Ensure connection lost callbacks are complte prior to disconnect end

Encountered this valgrind error while testing some code the other day:

==2568== Memcheck, a memory error detector
==2568== Copyright (C) 2002-2017, and GNU GPL'd, by Julian Seward et al.
==2568== Using Valgrind-3.16.1 and LibVEX; rerun with -h for copyright info
==2568== Command: ./test
==2568== Parent PID: 2560
==2568==
==2568== Thread 7:
==2568== Invalid read of size 8
==2568==    at 0x4CDA970: connectionLost_call (in /usr/lib/libpaho-mqtt3cs.so.1.3.9)
==2568==    by 0x500AEA6: start_thread (pthread_create.c:477)
==2568==    by 0x5121A2E: clone (clone.S:95)
==2568==  Address 0x91e8238 is 40 bytes inside a block of size 176 free'd
==2568==    at 0x48399AB: free (in /usr/lib/x86_64-linux-gnu/valgrind/vgpreload_memcheck-amd64-linux.so)
==2568==    by 0x4CF8B0F: myfree (in /usr/lib/libpaho-mqtt3cs.so.1.3.9)
==2568==    by 0x4CF3947: ListUnlink (in /usr/lib/libpaho-mqtt3cs.so.1.3.9)
==2568==    by 0x4CF3A16: ListRemove (in /usr/lib/libpaho-mqtt3cs.so.1.3.9)
==2568==    by 0x4CDA612: MQTTClient_destroy (in /usr/lib/libpaho-mqtt3cs.so.1.3.9)
==2568==    by 0x142BC0: comms_run (comms_connect.c:436)
==2568==    by 0x500AEA6: start_thread (pthread_create.c:477)
==2568==    by 0x5121A2E: clone (clone.S:95)
==2568==  Block was alloc'd at
==2568==    at 0x483877F: malloc (in /usr/lib/x86_64-linux-gnu/valgrind/vgpreload_memcheck-amd64-linux.so)
==2568==    by 0x4CF8725: mymalloc (in /usr/lib/libpaho-mqtt3cs.so.1.3.9)
==2568==    by 0x4CD9F58: MQTTClient_createWithOptions (in /usr/lib/libpaho-mqtt3cs.so.1.3.9)
==2568==    by 0x1428FB: comms_run (comms_connect.c:404)
==2568==    by 0x500AEA6: start_thread (pthread_create.c:477)
==2568==    by 0x5121A2E: clone (clone.S:95)
==2568==

It indicates a use after free of the MQTTClient data structure in the
connectionLost_call.

A bit of digging revealed that the problem was that my code Called
MQTTClient_disconnect and MQTTClient_destroy in rapid succession.
Because MQTTClient_disconnect does this:

Thread_start(connectionLost_call, m);

Its entirely possible for connectionLost_call to be executed after the
return from MQTTClient_disconnect completes, at which time the Client
may be destroyed/freed, leading to the above valgrind error.

fix is pretty straightforward. We could use a mutex and condition
variable to serialize the two threads, but Windows doesn't have support
for condition variables currently, so just use one time semaphore to
ensure serialization
Signed-off-by: NNeil Horman <nhorman@gmail.com>

convert to use of semaphore

src/MQTTClient.c

@@ -88,6 +88,11 @@
 const char *client_timestamp_eye = "MQTTClientV3_Timestamp " BUILD_TIMESTAMP;
 const char *client_version_eye = "MQTTClientV3_Version " CLIENT_VERSION;
 
+struct conlost_sync_data {
+	sem_type sem;
+	void *m;
+};
+
 int MQTTClient_init(void);
 
 void MQTTClient_global_init(MQTTClient_init_options* inits)
@@ -701,9 +706,12 @@ static int clientSockCompare(void* a, void* b)
  */
 static thread_return_type WINAPI connectionLost_call(void* context)
 {
-	MQTTClients* m = (MQTTClients*)context;
+	struct conlost_sync_data *data = (struct conlost_sync_data *)context;
+	MQTTClients* m = (MQTTClients *)data->m;
 
 	(*(m->cl))(m->context, NULL);
+
+	Thread_post_sem(data->sem);
 	return 0;
 }
 
@@ -1915,6 +1923,9 @@ static int MQTTClient_disconnect1(MQTTClient handle, int timeout, int call_conne
 	START_TIME_TYPE start;
 	int rc = MQTTCLIENT_SUCCESS;
 	int was_connected = 0;
+	struct conlost_sync_data sync = {
+		NULL, m
+	};
 
 	FUNC_ENTRY;
 	if (m == NULL || m->c == NULL)
@@ -1944,8 +1955,11 @@ exit:
 		MQTTClient_stop();
 	if (call_connection_lost && m->cl && was_connected)
 	{
+		sync.sem = Thread_create_sem(&rc);
 		Log(TRACE_MIN, -1, "Calling connectionLost for client %s", m->c->clientID);
-		Thread_start(connectionLost_call, m);
+		Thread_start(connectionLost_call, &sync);
+		Thread_wait_sem(sync.sem, 5000);
+		Thread_destroy_sem(sync.sem);
 	}
 	FUNC_EXIT_RC(rc);
 	return rc;