// Licensed to the .NET Foundation under one or more agreements.
// The .NET Foundation licenses this file to you under the MIT license.

#nullable disable

using System.Diagnostics;
using System.Runtime;

namespace Microsoft.NET.Sdk.Razor.Tool
{
    // Heavily influenced by:
    // https://github.com/dotnet/roslyn/blob/14aed138a01c448143b9acf0fe77a662e3dfe2f4/src/Compilers/Server/ServerShared/ServerDispatcher.cs#L15
    internal class DefaultRequestDispatcher : RequestDispatcher
    {
        private readonly CancellationToken _cancellationToken;
        private readonly CompilerHost _compilerHost;
        private readonly ConnectionHost _connectionHost;
        private readonly EventBus _eventBus;

        private KeepAlive _keepAlive;
        private State _state;
        private Task _timeoutTask;
        private Task _gcTask;
        private Task<Connection> _listenTask;
        private CancellationTokenSource _listenCancellationTokenSource;
        private List<Task<ConnectionResult>> _connections = new();

        public DefaultRequestDispatcher(
            ConnectionHost connectionHost,
            CompilerHost compilerHost,
            CancellationToken cancellationToken,
            EventBus eventBus = null,
            TimeSpan? keepAlive = null)
        {
            _connectionHost = connectionHost;
            _compilerHost = compilerHost;
            _cancellationToken = cancellationToken;

            _eventBus = eventBus ?? EventBus.Default;

            var keepAliveTimeout = DefaultServerKeepAlive;
            if (keepAlive.HasValue)
            {
                keepAliveTimeout = keepAlive.Value;
            }
            _keepAlive = new KeepAlive(keepAliveTimeout, isDefault: true);
        }

        // The server accepts connections until we reach a state that requires a shutdown. At that
        // time no new connections will be accepted and the server will drain existing connections.
        //
        // The idea is that it's better to let clients fallback to in-proc (and slow down) than it is to keep
        // running in an undesired state.
        public override void Run()
        {
            _state = State.Running;

            try
            {
                Listen();

                do
                {
                    Debug.Assert(_listenTask != null);

                    MaybeCreateTimeoutTask();
                    MaybeCreateGCTask();
                    WaitForAnyCompletion(_cancellationToken);
                    CheckCompletedTasks(_cancellationToken);
                }
                while (_connections.Count > 0 || _state == State.Running);
            }
            finally
            {
                _state = State.Completed;
                _gcTask = null;
                _timeoutTask = null;

                if (_listenTask != null)
                {
                    CloseListenTask();
                }
            }
        }


        private void CheckCompletedTasks(CancellationToken cancellationToken)
        {
            if (cancellationToken.IsCancellationRequested)
            {
                HandleCancellation();
                return;
            }

            if (_listenTask.IsCompleted)
            {
                HandleCompletedListenTask(cancellationToken);
            }

            if (_timeoutTask?.IsCompleted == true)
            {
                HandleCompletedTimeoutTask();
            }

            if (_gcTask?.IsCompleted == true)
            {
                HandleCompletedGCTask();
            }

            HandleCompletedConnections();
        }

        private void HandleCancellation()
        {
            Debug.Assert(_listenTask != null);

            // If cancellation has been requested then the server needs to be in the process
            // of shutting down.
            _state = State.ShuttingDown;

            CloseListenTask();

            try
            {
                Task.WaitAll(_connections.ToArray());
            }
            catch
            {
                // It's expected that some will throw exceptions, in particular OperationCanceledException.  It's
                // okay for them to throw so long as they complete.
            }

            HandleCompletedConnections();
            Debug.Assert(_connections.Count == 0);
        }

        /// <summary>
        /// The server farms out work to Task values and this method needs to wait until at least one of them
        /// has completed.
        /// </summary>
        private void WaitForAnyCompletion(CancellationToken cancellationToken)
        {
            var all = new List<Task>(_connections.Count + 3);
            all.AddRange(_connections);
            all.Add(_timeoutTask);
            all.Add(_listenTask);
            all.Add(_gcTask);

            try
            {
                var waitArray = all.Where(x => x != null).ToArray();
                Task.WaitAny(waitArray, cancellationToken);
            }
            catch (OperationCanceledException)
            {
                // Thrown when the provided cancellationToken is cancelled.  This is handled in the caller,
                // here it just serves to break out of the WaitAny call.
            }
        }

        private void Listen()
        {
            Debug.Assert(_listenTask == null);
            Debug.Assert(_timeoutTask == null);

            _listenCancellationTokenSource = new CancellationTokenSource();
            _listenTask = _connectionHost.WaitForConnectionAsync(_listenCancellationTokenSource.Token);
            _eventBus.ConnectionListening();
        }

        private void CloseListenTask()
        {
            Debug.Assert(_listenTask != null);

            _listenCancellationTokenSource.Cancel();
            _listenCancellationTokenSource = null;
            _listenTask = null;
        }

        private void HandleCompletedListenTask(CancellationToken cancellationToken)
        {
            _eventBus.ConnectionReceived();

            // Don't accept any new connections once we're in shutdown mode, instead gracefully reject the request.
            // This should cause the client to run in process.
            var accept = _state == State.Running;
            var connectionTask = AcceptConnection(_listenTask, accept, cancellationToken);
            _connections.Add(connectionTask);

            // Timeout and GC are only done when there are no active connections.  Now that we have a new
            // connection cancel out these tasks.
            _timeoutTask = null;
            _gcTask = null;

            // Begin listening again for new connections.
            _listenTask = null;
            Listen();
        }

        private void HandleCompletedTimeoutTask()
        {
            _eventBus.KeepAliveReached();
            _listenCancellationTokenSource.Cancel();
            _timeoutTask = null;
            _state = State.ShuttingDown;
        }

        private void HandleCompletedGCTask()
        {
            _gcTask = null;
            for (var i = 0; i < 10; i++)
            {
                GC.Collect();
                GC.WaitForPendingFinalizers();
            }

            GCSettings.LargeObjectHeapCompactionMode = GCLargeObjectHeapCompactionMode.CompactOnce;
            GC.Collect();
        }

        private void MaybeCreateTimeoutTask()
        {
            // If there are no active clients running then the server needs to be in a timeout mode.
            if (_connections.Count == 0 && _timeoutTask == null)
            {
                Debug.Assert(_listenTask != null);
                _timeoutTask = Task.Delay(_keepAlive.TimeSpan);
            }
        }

        private void MaybeCreateGCTask()
        {
            if (_connections.Count == 0 && _gcTask == null)
            {
                _gcTask = Task.Delay(GCTimeout);
            }
        }

        /// <summary>
        /// Checks the completed connection objects.
        /// </summary>
        /// <returns>False if the server needs to begin shutting down</returns>
        private void HandleCompletedConnections()
        {
            var shutdown = false;
            var processedCount = 0;
            var i = 0;
            while (i < _connections.Count)
            {
                var current = _connections[i];
                if (!current.IsCompleted)
                {
                    i++;
                    continue;
                }

                _connections.RemoveAt(i);
                processedCount++;

                var result = current.Result;
                if (result.KeepAlive.HasValue)
                {
                    var updated = _keepAlive.Update(result.KeepAlive.Value);
                    if (updated.Equals(_keepAlive))
                    {
                        _eventBus.UpdateKeepAlive(updated.TimeSpan);
                    }
                }

                switch (result.CloseReason)
                {
                    case ConnectionResult.Reason.CompilationCompleted:
                    case ConnectionResult.Reason.CompilationNotStarted:
                        // These are all normal end states.  Nothing to do here.
                        break;

                    case ConnectionResult.Reason.ClientDisconnect:
                        // Have to assume the worst here which is user pressing Ctrl+C at the command line and
                        // hence wanting all compilation to end.
                        _eventBus.ConnectionRudelyEnded();
                        shutdown = true;
                        break;

                    case ConnectionResult.Reason.ClientException:
                    case ConnectionResult.Reason.ClientShutdownRequest:
                        _eventBus.ConnectionRudelyEnded();
                        shutdown = true;
                        break;

                    default:
                        throw new InvalidOperationException($"Unexpected enum value {result.CloseReason}");
                }
            }

            if (processedCount > 0)
            {
                _eventBus.ConnectionCompleted(processedCount);
            }

            if (shutdown)
            {
                _state = State.ShuttingDown;
            }
        }

        internal async Task<ConnectionResult> AcceptConnection(Task<Connection> task, bool accept, CancellationToken cancellationToken)
        {
            Connection connection;
            try
            {
                connection = await task;
            }
            catch (Exception ex)
            {
                // Unable to establish a connection with the client.  The client is responsible for
                // handling this case. Nothing else for us to do here.
                ServerLogger.LogException(ex, "Error creating client named pipe");
                return new ConnectionResult(ConnectionResult.Reason.CompilationNotStarted);
            }

            try
            {
                using (connection)
                {
                    ServerRequest request;
                    try
                    {
                        ServerLogger.Log("Begin reading request.");
                        request = await ServerRequest.ReadAsync(connection.Stream, cancellationToken).ConfigureAwait(false);
                        ServerLogger.Log("End reading request.");
                    }
                    catch (Exception e)
                    {
                        ServerLogger.LogException(e, "Error reading build request.");
                        return new ConnectionResult(ConnectionResult.Reason.CompilationNotStarted);
                    }

                    if (request.IsShutdownRequest())
                    {
                        // Reply with the PID of this process so that the client can wait for it to exit.
                        var response = new ShutdownServerResponse(Environment.ProcessId);
                        await response.WriteAsync(connection.Stream, cancellationToken);

                        // We can safely disconnect the client, then when this connection gets cleaned up by the event loop
                        // the server will go to a shutdown state.
                        return new ConnectionResult(ConnectionResult.Reason.ClientShutdownRequest);
                    }
                    else if (!accept)
                    {
                        // We're already in shutdown mode, respond gracefully so the client can run in-process.
                        var response = new RejectedServerResponse();
                        await response.WriteAsync(connection.Stream, cancellationToken).ConfigureAwait(false);

                        return new ConnectionResult(ConnectionResult.Reason.CompilationNotStarted);
                    }
                    else
                    {
                        // If we get here then this is a real request that we will accept and process.
                        //
                        // Kick off both the compilation and a task to monitor the pipe for closing.
                        var buildCancelled = CancellationTokenSource.CreateLinkedTokenSource(cancellationToken);

                        var watcher = connection.WaitForDisconnectAsync(buildCancelled.Token);
                        var worker = ExecuteRequestAsync(request, buildCancelled.Token);

                        // await will end when either the work is complete or the connection is closed.
                        await Task.WhenAny(worker, watcher);

                        // Do an 'await' on the completed task, preference being compilation, to force
                        // any exceptions to be realized in this method for logging.
                        ConnectionResult.Reason reason;
                        if (worker.IsCompleted)
                        {
                            var response = await worker;

                            try
                            {
                                ServerLogger.Log("Begin writing response.");
                                await response.WriteAsync(connection.Stream, cancellationToken);
                                ServerLogger.Log("End writing response.");

                                reason = ConnectionResult.Reason.CompilationCompleted;

                                _eventBus.CompilationCompleted();
                            }
                            catch
                            {
                                reason = ConnectionResult.Reason.ClientDisconnect;
                            }
                        }
                        else
                        {
                            await watcher;
                            reason = ConnectionResult.Reason.ClientDisconnect;
                        }

                        // Begin the tear down of the Task which didn't complete.
                        buildCancelled.Cancel();

                        return new ConnectionResult(reason, request.KeepAlive);
                    }
                }
            }
            catch (Exception ex)
            {
                ServerLogger.LogException(ex, "Error handling connection");
                return new ConnectionResult(ConnectionResult.Reason.ClientException);
            }
        }

        private Task<ServerResponse> ExecuteRequestAsync(ServerRequest buildRequest, CancellationToken cancellationToken)
        {
            Func<ServerResponse> func = () =>
            {
                ServerLogger.Log("Begin processing request");

                var response = _compilerHost.Execute(buildRequest, cancellationToken);

                ServerLogger.Log("End processing request");
                return response;
            };

            var task = new Task<ServerResponse>(func, cancellationToken, TaskCreationOptions.LongRunning);
            task.Start();
            return task;
        }

        private enum State
        {
            /// <summary>
            /// Server running and accepting all requests
            /// </summary>
            Running,

            /// <summary>
            /// Server processing existing requests, responding to shutdown commands but is not accepting
            /// new build requests.
            /// </summary>
            ShuttingDown,

            /// <summary>
            /// Server is done.
            /// </summary>
            Completed,
        }

        private struct KeepAlive
        {
            public TimeSpan TimeSpan;
            public bool IsDefault;

            public KeepAlive(TimeSpan timeSpan, bool isDefault)
            {
                TimeSpan = timeSpan;
                IsDefault = isDefault;
            }

            public KeepAlive Update(TimeSpan timeSpan)
            {
                if (IsDefault || timeSpan > TimeSpan)
                {
                    return new KeepAlive(timeSpan, isDefault: false);
                }

                return this;
            }
        }
    }
}