res_pjsip_pubsub: Fix persistent subscriptions not surviving graceful shutdown

If you do a 'core (shutdown|restart) graceful' persistent subscriptions won't 
survive.  If you do a 'core (shutdown|restart) now' or asterisk terminates for 
some reason, they do.  Here's why...

When asterisk shuts down gracefully, it sends a 'NOTIFY/terminated' to 
subscribers for each subscription.  This not only tells the subscribers that the 
dialog/state machine is done, it also frees the last reference to the 
subscription tree which causes the persistent subscription to get deleted from 
astdb.  When asterisk restarts, nothing's left.  Just preventing the delete from 
astdb doesn't work because we already told the subscriber to terminate the 
dialog so we can't restart it even if it was still in astdb.  Everything works 
OK if asterisk terminates unexpectedly because we never send the 'terminated' 
message so on restart, the subscription is still in astdb and the subscriber is 
none the wiser.

This patch suppresses the sending of 'NOTIFY/terminated' on shutdown for 
persistent connections.

Tested-by: George Joseph

Review: https://reviewboard.asterisk.org/r/4318/
........

Merged revisions 430397 from http://svn.asterisk.org/svn/asterisk/branches/13


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@430398 65c4cc65-6c06-0410-ace0-fbb531ad65f3

res/res_pjsip_pubsub.c

@@ -2060,6 +2060,12 @@ static int send_notify(struct sip_subscription_tree *sub_tree, unsigned int forc
 	pjsip_evsub *evsub = sub_tree->evsub;
 	pjsip_tx_data *tdata;
 
+	if (ast_shutting_down()
+		&& sub_tree->root->subscription_state == PJSIP_EVSUB_STATE_TERMINATED
+		&& sub_tree->persistence) {
+		return 0;
+	}
+
 	if (pjsip_evsub_notify(evsub, sub_tree->root->subscription_state,
 				NULL, NULL, &tdata) != PJ_SUCCESS) {
 		return -1;