hash-of-wisdom/internal/server/tcp.go

package server

import (
	"context"
	"fmt"
	"io"
	"log/slog"
	"net"
	"sync"
	"time"

	"hash-of-wisdom/internal/application"
	"hash-of-wisdom/internal/lib/sl"
	"hash-of-wisdom/internal/metrics"
	"hash-of-wisdom/internal/protocol"
	"hash-of-wisdom/internal/service"
)

// TCPServer handles TCP connections for the Word of Wisdom protocol
type TCPServer struct {
	config            *Config
	wisdomApplication *application.WisdomApplication
	decoder           *protocol.MessageDecoder
	listener          net.Listener
	logger            *slog.Logger
	wg                sync.WaitGroup
	cancel            context.CancelFunc
}

// Option is a functional option for configuring TCPServer
type option func(*TCPServer)


// WithLogger sets a custom logger
func WithLogger(logger *slog.Logger) option {
	return func(s *TCPServer) {
		s.logger = logger
	}
}

// NewTCPServer creates a new TCP server with required configuration
func NewTCPServer(wisdomService *service.WisdomService, config *Config, opts ...option) *TCPServer {
	server := &TCPServer{
		config:            config,
		wisdomApplication: application.NewWisdomApplication(wisdomService),
		decoder:           protocol.NewMessageDecoder(),
		logger:            slog.Default(),
	}

	for _, opt := range opts {
		opt(server)
	}

	return server
}


// Start starts the TCP server
func (s *TCPServer) Start(ctx context.Context) error {
	listener, err := net.Listen("tcp", s.config.Address)
	if err != nil {
		return fmt.Errorf("failed to listen on %s: %w", s.config.Address, err)
	}

	s.listener = listener
	s.logger.Info("tcp server started", "address", s.config.Address)

	// Create cancellable context for server lifecycle
	serverCtx, cancel := context.WithCancel(ctx)
	s.cancel = cancel

	go s.acceptLoop(serverCtx)
	return nil
}

// Stop gracefully stops the server
func (s *TCPServer) Stop() error {
	s.logger.Info("stopping tcp server")

	// Cancel server context to stop accept loop and active connections
	if s.cancel != nil {
		s.cancel()
	}

	if s.listener != nil {
		s.listener.Close()
	}

	s.wg.Wait()
	s.logger.Info("tcp server stopped")
	return nil
}

// Address returns the server's listening address
func (s *TCPServer) Address() string {
	if s.listener != nil {
		return s.listener.Addr().String()
	}
	return s.config.Address
}

// acceptLoop accepts and handles incoming connections
func (s *TCPServer) acceptLoop(ctx context.Context) {
	for {
		select {
		case <-ctx.Done():
			return
		default:
		}

		rawConn, err := s.listener.Accept()
		if err != nil {
			select {
			case <-ctx.Done():
				return
			default:
				s.logger.Error("accept error", sl.Err(err))
				continue
			}
		}

		s.wg.Add(1)
		go func() {
			defer s.wg.Done()
			s.handleConnection(ctx, rawConn)
		}()
	}
}

// handleConnection handles a single client connection
func (s *TCPServer) handleConnection(ctx context.Context, rawConn net.Conn) {
	defer rawConn.Close()

	// Track active connections
	metrics.ActiveConnections.Inc()
	defer metrics.ActiveConnections.Dec()

	connLogger := s.logger.With("remote_addr", rawConn.RemoteAddr().String())
	connLogger.Info("connection accepted")

	// Create connection-scoped context for overall timeout
	connCtx, cancel := context.WithTimeout(ctx, s.config.Timeouts.Connection)
	defer cancel()

	if err := s.processConnection(connCtx, rawConn, connLogger); err != nil {
		connLogger.Error("connection error", sl.Err(err))
	} else {
		connLogger.Debug("connection completed successfully")
	}
}

// deadlineConn wraps a connection to automatically set deadlines on each read/write
type deadlineConn struct {
	net.Conn
	rto, wto time.Duration
}

func (d *deadlineConn) Read(p []byte) (int, error) {
	_ = d.SetReadDeadline(time.Now().Add(d.rto))
	return d.Conn.Read(p)
}

func (d *deadlineConn) Write(p []byte) (int, error) {
	_ = d.SetWriteDeadline(time.Now().Add(d.wto))
	return d.Conn.Write(p)
}

// processConnection handles the protocol message exchange
func (s *TCPServer) processConnection(ctx context.Context, conn net.Conn, logger *slog.Logger) error {
	// Set overall connection deadline
	globalDL := time.Now().Add(s.config.Timeouts.Connection)
	if err := conn.SetDeadline(globalDL); err != nil {
		return fmt.Errorf("failed to set connection deadline: %w", err)
	}

	// Create deadline wrapper for automatic timeout handling
	dc := &deadlineConn{
		Conn: conn,
		rto:  s.config.Timeouts.Read,
		wto:  s.config.Timeouts.Write,
	}

	// Use LimitReader to prevent reading more than the protocol-defined maximum message size
	// This protects against malicious clients that lie about payload length in headers
	limitedReader := io.LimitReader(dc, int64(protocol.MaxPayloadSize)+protocol.HeaderSize)

	// Read incoming message through limited reader
	logger.Debug("reading message from client")
	msg, err := s.decoder.Decode(limitedReader)
	if err != nil {
		if err == io.EOF {
			logger.Debug("client closed connection gracefully")
			return nil
		}
		metrics.RequestErrors.WithLabelValues("decode_error").Inc()
		logger.Error("failed to decode message", sl.Err(err))
		return fmt.Errorf("decode error: %w", err)
	}

	logger.Debug("message decoded", "type", msg.Type, "payload_length", msg.PayloadLength)

	// Track all requests
	metrics.RequestsTotal.Inc()

	// Process message through application layer with timing
	start := time.Now()
	response, err := s.wisdomApplication.HandleMessage(ctx, msg)
	duration := time.Since(start)
	metrics.RequestDuration.Observe(duration.Seconds())

	if err != nil {
		metrics.RequestErrors.WithLabelValues("internal_error").Inc()
		logger.Error("application error", sl.Err(err))
		return fmt.Errorf("application error: %w", err)
	}

	// Check if response is an error response
	if errorResp, isError := response.(*protocol.ErrorResponse); isError {
		metrics.RequestErrors.WithLabelValues(string(errorResp.Code)).Inc()
	} else {
		// Track quotes served for successful solution requests
		if msg.Type == protocol.SolutionRequestType {
			metrics.QuotesServed.Inc()
		}
	}

	logger.Debug("sending response to client")
	// Send response using the response's own Encode method
	if err := response.Encode(dc); err != nil {
		logger.Error("failed to encode response", sl.Err(err))
		return fmt.Errorf("encode error: %w", err)
	}

	logger.Debug("response sent successfully")
	return nil
}