Crash when adding Nodes with Multithreading enabled

[Fischer-Thomas]

Description

Currently when working with the flag for multithreading enabled there is a problem when adding nodes to the server. The UA_NodeMap_releaseNode asserts that entry->refCount > 0 failed and shortly after the application crashes. Please see the attachment for the callstack.

Background Information / Reproduction Steps

1. Enable UA_ENABLE_MULTITHREADING when building the server
2. Start the server.
3. Add a node to the server with the second thread

Checklist

Please provide the following information:

• [ ] open62541 Version (release number or git tag):
• [ ] Other OPC UA SDKs used (client or server):
• [ ] Operating system:
• [ ] Logs (with UA_LOGLEVEL set as low as necessary) attached
• [ ] Wireshark network dump attached
• [ ] Self-contained code example attached
• [ ] Critical issue

[Pro]

Note that Multithreading support is far from being in beta stadium. Probably this issue will not be fixed for a long time, but feel free to create a PR yourself.

You will have many similar issues like this one. The current recommended way for a multithreaded application is that you use your own mutex to make sure you have mutual exclusion on the UA_Server pointer.