Display error if broker unable to open the log file.

Thanks to Matthew Treinish.

Bug: https://github.com/eclipse/mosquitto/issues/234

ChangeLog.txt

@@ -11,6 +11,8 @@ Broker:
   #189.
 - Fix persistence saving of subscription QoS on big-endian machines.
 - Fix will retained flag handling on Windows. Closes #222.
+- Broker now displays an error if it is unable to open the log file. Closes
+  #234.
 
 Client library:
 - Support OpenSSL 1.1.0.

src/logging.c

@@ -71,6 +71,8 @@ int mqtt3_log_init(struct mqtt3_config *config)
 		}
 		config->log_fptr = _mosquitto_fopen(config->log_file, "at");
 		if(!config->log_fptr){
+			log_destinations = MQTT3_LOG_STDERR;
+			log_priorities = MOSQ_LOG_ERR;
 			_mosquitto_log_printf(NULL, MOSQ_LOG_ERR, "Error: Unable to open log file %s for writing.", config->log_file);
 			return MOSQ_ERR_INVAL;
 		}

src/mosquitto.c

@@ -287,7 +287,10 @@ int main(int argc, char *argv[])
 
 	/* Initialise logging only after initialising the database in case we're
 	 * logging to topics */
-	mqtt3_log_init(&config);
+	if(mqtt3_log_init(&config)){
+		rc = 1;
+		return rc;
+	}
 	_mosquitto_log_printf(NULL, MOSQ_LOG_INFO, "mosquitto version %s (build date %s) starting", VERSION, TIMESTAMP);
 	if(config.config_file){
 		_mosquitto_log_printf(NULL, MOSQ_LOG_INFO, "Config loaded from %s.", config.config_file);