From 64de2ba4ea7b6da390c84f60933ecd0eff5f9f67 Mon Sep 17 00:00:00 2001
From: Jon Shallow <supjps-libcoap@jpshallow.com>
Date: Mon, 21 Nov 2022 10:46:14 +0000
Subject: [PATCH] resource.c: Correctly handle observe delete by server
 application

If the request handler returns an error code to an observe request
then following the deletion of the observe, no longer refer to the
now deleted object.
---
 src/resource.c | 19 ++++++++++++-------
 1 file changed, 12 insertions(+), 7 deletions(-)

diff --git a/src/resource.c b/src/resource.c
index 8ff28175..487cc433 100644
--- a/src/resource.c
+++ b/src/resource.c
@@ -876,11 +876,13 @@ coap_notify_observers(coap_context_t *context, coap_resource_t *r,
   coap_string_t *query;
   coap_block_b_t block;
   coap_tick_t now;
+  coap_session_t *obs_session;
 
   if (r->observable && (r->dirty || r->partiallydirty)) {
     r->partiallydirty = 0;
 
     LL_FOREACH_SAFE(r->subscribers, obs, otmp) {
+      obs_session = obs->session;
       if (r->dirty == 0 && obs->dirty == 0) {
         /*
          * running this resource due to partiallydirty, but this observation's
@@ -982,6 +984,7 @@ coap_notify_observers(coap_context_t *context, coap_resource_t *r,
         }
         if (COAP_RESPONSE_CLASS(response->code) > 2) {
           coap_delete_observer(r, obs->session, &token);
+          obs = NULL;
         }
         break;
       case COAP_DELETING_RESOURCE:
@@ -992,16 +995,18 @@ coap_notify_observers(coap_context_t *context, coap_resource_t *r,
         break;
       }
 
-      if (response->type == COAP_MESSAGE_CON ||
-          (r->flags & COAP_RESOURCE_FLAGS_NOTIFY_NON_ALWAYS)) {
-        obs->non_cnt = 0;
-      } else {
-        obs->non_cnt++;
+      if (obs) {
+        if (response->type == COAP_MESSAGE_CON ||
+            (r->flags & COAP_RESOURCE_FLAGS_NOTIFY_NON_ALWAYS)) {
+          obs->non_cnt = 0;
+        } else {
+          obs->non_cnt++;
+        }
       }
 
-      mid = coap_send_internal( obs->session, response );
+      mid = coap_send_internal(obs_session, response);
 
-      if (COAP_INVALID_MID == mid) {
+      if (COAP_INVALID_MID == mid && obs) {
         coap_subscription_t *s;
         coap_log(LOG_DEBUG,
                  "coap_check_notify: sending failed, resource stays "