nucleic-windows/NucleicApp/Services/HostConnection.cs

using NucleicProtocol.Interop;

namespace NucleicApp.Services;

public enum HostConnectionState
{
    Stopped,
    Connecting,
    Connected,
    Reconnecting,
    Failed,
}

/// Owns one native client handle for one host. Native callbacks arrive on the DLL's dedicated
/// thread and are marshalled onto the renderer dispatcher before state is projected.
public sealed class HostConnection : IAsyncDisposable
{
    private readonly object gate = new();
    private readonly RendererStore store;
    private readonly IProtocolClientFactory clientFactory;
    private readonly IRendererDispatcher dispatcher;
    private readonly string identityDirectory;
    private readonly string deviceLabel;
    private IProtocolClient? client;
    private CancellationTokenSource? lifetime;
    private Task? retryTask;
    private string? rendezvousPath;
    private int retryAttempt;

    public HostConnectionState State { get; private set; } = HostConnectionState.Stopped;
    public ProtocolState? LastProtocolState { get; private set; }
    public event Action<HostConnectionState>? StateChanged;

    public HostConnection(
        RendererStore store, string identityDirectory,
        IProtocolClientFactory? clientFactory = null,
        IRendererDispatcher? dispatcher = null,
        string deviceLabel = "Nucleic for Windows")
    {
        this.store = store;
        this.identityDirectory = identityDirectory;
        this.clientFactory = clientFactory ?? new ProtocolClientFactory();
        this.dispatcher = dispatcher ?? InlineRendererDispatcher.Instance;
        this.deviceLabel = deviceLabel;
    }

    public async Task StartLocalAsync(string rendezvousPath, CancellationToken cancellationToken = default)
    {
        lock (gate)
        {
            if (lifetime is not null) throw new InvalidOperationException("connection is already started");
            var createdLifetime = CancellationTokenSource.CreateLinkedTokenSource(cancellationToken);
            IProtocolClient? createdClient = null;
            try
            {
                createdClient = clientFactory.Create(new ProtocolClientConfiguration
                {
                    IdentityDirectory = identityDirectory,
                    DeviceLabel = deviceLabel,
                    Channel = BuildChannel.Current,
                });
                createdClient.MessageJsonReceived += OnMessage;
                createdClient.StateJsonReceived += OnState;
                this.rendezvousPath = rendezvousPath;
                lifetime = createdLifetime;
                client = createdClient;
            }
            catch
            {
                createdClient?.Dispose();
                createdLifetime.Dispose();
                throw;
            }
        }
        if (await ConnectNowAsync(initial: true, lifetime.Token).ConfigureAwait(false)
            != ProtocolResult.Ok)
        {
            ScheduleReconnect();
        }
    }

    public ProtocolResult SendIntent(string json)
    {
        lock (gate) return client?.SendIntent(json) ?? ProtocolResult.InvalidState;
    }

    private async Task<ProtocolResult> ConnectNowAsync(
        bool initial, CancellationToken cancellationToken)
    {
        IProtocolClient active;
        string path;
        lock (gate)
        {
            active = client ?? throw new InvalidOperationException("connection has no client");
            path = rendezvousPath ?? throw new InvalidOperationException("connection has no rendezvous");
        }
        SetState(initial ? HostConnectionState.Connecting : HostConnectionState.Reconnecting);
        var record = await RendezvousFile.ReadAsync(path, cancellationToken).ConfigureAwait(false);
        var result = active.ConnectLocal(record);
        if (result != ProtocolResult.Ok)
            SetState(HostConnectionState.Failed);
        return result;
    }

    private void OnMessage(string json) => dispatcher.Post(() => store.Apply(json));

    private void OnState(string json)
    {
        ProtocolState state;
        try { state = ProtocolState.Parse(json); }
        catch { return; }
        dispatcher.Post(() =>
        {
            LastProtocolState = state;
            switch (state.State)
            {
                case "connecting": SetState(HostConnectionState.Connecting); break;
                case "ready":
                    retryAttempt = 0;
                    SetState(HostConnectionState.Connected);
                    _ = SendIntent(ClientIntents.ListSessions());
                    _ = SendIntent(ClientIntents.ListDashboard());
                    _ = SendIntent(ClientIntents.ListPeers());
                    break;
                case "failed": SetState(HostConnectionState.Failed); ScheduleReconnect(); break;
                case "closed": ScheduleReconnect(); break;
            }
        });
    }

    private void ScheduleReconnect()
    {
        lock (gate)
        {
            if (lifetime is null || lifetime.IsCancellationRequested || retryTask is { IsCompleted: false })
                return;
            var cancellationToken = lifetime.Token;
            var delay = TimeSpan.FromMilliseconds(Math.Min(30_000, 250 * Math.Pow(2, retryAttempt++)));
            retryTask = Task.Run(async () =>
            {
                while (!cancellationToken.IsCancellationRequested)
                {
                    try
                    {
                        await Task.Delay(delay, cancellationToken).ConfigureAwait(false);
                        if (await ConnectNowAsync(initial: false, cancellationToken)
                            .ConfigureAwait(false) == ProtocolResult.Ok)
                        {
                            return;
                        }
                    }
                    catch (OperationCanceledException) { return; }
                    catch { SetState(HostConnectionState.Failed); }

                    // A synchronous admission failure (notably InvalidState while the native
                    // close callback is just ahead of its task cleanup) must not strand the
                    // connection. The old recursive scheduler observed its own retry task as
                    // active and silently declined to schedule another attempt.
                    delay = TimeSpan.FromMilliseconds(
                        Math.Min(30_000, 250 * Math.Pow(2, retryAttempt++)));
                }
            }, cancellationToken);
        }
    }

    private void SetState(HostConnectionState next)
    {
        if (State == next) return;
        State = next;
        StateChanged?.Invoke(next);
    }

    public async ValueTask DisposeAsync()
    {
        CancellationTokenSource? cancellation;
        Task? retry;
        IProtocolClient? closing;
        lock (gate)
        {
            cancellation = lifetime;
            lifetime = null;
            retry = retryTask;
            retryTask = null;
            closing = client;
            client = null;
        }
        if (cancellation is not null) await cancellation.CancelAsync().ConfigureAwait(false);
        if (retry is not null) try { await retry.ConfigureAwait(false); } catch (OperationCanceledException) { }
        closing?.Dispose();
        cancellation?.Dispose();
        SetState(HostConnectionState.Stopped);
    }
}

internal static class BuildChannel
{
    public static string Current
    {
        get
        {
#if NUCLEIC_STABLE
            return "release";
#elif NUCLEIC_RC
            return "rc";
#elif NUCLEIC_BETA
            return "beta";
#elif NUCLEIC_CANARY
            return "canary";
#else
            return "local";
#endif
        }
    }

    public static string Suffix => Current switch
    {
        "release" => string.Empty,
        "rc" => "-rc",
        "beta" => "-beta",
        "canary" => "-canary",
        _ => "-local",
    };
}