MonitoringService: fix SetKernel/CollectAsync race with lock-free state snapshot

[Copilot]

• Remove SemaphoreSlim _kernelLock and all associated lock/acquire/release code
• Bundle _kernel + all _prev* + device-state into a single inner KernelState class
• Replace eleven separate instance fields with a single volatile KernelState _state
• SetKernel(): build new KernelState fully, then atomically assign _state = newState (no blocking, no locks)
• CollectAsync(): capture _state once per tick into a local; use it throughout — no lock needed
• SubscribeDevice/OnIoRequestCompleted close over their own KernelState — IO events always go to the correct state
• Reorder SetKernel: publish _state = newState before Clear() calls so CollectAsync sees the new state before metrics are wiped
• Build passes with 0 warnings, 0 errors; CodeQL finds 0 alerts

Original prompt

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This section details on the original issue you should resolve

<issue_title>copilot: Synchronize SetKernel/CollectAsync shared state in MonitoringService</issue_title>
<issue_description>Context: Copilot review comment on PR #31
#31 (comment)

File: src/ProcSim.Core/Monitoring/MonitoringService.cs

Offending snippet (reads shared state without synchronization)

// MonitoringService.cs (main @ 87e19c808f6e9a21213fd45805a481fab355d34e)

private async Task CollectAsync(CancellationToken ct)
{
    while (await _timer.WaitForNextTickAsync(ct))
    {
        if (_kernel is null || _kernel.GlobalCycle == 0)
            continue;

        DateTime ts = DateTime.UtcNow;
        ulong nowTick = _kernel.GlobalCycle;

        foreach (Cpu cpu in _kernel.Cpus.Values)
        {
            CpuSnapshot prev = _prevCpu[cpu.Id];
            ...
        }
    }
}

Related mutation/reset happens in SetKernel():

public void SetKernel(Kernel kernel)
{
    _kernel = kernel;

    _prevCpu.Clear();
    _prevProcCycles.Clear();
    _prevChan.Clear();
    ...
}

Copilot notes that CollectAsync reads shared mutable state (_kernel and various *_prev* dictionaries) without synchronization, while SetKernel() can run concurrently (sets _kernel and clears dictionaries). This can lead to observing partially-reset state and runtime exceptions (e.g., _kernel becoming null between reads, or metric collection indexing into cleared dictionaries).

Acceptance criteria

• Prevent races between SetKernel() and metric collection (CollectAsync and helpers).
• Ensure metric collection uses a stable snapshot of the kernel + per-kernel metric state (or synchronized access).
• No possible NullReferenceException / KeyNotFoundException due to concurrent resets.
• Tests/build/CI pass.</issue_description>

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

• Fixes copilot: Synchronize SetKernel/CollectAsync shared state in MonitoringService #32

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[Copilot]

Pull request overview

This PR fixes a race condition in MonitoringService between SetKernel() (which could clear and re-initialize shared state dictionaries) and CollectAsync() (which reads that same shared state concurrently). The fix introduces a KernelState inner class that bundles the kernel reference with all associated per-tick state, then publishes it as a single volatile reference swap, so CollectAsync always operates on a coherent snapshot.

Changes:

• Added a KernelState inner class bundling Kernel, all _prev* dicts, and IO tracking collections into one object.
• Replaced eleven separate mutable instance fields with a single private volatile KernelState _state; SetKernel builds the new state off the hot path and publishes it atomically.
• Updated CollectAsync and all Collect* / SubscribeDevice / OnIoRequestCompleted methods to pass KernelState state as a parameter, making event handlers close over their creation-time state.

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[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud

[Copilot]

Pull request overview

Copilot reviewed 1 out of 1 changed files in this pull request and generated no new comments.

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