Don't try to start DLT logging if DLT unavailable.

This is to avoid a long delay when shutting down the broker.

Closes #1735. Thanks to Colin Law.

ChangeLog.txt

@@ -17,6 +17,8 @@ Broker:
   Closes #1689. Closes #1741.
 - Log protocol error message where appropriate from a bad UNSUBSCRIBE, rather
   than the generic "socket error".
+- Don't try to start DLT logging if DLT unavailable, to avoid a long delay
+  when shutting down the broker. Closes #1735.
 
 Client library:
 - Improved documentation around connect callback return codes. Close #1730.

src/logging.c

@@ -24,6 +24,7 @@ Contributors:
 #include <time.h>
 
 #ifdef WITH_DLT
+#include <sys/stat.h>
 #include <dlt/dlt.h>
 #endif
 
@@ -55,6 +56,29 @@ static int log_priorities = MOSQ_LOG_ERR | MOSQ_LOG_WARNING | MOSQ_LOG_NOTICE |
 
 #ifdef WITH_DLT
 static DltContext dltContext;
+static bool dlt_allowed = false;
+
+void dlt_fifo_check(void)
+{
+	struct stat statbuf;
+	int fd;
+
+	/* If we start DLT but the /tmp/dlt fifo doesn't exist, or isn't available
+	 * for writing then there is a big delay when we try and close the log
+	 * later, so check for it first. This has the side effect of not letting
+	 * people using DLT create the fifo after Mosquitto has started, but at the
+	 * benefit of not having a massive delay for everybody else. */
+	memset(&statbuf, 0, sizeof(statbuf));
+	if(stat("/tmp/dlt", &statbuf) == 0){
+		if(S_ISFIFO(statbuf.st_mode)){
+			fd = open("/tmp/dlt", O_NONBLOCK | O_WRONLY);
+			if(fd != -1){
+				dlt_allowed = true;
+				close(fd);
+			}
+		}
+	}
+}
 #endif
 
 static int get_time(struct tm **ti)
@@ -120,8 +144,11 @@ int log__init(struct mosquitto__config *config)
 		restore_privileges();
 	}
 #ifdef WITH_DLT
-	DLT_REGISTER_APP("MQTT","mosquitto log");
+	dlt_fifo_check();
-	dlt_register_context(&dltContext, "MQTT", "mosquitto DLT context");
+	if(dlt_allowed){
+		DLT_REGISTER_APP("MQTT","mosquitto log");
+		dlt_register_context(&dltContext, "MQTT", "mosquitto DLT context");
+	}
 #endif
 	return rc;
 }
@@ -143,8 +170,10 @@ int log__close(struct mosquitto__config *config)
 	}
 
 #ifdef WITH_DLT
-	dlt_unregister_context(&dltContext);
+	if(dlt_allowed){
-	DLT_UNREGISTER_APP();
+		dlt_unregister_context(&dltContext);
+		DLT_UNREGISTER_APP();
+	}
 #endif
 	/* FIXME - do something for all destinations! */
 	return MOSQ_ERR_SUCCESS;