Fix use of `MOSQ_OPT_TLS_ENGINE` being unable to be used.

This was due to the openssl ctx not being initialised until starting to connect.

Closes #2537. Thanks to chessing-c4.

ChangeLog.txt

@@ -11,6 +11,8 @@ Broker:
 Client library:
 - Fix threads library detection on Windows under cmake. Bumps the minimum
   cmake version to 3.1, which is still ancient.
+- Fix use of `MOSQ_OPT_TLS_ENGINE` being unable to be used due to the openssl
+  ctx not being initialised until starting to connect. Closes #2537.
 
 Clients:
 - Fix mosquitto_pub incorrectly reusing topic aliases when reconnecting.

include/mosquitto.h

@@ -1565,6 +1565,9 @@ libmosq_EXPORT int mosquitto_int_option(struct mosquitto *mosq, enum mosq_opt_t
  *	MOSQ_OPT_TLS_ENGINE - Configure the client for TLS Engine support.
  *	          Pass a TLS Engine ID to be used when creating TLS
  *	          connections. Must be set before <mosquitto_connect>.
+ *	          Must be a valid engine, and note that the string will not be used
+ *	          until a connection attempt is made so this function will return
+ *	          success even if an invalid engine string is passed.
  *
  *	MOSQ_OPT_TLS_KEYFORM - Configure the client to treat the keyfile
  *	          differently depending on its type.  Must be set

lib/options.c

@@ -284,14 +284,17 @@ int mosquitto_string_option(struct mosquitto *mosq, enum mosq_opt_t option, cons
 	switch(option){
 		case MOSQ_OPT_TLS_ENGINE:
 #if defined(WITH_TLS) && !defined(OPENSSL_NO_ENGINE)
-			eng = ENGINE_by_id(value);
+			mosquitto__free(mosq->tls_engine);
-			if(!eng){
+			if(value){
-				return MOSQ_ERR_INVAL;
+				eng = ENGINE_by_id(value);
-			}
+				if(!eng){
-			ENGINE_free(eng); /* release the structural reference from ENGINE_by_id() */
+					return MOSQ_ERR_INVAL;
-			mosq->tls_engine = mosquitto__strdup(value);
+				}
-			if(!mosq->tls_engine){
+				ENGINE_free(eng); /* release the structural reference from ENGINE_by_id() */
-				return MOSQ_ERR_NOMEM;
+				mosq->tls_engine = mosquitto__strdup(value);
+				if(!mosq->tls_engine){
+					return MOSQ_ERR_NOMEM;
+				}
 			}
 			return MOSQ_ERR_SUCCESS;
 #else