Problem

When implementing an OPC UA server with event history support, the HistoryRead bit (0x04) in the EventNotifier attribute of the Server object was not being set, even when AccessHistoryEventsCapability was configured to true in the ServerCapabilities. This prevented OPC UA clients from discovering that the server supports historical event reads on the Server object.

According to the OPC UA specification, when a server supports history read capabilities, the Server object's EventNotifier should have the HistoryRead bit set to indicate this capability to clients.

Solution

This PR implements a systematic approach to synchronize the Server object's EventNotifier with the configured history capabilities:

1. New Virtual Method in DiagnosticsNodeManager

Added UpdateServerEventNotifier() method that:

• Reads the HistoryServerCapabilities configuration
• Sets the HistoryRead bit (0x04) when AccessHistoryEventsCapability or AccessHistoryDataCapability is enabled
• Sets the HistoryWrite bit (0x08) when any insert/update/replace event or data capability is enabled
• Clears these bits when the corresponding capabilities are disabled
• Is declared as public virtual to allow customization in derived classes

2. Automatic Initialization in ServerInternalData

Modified CreateServerObject() to:

• Call GetDefaultHistoryCapabilities() to ensure history capabilities are initialized
• Call UpdateServerEventNotifier() to update the Server EventNotifier based on the configured capabilities
• This ensures the Server object's EventNotifier is properly set during server startup

3. Integration Test

Added ServerEventNotifierHistoryReadBit test in ReferenceServerTest.cs that verifies:

• The Server object's EventNotifier can be read correctly
• The correlation between history capabilities and EventNotifier bits
• Proper handling of both AccessHistoryEventsCapability and AccessHistoryDataCapability

Example Usage

After this fix, servers with history capabilities enabled will automatically have the correct EventNotifier bits set:

// Configure history capabilities
var historyCapabilities = diagnosticsNodeManager.GetDefaultHistoryCapabilities();
historyCapabilities.AccessHistoryEventsCapability.Value = true;

// During server initialization, UpdateServerEventNotifier() is called automatically
// The Server object's EventNotifier will now have bit 0x04 (HistoryRead) set

For runtime updates:

// Modify capabilities after initialization
historyCapabilities.AccessHistoryDataCapability.Value = true;

// Manually trigger update
diagnosticsNodeManager.UpdateServerEventNotifier();

Compatibility

• ✅ Backward compatible - no breaking changes to existing APIs
• ✅ Virtual method allows for customization in derived classes
• ✅ Automatic initialization works transparently for existing servers
• ✅ All existing tests pass (23/23 ReferenceServerTests)
• ✅ No security vulnerabilities detected

Addresses

Fixes the issue described in the original problem statement where setting AccessHistoryEventsCapability to true should result in the HistoryRead bit being set on the EventNotifier attribute of the Server object.

Co-authored-by: @romanett

Fixes #2081

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