strans/accept: fix cancel/rejection (#423)

In sipsess_reply_2xx, the reply is sent using a server transaction which
sets the st to NULL when replying with scode >=200. This is problematic
when sending 2xx responses to PRACKs after which sess->st was NULL.
Now, st is not passed to sip_treplyf if the request was PRACK, so st is
not set to NULL.

src/sipsess/accept.c

@@ -197,9 +197,7 @@ int sipsess_answer(struct sipsess *sess, uint16_t scode, const char *reason,
 	va_list ap;
 	int err;
 
-	if (!sess || (!sess->st
-	    && (sess->established || sess->awaiting_answer))
-	    || !sess->msg || scode < 200 || scode > 299)
+	if (!sess || !sess->st || !sess->msg || scode < 200 || scode > 299)
 		return EINVAL;
 
 	va_start(ap, fmt);

src/sipsess/reply.c

@@ -89,6 +89,7 @@ int sipsess_reply_2xx(struct sipsess *sess, const struct sip_msg *msg,
 {
 	struct sipsess_reply *reply;
 	struct sip_contact contact;
+	bool is_prack = false;
 	int err = ENOMEM;
 
 	reply = mem_zalloc(sizeof(*reply), destructor);
@@ -101,9 +102,10 @@ int sipsess_reply_2xx(struct sipsess *sess, const struct sip_msg *msg,
 	reply->msg  = mem_ref((void *)msg);
 	reply->sess = sess;
 
+	is_prack = !pl_strcmp(&msg->met, "PRACK");
 	sip_contact_set(&contact, sess->cuser, &msg->dst, msg->tp);
 
-	err = sip_treplyf(&sess->st, &reply->mb, sess->sip,
+	err = sip_treplyf(is_prack ? NULL : &sess->st, &reply->mb, sess->sip,
 			  msg, true, scode, reason,
 			  "%H"
 			  "%v"
@@ -123,7 +125,7 @@ int sipsess_reply_2xx(struct sipsess *sess, const struct sip_msg *msg,
 	if (err)
 		goto out;
 
-	if (pl_strcmp(&msg->met, "PRACK")) {
+	if (!is_prack) {
 		tmr_start(&reply->tmr, 64 * SIP_T1, tmr_handler, reply);
 		tmr_start(&reply->tmrg, SIP_T1, retransmit_handler, reply);
 	}
@@ -138,7 +140,9 @@ int sipsess_reply_2xx(struct sipsess *sess, const struct sip_msg *msg,
 
  out:
 	if (err) {
-		sess->st = mem_deref(sess->st);
+		if (!is_prack)
+			sess->st = mem_deref(sess->st);
+
 		mem_deref(reply);
 	}