applications: serial_lte_modem: Fix datamode quit_str handling

Fixes an issue where non-default quit strings, with non-repeating
characters, such as "abc" and "aabbcc", did not interpret the
start of the quit string properly, if the last characters of the
data collided with the quit string start. For example, "*aabc"
and "*aaabbcc" with our example quit strings.

Signed-off-by: Markus Lassila <[email protected]>

Author: MarkusLassila

applications/serial_lte_modem/src/slm_at_host.c

@@ -220,21 +220,14 @@ K_TIMER_DEFINE(inactivity_timer, inactivity_timer_handler, NULL);
 /* Search for quit_str and send data prior to that. Tracks quit_str over several calls. */
 static size_t raw_rx_handler(const uint8_t *buf, const size_t len)
 {
+	k_mutex_lock(&mutex_data, K_FOREVER);
+
 	const char *const quit_str = CONFIG_SLM_DATAMODE_TERMINATOR;
 	size_t processed;
 	bool quit_str_match = false;
-	bool prev_quit_str_match = false;
-	uint8_t quit_str_match_count;
-	uint8_t prev_quit_str_match_count;
-
-	k_mutex_lock(&mutex_data, K_FOREVER);
-
-	/* Initialize from previous time.*/
-	prev_quit_str_match_count = quit_str_partial_match;
-	quit_str_match_count = prev_quit_str_match_count;
-	if (prev_quit_str_match_count != 0) {
-		prev_quit_str_match = true;
-	}
+	uint8_t quit_str_match_count = quit_str_partial_match;
+	uint8_t prev_quit_str_match_count = quit_str_partial_match;
+	uint8_t prev_quit_str_match_count_original = quit_str_partial_match;
 
 	/* Find quit_str or partial match at the end of the buffer. */
 	for (processed = 0; processed < len && quit_str_match == false; processed++) {
@@ -243,22 +236,37 @@ static size_t raw_rx_handler(const uint8_t *buf, const size_t len)
 			if (quit_str_match_count == strlen(quit_str)) {
 				quit_str_match = true;
 			}
-		} else {
-			/* No match. Possible previous partial quit_str match is data. */
-			quit_str_match_count = 0;
-			prev_quit_str_match = false;
+		} else if (quit_str_match_count > 0) {
+			/* Check if we match a beginning of a new quit_str.
+			 * We either match the first character, or in the edge case of
+			 * quit_str starting with multiple same characters, e.g. "aaabbb",
+			 * we match all but the current character (with input aaaa).
+			 */
+			for (int i = 0; i < quit_str_match_count; i++) {
+				if (buf[processed] != quit_str[i]) {
+					quit_str_match_count = i;
+					break;
+				}
+			}
+			if (quit_str_match_count == 0) {
+				/* No match.
+				 * Previous partial quit_str is data.
+				 */
+				prev_quit_str_match_count = 0;
+			} else if (prev_quit_str_match_count > 0) {
+				/* Partial match.
+				 * Part of the previous partial quit_str is data.
+				 */
+				prev_quit_str_match_count--;
+			}
 		}
 	}
 
-	if (prev_quit_str_match == false) {
-		/* Write data which was previously interpreted as a possible partial quit_str. */
-		write_data_buf(quit_str, prev_quit_str_match_count);
+	/* Write data which was previously interpreted as a possible partial quit_str. */
+	write_data_buf(quit_str, prev_quit_str_match_count_original - prev_quit_str_match_count);
 
-		/* Write data from buf until the start of the possible (partial) quit_str. */
-		write_data_buf(buf, processed - quit_str_match_count);
-	} else {
-		/* Nothing to write this round.*/
-	}
+	/* Write data from buf until the start of the possible (partial) quit_str. */
+	write_data_buf(buf, processed - (quit_str_match_count - prev_quit_str_match_count));
 
 	if (quit_str_match) {
 		raw_send(SLM_DATAMODE_FLAGS_NONE);