res_rtp_asterisk: Avoid merging command and regular T.140 text packets

When realtime text packets are to be sent, the text is accumulated in a
buffer and sent regularly by a timer.  It can happen that commands such as
a backspace, CR, or LF get merged with regular text.  This breaks some
UAs.

The proposed change:
* We test if the current packet contains a command.  If so we send the
buffer immediately.
* We test if the buffer contained a command.  If so we send the buffer
immediately.
* We accumulate the text (or the command) in the buffer.

ASTERISK-27970

Change-Id: Ifbe993311410fa855cb8aa4a12084db75f413462

res/res_rtp_asterisk.c

@@ -6101,9 +6101,29 @@ static int rtp_red_init(struct ast_rtp_instance *instance, int buffer_time, int
 static int rtp_red_buffer(struct ast_rtp_instance *instance, struct ast_frame *frame)
 {
 	struct ast_rtp *rtp = ast_rtp_instance_get_data(instance);
+	struct rtp_red *red = rtp->red;
+
+	if (!red) {
+		return 0;
+	}
+
+	if (frame->datalen > 0) {
+		if (red->t140.datalen > 0) {
+			const unsigned char *primary = red->buf_data;
+
+			/* There is something already in the T.140 buffer */
+			if (primary[0] == 0x08 || primary[0] == 0x0a || primary[0] == 0x0d) {
+				/* Flush the previous T.140 packet if it is a command */
+				ast_rtp_write(instance, &rtp->red->t140);
+			} else {
+				primary = frame->data.ptr;
+				if (primary[0] == 0x08 || primary[0] == 0x0a || primary[0] == 0x0d) {
+					/* Flush the previous T.140 packet if we are buffering a command now */
+					ast_rtp_write(instance, &rtp->red->t140);
+				}
+			}
+		}
 
-	if (frame->datalen > -1) {
-		struct rtp_red *red = rtp->red;
 		memcpy(&red->buf_data[red->t140.datalen], frame->data.ptr, frame->datalen);
 		red->t140.datalen += frame->datalen;
 		red->t140.ts = frame->ts;