MINOR: htx/channel: Add a function to copy an HTX message in a channel's buffer

The channel_htx_copy_msg() function can now be used to copy an HTX message in a
channel's buffer. This function takes care to not overwrite existing data.

This patch depends on the commit "MINOR: htx: Add a function to append an HTX
message to another one". Both are mandatory to fix a bug in
http_reply_and_close() function. Be careful to backport both first.

include/proto/channel.h

@@ -935,6 +935,22 @@ static inline int32_t channel_htx_fwd_headers(struct channel *chn, struct htx *h
 	return pos;
 }
 
+/* Copy an HTX message stored in the buffer <msg> to the channel's one. We
+ * take care to not overwrite existing data in the channel. All the message is
+ * copied or nothing. It returns 1 on success and 0 on error.
+ */
+static inline int channel_htx_copy_msg(struct channel *chn, struct htx *htx, const struct buffer *msg)
+{
+	/* The channel buffer is empty, we can do a raw copy */
+	if (c_empty(chn)) {
+		chn->buf.data = msg->data;
+		memcpy(chn->buf.area, msg->area, msg->data);
+		return 1;
+	}
+
+	/* Otherwise, we need to append the HTX message */
+	return htx_append_msg(htx, htxbuf(msg));
+}
 /*
  * Advance the channel buffer's read pointer by <len> bytes. This is useful
  * when data have been read directly from the buffer. It is illegal to call