flags |= CF_READ_DONTWAIT; /* try to get back here ASAP */
-				return 0;
-			}
-			if (rep->buf->i <= rep->buf->size - global.tune.maxrewrite)
-				buffer_slow_realign(msg->chn->buf);
+		if (unlikely(!channel_reserved(rep))) {
+			/* some data has still not left the buffer, wake us once that's done */
+			if (rep->flags & (CF_SHUTW|CF_SHUTW_NOW|CF_WRITE_ERROR|CF_WRITE_TIMEOUT))
+				goto abort_response;
+			channel_dont_close(rep);
+			rep->flags |= CF_READ_DONTWAIT; /* try to get back here ASAP */
+			return 0;
 		}
 
+		if (unlikely(bi_end(rep->buf) < b_ptr(rep->buf, msg->next) ||
+		             bi_end(rep->buf) > rep->buf->data + rep->buf->size - global.tune.maxrewrite))
+			buffer_slow_realign(rep->buf);
+
 		if (likely(msg->next < rep->buf->i))
 			http_msg_analyzer(msg, &txn->hdr_idx);
 	}