using System;
using System.Threading;
using System.Threading.Tasks;
using MediaBrowser.Common.Configuration;
using MediaBrowser.Controller.Entities;
using Microsoft.Extensions.Hosting;
using Microsoft.Extensions.Logging;

namespace Emby.Server.Implementations.Configuration
{
    /// <summary>
    /// Applies the configuration invalidations published by the other instances sharing this
    /// configuration directory, dropping the local cache entry so the next read comes off the shared file.
    /// </summary>
    public sealed class ConfigurationInvalidationSubscriber : IHostedService
    {
        private readonly IConfigurationInvalidationBus _bus;
        private readonly IConfigurationManager _configurationManager;
        private readonly ILogger<ConfigurationInvalidationSubscriber> _logger;

        /// <summary>
        /// Initializes a new instance of the <see cref="ConfigurationInvalidationSubscriber"/> class.
        /// </summary>
        /// <param name="bus">The invalidation bus.</param>
        /// <param name="configurationManager">The configuration manager holding the cached configuration.</param>
        /// <param name="logger">The logger.</param>
        public ConfigurationInvalidationSubscriber(
            IConfigurationInvalidationBus bus,
            IConfigurationManager configurationManager,
            ILogger<ConfigurationInvalidationSubscriber> logger)
        {
            _bus = bus;
            _configurationManager = configurationManager;
            _logger = logger;
        }

        /// <inheritdoc />
        public Task StartAsync(CancellationToken cancellationToken)
        {
            _bus.Subscribe(Apply);
            return Task.CompletedTask;
        }

        /// <inheritdoc />
        public Task StopAsync(CancellationToken cancellationToken) => Task.CompletedTask;

        private void Apply(ConfigurationInvalidation invalidation)
        {
            try
            {
                // Applying re-raises the same update events a local save raises, so the in-process
                // consumers re-read. The scope tells those consumers that the write was somebody else's,
                // so the ones that answer an update by writing neither repeat it nor publish it back.
                using var scope = ConfigurationInvalidationContext.BeginApply();

                switch (invalidation.Scope)
                {
                    case ConfigurationInvalidationScope.SystemConfiguration:
                        _configurationManager.InvalidateCachedConfiguration(null);
                        break;
                    case ConfigurationInvalidationScope.NamedConfiguration when !string.IsNullOrEmpty(invalidation.Target):
                        _configurationManager.InvalidateCachedConfiguration(invalidation.Target);
                        break;
                    case ConfigurationInvalidationScope.LibraryOptions when !string.IsNullOrEmpty(invalidation.Target):
                        CollectionFolder.InvalidateLibraryOptions(invalidation.Target);
                        break;
                    case ConfigurationInvalidationScope.AllLibraryOptions:
                        CollectionFolder.InvalidateAllLibraryOptions();
                        break;
                    default:
                        return;
                }

                _logger.LogDebug("Applied {Scope} invalidation for {Target} from {OriginId}.", invalidation.Scope, invalidation.Target, invalidation.OriginId);
            }
            catch (Exception ex)
            {
                _logger.LogWarning(ex, "Failed to apply a {Scope} invalidation for {Target}.", invalidation.Scope, invalidation.Target);
            }
        }
    }
}