ChainedConfigurationSource doesn't support reloading manually

[davidfowl]

Description

This was found as a part of investigating dotnet/aspnetcore#36181. It seems like manually calling IConfigurationRoot.Reload on a chained configuration source doesn't work.

Configuration

var config = new ConfigurationBuilder()
        .Add(new RandomConfigurationSource())
        .Build();

var config2 = new ConfigurationBuilder()
            .AddConfiguration(config)
            .Build();

Console.WriteLine(config2["Random"]);

config2.Reload();

Console.WriteLine(config2["Random"]);

public class RandomConfigurationSource : IConfigurationSource
{
    public IConfigurationProvider Build(IConfigurationBuilder builder)
    {
        return new RandomConfigurationProvider();
    }
}

public class RandomConfigurationProvider : ConfigurationProvider
{
    public override void Load()
    {
        Data["Random"] = Guid.NewGuid().ToString();
    }
}

This should print 2 random guids, instead it prints the same guid. If you used a single configuration root (not chained), it would work.

Regression?

Nope

[OwnageIsMagic]

I'm actually (ab)using this behavior to reorder configuration providers, without rebuilding the whole tree.

var builder = WebApplication.CreateBuilder(Args);
builder.Configuration.InsertConsul( new Uri("..."),
 ^2, // insert before before env and args
 "config1", "config2", "config3"
);
// ...

static IConfigurationBuilder InsertConsul(this IConfigurationBuilder builder, Uri consulUri,
    Index insertPosition, params string[] cfgNodes)
{
    IConfigurationBuilder? oldBuilder = null;
    if (!insertPosition.Equals(^0)) // not to the end
    {
        oldBuilder = builder;
        builder = new ConfigurationBuilder();
    }

    foreach (var cfgNode in cfgNodes)
    {
        builder.AddConsul(cfgNode, options =>
        {
            options.ConsulConfigurationOptions = cco => cco.Address = consulUri;
        });
    }

    if (oldBuilder != null)
    {
        // WebApplication's ConfigurationManager eagerly rebuilds configuration on Sources modification
        // Create ChainedSource wrapper to add them as one item; as side effect it also disables reload
        // Otherwise (inserting directly into ConfigurationManager.Sources) it will
        // rebuild configuration tree on each Insert (makes 1+2+3+N blocking HTTP calls, instead of N)
        var source = new ChainedConfigurationSource { Configuration = builder.Build(), ShouldDisposeConfiguration = true };
        builder = oldBuilder;
        var sources = builder.Sources;
        sources.Insert(insertPosition.GetOffset(sources.Count), source);
    }

    return builder;
}

It would be nice to have some API to temporally pause eager configuration evaluation. Or some other way to reorder providers.
Can we just reorder providers and rebuild lazily (on first access)?

[rosebyte]

Triage: we should fix it.