Fix SEGFAULT in MQTT server

PUBLISHED_FROM=6455082726781aab819a7f8645e81adaa7fc7669

docs/c-api/mqtt_server.h/LIST_ENTRY.md

+---
+title: "LIST_ENTRY()"
+decl_name: "LIST_ENTRY"
+symbol_kind: "func"
+signature: |
+    LIST_ENTRY(mg_mqtt_session);
+---
+
+Broker 
+

docs/c-api/mqtt_server.h/intro.md

@@ -3,9 +3,7 @@ title: "MQTT Server API reference"
 symbol_kind: "intro"
 decl_name: "mqtt_server.h"
 items:
-  - { name: mg_mqtt_broker.md }
-  - { name: mg_mqtt_broker_init.md }
-  - { name: mg_mqtt_next.md }
+  - { name: LIST_ENTRY.md }
   - { name: struct_mg_mqtt_broker.md }
   - { name: struct_mg_mqtt_session.md }
 ---

docs/c-api/mqtt_server.h/mg_mqtt_broker.md

----
-title: "mg_mqtt_broker()"
-decl_name: "mg_mqtt_broker"
-symbol_kind: "func"
-signature: |
-  void mg_mqtt_broker(struct mg_connection *brk, int ev, void *data);
----
-
-Processes a MQTT broker message.
-
-The listening connection expects a pointer to an initialised
-`mg_mqtt_broker` structure in the `user_data` field.
-
-Basic usage:
-
-```c
-mg_mqtt_broker_init(&brk, NULL);
-
-if ((nc = mg_bind(&mgr, address, mg_mqtt_broker)) == NULL) {
-  // fail;
-}
-nc->user_data = &brk;
-```
-
-New incoming connections will receive a `mg_mqtt_session` structure
-in the connection `user_data`. The original `user_data` will be stored
-in the `user_data` field of the session structure. This allows the user
-handler to store user data before `mg_mqtt_broker` creates the session.
-
-Since only the MG_EV_ACCEPT message is processed by the listening socket,
-for most events the `user_data` will thus point to a `mg_mqtt_session`. 
-

docs/c-api/mqtt_server.h/mg_mqtt_broker_init.md

----
-title: "mg_mqtt_broker_init()"
-decl_name: "mg_mqtt_broker_init"
-symbol_kind: "func"
-signature: |
-  void mg_mqtt_broker_init(struct mg_mqtt_broker *brk, void *user_data);
----
-
-Initialises a MQTT broker. 
-

docs/c-api/mqtt_server.h/mg_mqtt_next.md

----
-title: "mg_mqtt_next()"
-decl_name: "mg_mqtt_next"
-symbol_kind: "func"
-signature: |
-  struct mg_mqtt_session *mg_mqtt_next(struct mg_mqtt_broker *brk,
-                                       struct mg_mqtt_session *s);
----
-
-Iterates over all MQTT session connections. Example:
-
-```c
-struct mg_mqtt_session *s;
-for (s = mg_mqtt_next(brk, NULL); s != NULL; s = mg_mqtt_next(brk, s)) {
-  // Do something
-}
-``` 
-

docs/c-api/mqtt_server.h/struct_mg_mqtt_broker.md

@@ -4,7 +4,7 @@ decl_name: "struct mg_mqtt_broker"
 symbol_kind: "struct"
 signature: |
   struct mg_mqtt_broker {
-    struct mg_mqtt_session *sessions; /* Session list */
+    LIST_HEAD(, mg_mqtt_session) sessions; /* Session list */
     void *user_data;                       /* User data */
   };
 ---

docs/c-api/mqtt_server.h/struct_mg_mqtt_session.md

@@ -5,11 +5,11 @@ symbol_kind: "struct"
 signature: |
   struct mg_mqtt_session {
     struct mg_mqtt_broker *brk;       /* Broker */
-    struct mg_mqtt_session *next, *prev; /* mg_mqtt_broker::sessions linkage */
+    LIST_ENTRY(mg_mqtt_session) link; /* mg_mqtt_broker::sessions linkage */
     struct mg_connection *nc;         /* Connection with the client */
     size_t num_subscriptions;         /* Size of `subscriptions` array */
-    struct mg_mqtt_topic_expression *subscriptions;
     void *user_data;                  /* User data */
+    struct mg_mqtt_topic_expression *subscriptions;
   };
 ---
 

mongoose.c

@@ -8595,7 +8595,7 @@ void mg_mqtt_disconnect(struct mg_connection *nc) {
  */
 
 /* Amalgamated: #include "mongoose/src/internal.h" */
-/* Amalgamated: #include "mongoose/src/mqtt-broker.h" */
+/* Amalgamated: #include "mongoose/src/mqtt-server.h" */
 
 #if MG_ENABLE_MQTT_BROKER
 
@@ -8609,16 +8609,11 @@ static void mg_mqtt_session_init(struct mg_mqtt_broker *brk,
 }
 
 static void mg_mqtt_add_session(struct mg_mqtt_session *s) {
-  s->next = s->brk->sessions;
-  s->brk->sessions = s;
-  s->prev = NULL;
-  if (s->next != NULL) s->next->prev = s;
+  LIST_INSERT_HEAD(&s->brk->sessions, s, link);
 }
 
 static void mg_mqtt_remove_session(struct mg_mqtt_session *s) {
-  if (s->prev == NULL) s->brk->sessions = s->next;
-  if (s->prev) s->prev->next = s->next;
-  if (s->next) s->next->prev = s->prev;
+  LIST_REMOVE(s, link);
 }
 
 static void mg_mqtt_destroy_session(struct mg_mqtt_session *s) {
@@ -8636,7 +8631,7 @@ static void mg_mqtt_close_session(struct mg_mqtt_session *s) {
 }
 
 void mg_mqtt_broker_init(struct mg_mqtt_broker *brk, void *user_data) {
-  brk->sessions = NULL;
+  LIST_EMPTY(&brk->sessions);
   brk->user_data = user_data;
 }
 
@@ -8662,7 +8657,6 @@ static void mg_mqtt_broker_handle_connect(struct mg_mqtt_broker *brk,
 
 static void mg_mqtt_broker_handle_subscribe(struct mg_connection *nc,
                                             struct mg_mqtt_message *msg) {
-
   struct mg_mqtt_session *ss = (struct mg_mqtt_session *) nc->user_data;
   uint8_t qoss[512];
   size_t qoss_len = 0;
@@ -8676,7 +8670,6 @@ static void mg_mqtt_broker_handle_subscribe(struct mg_connection *nc,
     qoss[qoss_len++] = qos;
   }
 
-
   ss->subscriptions = (struct mg_mqtt_topic_expression *) realloc(
       ss->subscriptions, sizeof(*ss->subscriptions) * qoss_len);
   for (pos = 0;
@@ -8721,7 +8714,8 @@ static void mg_mqtt_broker_handle_publish(struct mg_mqtt_broker *brk,
       if (mg_mqtt_match_topic_expression(s->subscriptions[i].topic,
                                          &msg->topic)) {
         char buf[100], *p = buf;
-        mg_asprintf(&p, sizeof(buf), "%.*s", (int) msg->topic.len, msg->topic.p);
+        mg_asprintf(&p, sizeof(buf), "%.*s", (int) msg->topic.len,
+                    msg->topic.p);
         if (p == NULL) {
           return;
         }
@@ -8748,6 +8742,7 @@ void mg_mqtt_broker(struct mg_connection *nc, int ev, void *data) {
   switch (ev) {
     case MG_EV_ACCEPT:
       mg_set_protocol_mqtt(nc);
+      nc->user_data = NULL;  // This is NOT a listening connection
       break;
     case MG_EV_MQTT_CONNECT:
       mg_mqtt_broker_handle_connect(brk, nc);
@@ -8768,7 +8763,7 @@ void mg_mqtt_broker(struct mg_connection *nc, int ev, void *data) {
 
 struct mg_mqtt_session *mg_mqtt_next(struct mg_mqtt_broker *brk,
                                      struct mg_mqtt_session *s) {
-  return s == NULL ? brk->sessions : s->next;
+  return s == NULL ? LIST_FIRST(&brk->sessions) : LIST_NEXT(s, link);
 }
 
 #endif /* MG_ENABLE_MQTT_BROKER */