res_pjsip_refer: Prevent sending duplicate headers.

res_pjsip_refer will attempt to add Referred-By or Replaces headers to
outbound INVITEs at times. If the INVITE gets challenged for
authentication, then we will resend the INVITE. Prior to this patch, the
Referred-By or Replaces header would be re-added to the outbound INVITE,
resulting in duplicated headers.

ASTERISK-25204 #close
Reported by Mark Michelson

Change-Id: I59fb5c08b4d253c0dba9ee3d3950b5025358222d

res/res_pjsip_refer.c

@@ -987,10 +987,41 @@ static int refer_incoming_request(struct ast_sip_session *session, pjsip_rx_data
 	}
 }
 
-static void refer_outgoing_request(struct ast_sip_session *session, struct pjsip_tx_data *tdata)
+/*!
+ * \brief Use the value of a channel variable as the value of a SIP header
+ *
+ * This looks up a variable name on a channel, then takes that value and adds
+ * it to an outgoing SIP request. If the header already exists on the message,
+ * then no action is taken.
+ *
+ * \pre chan is locked.
+ *
+ * \param chan The channel on which to find the variable.
+ * \param var_name The name of the channel variable to use.
+ * \param header_name The name of the SIP header to add to the outgoing message.
+ * \param tdata The outgoing SIP message on which to add the header
+ */
+static void add_header_from_channel_var(struct ast_channel *chan, const char *var_name, const char *header_name, pjsip_tx_data *tdata)
 {
-	const char *hdr;
+	const char *var_value;
+	pj_str_t pj_header_name;
+	pjsip_hdr *header;
 
+	var_value = pbx_builtin_getvar_helper(chan, var_name);
+	if (ast_strlen_zero(var_value)) {
+		return;
+	}
+
+	pj_cstr(&pj_header_name, header_name);
+	header = pjsip_msg_find_hdr_by_name(tdata->msg, &pj_header_name, NULL);
+	if (header) {
+		return;
+	}
+	ast_sip_add_header(tdata, header_name, var_value);
+}
+
+static void refer_outgoing_request(struct ast_sip_session *session, struct pjsip_tx_data *tdata)
+{
 	if (pjsip_method_cmp(&tdata->msg->line.req.method, &pjsip_invite_method)
 		|| !session->channel
 		|| session->inv_session->state != PJSIP_INV_STATE_NULL) {
@@ -998,15 +1029,8 @@ static void refer_outgoing_request(struct ast_sip_session *session, struct pjsip
 	}
 
 	ast_channel_lock(session->channel);
-	hdr = pbx_builtin_getvar_helper(session->channel, "SIPREPLACESHDR");
-	if (!ast_strlen_zero(hdr)) {
-		ast_sip_add_header(tdata, "Replaces", hdr);
-	}
-
-	hdr = pbx_builtin_getvar_helper(session->channel, "SIPREFERREDBYHDR");
-	if (!ast_strlen_zero(hdr)) {
-		ast_sip_add_header(tdata, "Referred-By", hdr);
-	}
+	add_header_from_channel_var(session->channel, "SIPREPLACESHDR", "Replaces", tdata);
+	add_header_from_channel_var(session->channel, "SIPREFERREDBYHDR", "Referred-By", tdata);
 	ast_channel_unlock(session->channel);
 }