sctp: fix outgoing SACK corruption on DATA chunk reception

[r-c-n]

On libpeer's own SCTP implementation, reception of a DATA chunk may result in a call to the onmessage callback, if available. Before running the callback, sctp_incoming_data() prepares a SACK chunk that is meant to be sent after the callback is done, but if the callback sends any data as a response, it'll overwrite the already formatted SACK chunk header, corrupting the SACK.

Fix it by moving the SACK send before the callback.

NOTE: similar problems may surface depending on what the onmessage, onopen and onclose do. I've only seen this happening in the SCTP_DATA case but I think it could also happen in the SCTP_COOKIE_ECHO case. I haven't tested it, though, so any opinion is appreciated.

Example

Before the fix, if the peer receives a DATA chunk, it first sends another DATA chunk as a response (similar to what

libpeer/examples/pico/main.c

Line 34 in 9319aa4

static void onmessage(char* msg, size_t len, void* userdata, uint16_t sid) {

does). Then, it sends the SACK chunk, whose buffer has been overwritten by the previous DATA send:

After the fix, on the same interchange, the peer sends the SACK first, then runs the onmessage callback to send the DATA chunk: