Fix client Close method to properly wait for outstanding publish requests

[Copilot]

Problem

The OPC UA client's Close method does not properly terminate outstanding publish requests before closing the session, which violates the OPC UA standard and can lead to ServiceFault responses from the server.

According to OPC UA Part 4, Section 5.6.4: "Clients are urged to wait for all outstanding requests to complete before submitting the CloseSession request."

Current Behavior

When closing a session with active subscriptions:

1. The session is closed immediately
2. Outstanding publish requests are left pending
3. The server may return ServiceFault responses for these orphaned requests

Expected Behavior

Before closing a session, the client should:

1. Stop sending new publish requests
2. Wait for outstanding publish requests to complete
3. Cancel any remaining requests that don't complete in time
4. Then send the CloseSession request

Solution

This PR adds proper handling of outstanding publish requests during session close:

1. New Configuration Property

Added PublishRequestCancelWaitTimeout property to control the wait behavior:

// Default: Wait up to 5 seconds before canceling
await session.CloseAsync();

// Cancel immediately without waiting
session.PublishRequestCancelWaitTimeout = 0;
await session.CloseAsync();

// Wait indefinitely for all requests to complete
session.PublishRequestCancelWaitTimeout = -1;
await session.CloseAsync();

// Custom timeout
session.PublishRequestCancelWaitTimeout = 10000; // 10 seconds
await session.CloseAsync();

2. Enhanced Close Flow

The CloseAsync method now:

1. Stops the keep-alive timer (prevents new publish requests)
2. Identifies outstanding publish requests
3. Waits for them to complete (up to configured timeout)
4. Cancels remaining requests using the OPC UA Cancel service
5. Proceeds with the CloseSession request

3. Implementation Details

• Helper Method: WaitForOrCancelOutstandingPublishRequestsAsync() handles the wait/cancel logic
• Cancellation Support: Properly handles CancellationToken for early termination
• Logging: Logs information about outstanding requests and cancellation actions
• Error Handling: Continues with session close even if cancel operations fail

Testing

• Added unit tests to verify outstanding requests are properly cancelled
• Added tests to verify property configuration works correctly
• All existing tests pass with no regressions
• Build completes with 0 warnings and 0 errors

Breaking Changes

None. This change is backward compatible with a sensible default timeout of 5 seconds.

Benefits

✅ Standards Compliant: Follows OPC UA Part 4 recommendations
✅ Reduces ServiceFaults: Prevents errors from orphaned publish requests
✅ Configurable: Applications can choose the appropriate wait strategy
✅ Backward Compatible: Default behavior is a reasonable compromise
✅ Reliable: Improves client stability during shutdown

Fixes #1821

Original prompt

---

This section details on the original issue you should resolve

<issue_title>The client close method does not properly terminate the outstanding requests</issue_title>
<issue_description>### Type of issue

• Bug
• Enhancement
• Compliance
• Question
• Help wanted

Current Behavior

Close method closes sessions without stopping/waiting outstanding request to properly terminated.

Expected Behavior

According to the OPCUA standard: Clients are urged to wait for all outstanding requests to complete before submitting the CloseSession request. (extracted from 5.6.4 CloseSesison OPC Unified Architecture, Part 4)

The client does not call any cancel service before calling the closeSession request. In consequence, the client can receive ServiceFault result for publish request that have not been properly terminated (by waiting the response or calling the cancel service on it)

I think that the the Close(int timeout) method should stop the monitorItems publishing and call the cancel service on outstanding requests before sending the closeSession request.

Regards

Steps To Reproduce

1. Compile Sample client
2. Connect to a server in none security
3. Create a session, a subscription, a monitorItems
4. Launch wireshark
5. Close the session
   6.The server mays has sent Fault answer to the last publish service request sent by the client.

Environment

- OS: Windows 10
- Environment:Visual studio 2019 16.11.3
- Runtime: NT 4.6.2
- Nuget Version:1.4.368.33
- Component:Opc.Ua.Client
- Server:ReferenceServer
- Client: ReferenceClient

Anything else?

Opc.Ua.Client\Session.cs\public virtual StatusCode Close(int timeout)</issue_description>

<agent_instructions>Please update the Client Library to cancel outstanding publish requests on Session Close. Please add a Property that defines a wait timeout before publish requests are cancelled.</agent_instructions>

Comments on the Issue (you are @copilot in this section)

@mregen @GregoireG-C , since subscription can be transferred, the Close method does not try to close subscriptions or cancel publish request by default. @romanett I do not fully agree to the explanation. Outstanding publish requests are not transferred to the new session. So on session close they shall all be cancelled or waited for completion by the client. Also the client shall stop publishing when close is called.

Fixes #1778

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[CLAassistant]

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1 out of 2 committers have signed the CLA.

✅ romanett
❌ Copilot
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[codecov]

Codecov Report

❌ Patch coverage is 84.25926% with 17 lines in your changes missing coverage. Please review.
✅ Project coverage is 57.91%. Comparing base (8152fee) to head (4490246).
⚠️ Report is 4 commits behind head on master.

Files with missing lines	Patch %	Lines
Libraries/Opc.Ua.Client/Session/Session.cs	85.84%	12 Missing and 3 partials ⚠️
...ibraries/Opc.Ua.Client/Session/TraceableSession.cs	0.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #3254      +/-   ##
==========================================
+ Coverage   57.85%   57.91%   +0.05%     
==========================================
  Files         365      365              
  Lines       79806    79882      +76     
  Branches    13882    13897      +15     
==========================================
+ Hits        46171    46261      +90     
+ Misses      29490    29479      -11     
+ Partials     4145     4142       -3     

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[salihgoncu]

Seems to be doing what is intended. - I have a couple of comments before going ahead and merging:

1. The timeout is fixed at 5 seconds. - Wouldn't it be good to have it configurable or enforcable? - Windows Service Manager has a default timeout of 30 seconds. A service doing lots of cleanup during shutdown may cause errors in SCM.
2. When the session close request comes to server, shouldn't the server invalidate any pending requests to that session? - Feels like it is more of a correct approach as the session closure can be either explicit by the client or implicit due to a loss of communication. - Regardless, the server needs to cleanup the resources. - This is just increasing the complexity at the client side.

[romanett]

@salihgoncu

1. The delay can be configured.
2. Yes this is true but the spec also says the Client should do this