GetRelationalModel throws "The model must be finalized" when two threads make the first call at once (check-then-act on RelationalModelFactory) #39214

// src/EFCore.Relational/Extensions/RelationalModelExtensions.cs (v10.0.12, unchanged on main)
public static IRelationalModel GetRelationalModel(this IModel model)
{
    var relationalModel = (IRelationalModel?)model.FindRuntimeAnnotationValue(RelationalAnnotationNames.RelationalModel);
    if (relationalModel == null)
    {
        var relationalModelFactory = (Func<IRelationalModel>?)model.FindRuntimeAnnotationValue(
                RelationalAnnotationNames.RelationalModelFactory)
            ?? throw new InvalidOperationException(CoreStrings.ModelNotFinalized(nameof(GetRelationalModel)));
        lock (relationalModelFactory)
        {
            relationalModel = model.GetOrAddRuntimeAnnotationValue(
                RelationalAnnotationNames.RelationalModel, f => f!(), relationalModelFactory);
            model.RemoveRuntimeAnnotation(RelationalAnnotationNames.RelationalModelFactory);
        }
    }

    return relationalModel;
}

using Microsoft.EntityFrameworkCore;
using Microsoft.EntityFrameworkCore.Infrastructure;
using Microsoft.EntityFrameworkCore.Metadata;
using Microsoft.EntityFrameworkCore.Metadata.Internal;

var model = new PausingModel();
var relational = new RelationalModel(model);
model.AddRuntimeAnnotation(RelationalAnnotationNames.RelationalModelFactory, (Func<IRelationalModel>)(() => relational));
Exception? failure = null;
var b = new Thread(() =>
{
    try { model.GetRelationalModel(); }
    catch (Exception e) { failure = e; }
});
model.Held = b;
b.Start();
model.Reached.Wait();                 // B has read RelationalModel == null
var a = model.GetRelationalModel();   // A: the first touch completes
model.Release.Set();
b.Join();
Console.WriteLine($"A got the relational model: {ReferenceEquals(a, relational)}");
Console.WriteLine($"B: {failure?.GetType().Name}: {failure?.Message}");

internal sealed class PausingModel : RuntimeModel
{
    public Thread? Held;
    public readonly ManualResetEventSlim Reached = new();
    public readonly ManualResetEventSlim Release = new();

    public override Annotation? FindRuntimeAnnotation(string name)
    {
        var annotation = base.FindRuntimeAnnotation(name);
        if (name == RelationalAnnotationNames.RelationalModel && annotation is null && Thread.CurrentThread == Held)
        {
            Reached.Set();
            Release.Wait();
        }

        return annotation;
    }
}

var relationalModel = (IRelationalModel?)model.FindRuntimeAnnotationValue(RelationalAnnotationNames.RelationalModel);
if (relationalModel == null)
{
    var relationalModelFactory = (Func<IRelationalModel>?)model.FindRuntimeAnnotationValue(
        RelationalAnnotationNames.RelationalModelFactory);
    if (relationalModelFactory == null)
    {
        // another thread may have completed the first touch between the two reads
        return (IRelationalModel?)model.FindRuntimeAnnotationValue(RelationalAnnotationNames.RelationalModel)
            ?? throw new InvalidOperationException(CoreStrings.ModelNotFinalized(nameof(GetRelationalModel)));
    }

    lock (relationalModelFactory)
    {
        relationalModel = model.GetOrAddRuntimeAnnotationValue(
            RelationalAnnotationNames.RelationalModel, f => f!(), relationalModelFactory);
        model.RemoveRuntimeAnnotation(RelationalAnnotationNames.RelationalModelFactory);
    }
}

return relationalModel;