using System.Runtime.ExceptionServices;

namespace Umbraco.Cms.Core.Cache;

public static class SafeLazy
{
    // an object that represent a value that has not been created yet
    internal static readonly object ValueNotCreated = new();

    public static Lazy<object?> GetSafeLazy(Func<object?> getCacheItem) =>

        // try to generate the value and if it fails,
        // wrap in an ExceptionHolder - would be much simpler
        // to just use lazy.IsValueFaulted alas that field is
        // internal
        new Lazy<object?>(() =>
        {
            try
            {
                return getCacheItem();
            }
            catch (Exception e)
            {
                return new ExceptionHolder(ExceptionDispatchInfo.Capture(e));
            }
        });

    public static object? GetSafeLazyValue(Lazy<object?>? lazy, bool onlyIfValueIsCreated = false)
    {
        // if onlyIfValueIsCreated, do not trigger value creation
        // must return something, though, to differentiate from null values
        if (onlyIfValueIsCreated && lazy?.IsValueCreated == false)
        {
            return ValueNotCreated;
        }

        // if execution has thrown then lazy.IsValueCreated is false
        // and lazy.IsValueFaulted is true (but internal) so we use our
        // own exception holder (see Lazy<T> source code) to return null
        if (lazy?.Value is ExceptionHolder)
        {
            return null;
        }

        // we have a value and execution has not thrown so returning
        // here does not throw - unless we're re-entering, take care of it
        try
        {
            return lazy?.Value;
        }
        catch (InvalidOperationException e)
        {
            throw new InvalidOperationException(
                "The method that computes a value for the cache has tried to read that value from the cache.", e);
        }
    }

    public class ExceptionHolder
    {
        public ExceptionHolder(ExceptionDispatchInfo e) => Exception = e;

        public ExceptionDispatchInfo Exception { get; }
    }
}