[463479] Make _mosquitto_mid_generate() thread safe.

Thanks to bdwalker.

Bug: https://bugs.eclipse.org/bugs/show_bug.cgi?id=463479

ChangeLog.txt

@@ -17,6 +17,7 @@ Client library:
 - Fix crash on multiple calls to mosquitto_lib_init/mosquitto_lib_cleanup.
   Closes #462780.
 - Allow longer paths on Windows. Closes #462781.
+- Make _mosquitto_mid_generate() thread safe. Closes #463479.
 
 
 1.4 - 20150218

lib/mosquitto.c

@@ -211,6 +211,7 @@ int mosquitto_reinitialise(struct mosquitto *mosq, const char *id, bool clean_se
 	pthread_mutex_init(&mosq->msgtime_mutex, NULL);
 	pthread_mutex_init(&mosq->in_message_mutex, NULL);
 	pthread_mutex_init(&mosq->out_message_mutex, NULL);
+	pthread_mutex_init(&mosq->mid_mutex, NULL);
 	mosq->thread_id = pthread_self();
 #endif
 
@@ -293,6 +294,7 @@ void _mosquitto_destroy(struct mosquitto *mosq)
 		pthread_mutex_destroy(&mosq->msgtime_mutex);
 		pthread_mutex_destroy(&mosq->in_message_mutex);
 		pthread_mutex_destroy(&mosq->out_message_mutex);
+		pthread_mutex_destroy(&mosq->mid_mutex);
 	}
 #endif
 	if(mosq->sock != INVALID_SOCKET){

lib/mosquitto_internal.h

@@ -187,6 +187,7 @@ struct mosquitto {
 	pthread_mutex_t state_mutex;
 	pthread_mutex_t in_message_mutex;
 	pthread_mutex_t out_message_mutex;
+	pthread_mutex_t mid_mutex;
 	pthread_t thread_id;
 #endif
 	bool clean_session;

lib/util_mosq.c

@@ -147,12 +147,23 @@ void _mosquitto_check_keepalive(struct mosquitto *mosq)
 
 uint16_t _mosquitto_mid_generate(struct mosquitto *mosq)
 {
+	/* FIXME - this would be better with atomic increment, but this is safer
+	 * for now for a bug fix release.
+	 *
+	 * If this is changed to use atomic increment, callers of this function
+	 * will have to be aware that they may receive a 0 result, which may not be
+	 * used as a mid.
+	 */
+	uint16_t mid;
 	assert(mosq);
 
+	pthread_mutex_lock(&mosq->mid_mutex);
 	mosq->last_mid++;
 	if(mosq->last_mid == 0) mosq->last_mid++;
+	mid = mosq->last_mid;
+	pthread_mutex_unlock(&mosq->mid_mutex);
 	
-	return mosq->last_mid;
+	return mid;
 }
 
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