ATHandler::consume_to_tag method doesn't detect tags in some cases

[vznncv]

Description

I'm trying to make a driver for a sim5320 using cellular framework, but ATHandler doesn't parse some responses correctly. After some investigation, I found that problem is caused by ATHandler::consume_to_tag method.

If sequence from buffer contains tag and a symbol before tag coincides with zero symbol of the tag, then the tag won't be detected. For example, if it consumes to a tag "\r\n" in a sequence "\r\r\nOK", the tag won't be found.

Problem explanation. The method from the current master branch (SHA: 8b62315) is the following:

bool ATHandler::consume_to_tag(const char *tag, bool consume_tag)
{
size_t match_pos = 0;

while (true) {
int c = get_char();
if (c == -1) {
break;
} else if (c == tag[match_pos]) {
match_pos++;
if (match_pos == strlen(tag)) {
if (!consume_tag) {
_recv_pos -= strlen(tag);
}
return true;
}
} else if (match_pos) {
match_pos = 0;
}
}
tr_debug("consume_to_tag not found");
return false;
}

Suppose we have want to consume to a tag "\r\n" in a sequence "\r\r\nOK":

1. iteration 1 of the while loop: the \r is compared with \r, symbols are matched and match_pos is increased
2. iteration 2 of the while loop: the \r is compared with \n, symbols aren't matched and match_pos is set to zero, but the code doesn't check if \r is matched to the zero symbol of the tag (i.e. \r).
3. other iterations: the tag won't be found, as its zero symbol is skipped

SHA of the Mbed OS: 8b62315

Issue request type

[ ] Question
[ ] Enhancement
[x] Bug

[mirelachirica]

Internal issue generated.

[mirelachirica]

PR: https://github.com/ARMmbed/mbed-os/pull/8350/files

[ciarmcom]

Internal Jira reference: https://jira.arm.com/browse/MBOCUSTRIA-37

[cmonr]

Marking as closed since #8350 has been merged.