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);
- if(!eng){
- return MOSQ_ERR_INVAL;
- }
- ENGINE_free(eng); /* release the structural reference from ENGINE_by_id() */
- mosq->tls_engine = mosquitto__strdup(value);
- if(!mosq->tls_engine){
- return MOSQ_ERR_NOMEM;
+ mosquitto__free(mosq->tls_engine);
+ if(value){
+ eng = ENGINE_by_id(value);
+ if(!eng){
+ return MOSQ_ERR_INVAL;
+ }
+ ENGINE_free(eng); /* release the structural reference from ENGINE_by_id() */
+ mosq->tls_engine = mosquitto__strdup(value);
+ if(!mosq->tls_engine){
+ return MOSQ_ERR_NOMEM;
+ }
  }
  return MOSQ_ERR_SUCCESS;
 #else