feat(el): Expose the EventLoop mutex in its API

arch/eventloop_posix.c

@@ -417,6 +417,18 @@ UA_EventLoopPOSIX_free(UA_EventLoopPOSIX *el) {
     return UA_STATUSCODE_GOOD;
 }
 
+static void
+UA_EventLoopPOSIX_lock(UA_EventLoop *public_el) {
+    UA_EventLoopPOSIX *el = (UA_EventLoopPOSIX*)public_el;
+    UA_LOCK(&el->elMutex);
+}
+
+static void
+UA_EventLoopPOSIX_unlock(UA_EventLoop *public_el) {
+    UA_EventLoopPOSIX *el = (UA_EventLoopPOSIX*)public_el;
+    UA_UNLOCK(&el->elMutex);
+}
+
 UA_EventLoop *
 UA_EventLoop_new_POSIX(const UA_Logger *logger) {
     UA_EventLoopPOSIX *el = (UA_EventLoopPOSIX*)
@@ -462,6 +474,9 @@ UA_EventLoop_new_POSIX(const UA_Logger *logger) {
         (UA_StatusCode (*)(UA_EventLoop*, UA_EventSource*))
         UA_EventLoopPOSIX_deregisterEventSource;
 
+    el->eventLoop.lock = UA_EventLoopPOSIX_lock;
+    el->eventLoop.unlock = UA_EventLoopPOSIX_unlock;
+
     return &el->eventLoop;
 }
 

include/open62541/plugin/eventloop.h

@@ -198,6 +198,18 @@ struct UA_EventLoop {
     /* Stops the EventSource before deregistrering it */
     UA_StatusCode
     (*deregisterEventSource)(UA_EventLoop *el, UA_EventSource *es);
+
+    /* Locking
+     * ~~~~~~~
+     *
+     * For multi-threading the EventLoop is protected by a mutex. The mutex is
+     * expected to be recursive (can be taken more than once from the same
+     * thread). A common approach to avoid deadlocks is to establish an absolute
+     * ordering between the locks. Where the "lower" locks needs to be taken
+     * before the "upper" lock. The EventLoop-mutex is exposed here to allow it
+     * to be taken from the outside. */
+    void (*lock)(UA_EventLoop *el);
+    void (*unlock)(UA_EventLoop *el);
 };
 
 /**