dcrouter/ts/mail/delivery/smtpserver/connection-manager.ts

/**
 * SMTP Connection Manager
 * Responsible for managing socket connections to the SMTP server
 */

import * as plugins from '../../../plugins.js';
import type { IConnectionManager, ISmtpServer } from './interfaces.js';
import { SmtpResponseCode, SMTP_DEFAULTS, SmtpState } from './constants.js';
import { SmtpLogger } from './utils/logging.js';
import { adaptiveLogger } from './utils/adaptive-logging.js';
import { getSocketDetails, formatMultilineResponse } from './utils/helpers.js';

/**
 * Manager for SMTP connections
 * Handles connection setup, event listeners, and lifecycle management
 * Provides resource management, connection tracking, and monitoring
 */
export class ConnectionManager implements IConnectionManager {
  /**
   * Reference to the SMTP server instance
   */
  private smtpServer: ISmtpServer;
  
  /**
   * Set of active socket connections
   */
  private activeConnections: Set<plugins.net.Socket | plugins.tls.TLSSocket> = new Set();
  
  /**
   * Connection tracking for resource management
   */
  private connectionStats = {
    totalConnections: 0,
    activeConnections: 0,
    peakConnections: 0,
    rejectedConnections: 0,
    closedConnections: 0,
    erroredConnections: 0,
    timedOutConnections: 0
  };

  /**
   * Per-IP connection tracking for rate limiting
   */
  private ipConnections: Map<string, {
    count: number;
    firstConnection: number;
    lastConnection: number;
  }> = new Map();
  
  /**
   * Resource monitoring interval
   */
  private resourceCheckInterval: NodeJS.Timeout | null = null;
  
  /**
   * Track cleanup timers so we can clear them
   */
  private cleanupTimers: Set<NodeJS.Timeout> = new Set();
  
  /**
   * SMTP server options with enhanced resource controls
   */
  private options: {
    hostname: string;
    maxConnections: number;
    socketTimeout: number;
    maxConnectionsPerIP: number;
    connectionRateLimit: number;
    connectionRateWindow: number;
    bufferSizeLimit: number;
    resourceCheckInterval: number;
  };
  
  /**
   * Creates a new connection manager with enhanced resource management
   * @param smtpServer - SMTP server instance
   */
  constructor(smtpServer: ISmtpServer) {
    this.smtpServer = smtpServer;
    
    // Get options from server
    const serverOptions = this.smtpServer.getOptions();
    
    // Default values for resource management - adjusted for production scalability
    const DEFAULT_MAX_CONNECTIONS_PER_IP = 50; // Increased to support high-concurrency scenarios
    const DEFAULT_CONNECTION_RATE_LIMIT = 200; // Increased for production load handling
    const DEFAULT_CONNECTION_RATE_WINDOW = 60 * 1000; // 60 seconds window
    const DEFAULT_BUFFER_SIZE_LIMIT = 10 * 1024 * 1024; // 10 MB
    const DEFAULT_RESOURCE_CHECK_INTERVAL = 30 * 1000; // 30 seconds
    
    this.options = {
      hostname: serverOptions.hostname || SMTP_DEFAULTS.HOSTNAME,
      maxConnections: serverOptions.maxConnections || SMTP_DEFAULTS.MAX_CONNECTIONS,
      socketTimeout: serverOptions.socketTimeout || SMTP_DEFAULTS.SOCKET_TIMEOUT,
      maxConnectionsPerIP: DEFAULT_MAX_CONNECTIONS_PER_IP,
      connectionRateLimit: DEFAULT_CONNECTION_RATE_LIMIT,
      connectionRateWindow: DEFAULT_CONNECTION_RATE_WINDOW,
      bufferSizeLimit: DEFAULT_BUFFER_SIZE_LIMIT,
      resourceCheckInterval: DEFAULT_RESOURCE_CHECK_INTERVAL
    };
    
    // Start resource monitoring
    this.startResourceMonitoring();
  }
  
  /**
   * Start resource monitoring interval to check resource usage
   */
  private startResourceMonitoring(): void {
    // Clear any existing interval
    if (this.resourceCheckInterval) {
      clearInterval(this.resourceCheckInterval);
    }
    
    // Set up new interval
    this.resourceCheckInterval = setInterval(() => {
      this.monitorResourceUsage();
    }, this.options.resourceCheckInterval);
  }
  
  /**
   * Monitor resource usage and log statistics
   */
  private monitorResourceUsage(): void {
    // Calculate memory usage
    const memoryUsage = process.memoryUsage();
    const memoryUsageMB = {
      rss: Math.round(memoryUsage.rss / 1024 / 1024),
      heapTotal: Math.round(memoryUsage.heapTotal / 1024 / 1024),
      heapUsed: Math.round(memoryUsage.heapUsed / 1024 / 1024),
      external: Math.round(memoryUsage.external / 1024 / 1024)
    };
    
    // Calculate connection rate metrics
    const activeIPs = Array.from(this.ipConnections.entries())
      .filter(([_, data]) => data.count > 0).length;
      
    const highVolumeIPs = Array.from(this.ipConnections.entries())
      .filter(([_, data]) => data.count > this.options.connectionRateLimit / 2).length;
    
    // Log resource usage with more detailed metrics
    SmtpLogger.info('Resource usage stats', {
      connections: {
        active: this.activeConnections.size,
        total: this.connectionStats.totalConnections,
        peak: this.connectionStats.peakConnections,
        rejected: this.connectionStats.rejectedConnections,
        closed: this.connectionStats.closedConnections,
        errored: this.connectionStats.erroredConnections,
        timedOut: this.connectionStats.timedOutConnections
      },
      memory: memoryUsageMB,
      ipTracking: {
        uniqueIPs: this.ipConnections.size,
        activeIPs: activeIPs,
        highVolumeIPs: highVolumeIPs
      },
      resourceLimits: {
        maxConnections: this.options.maxConnections,
        maxConnectionsPerIP: this.options.maxConnectionsPerIP,
        connectionRateLimit: this.options.connectionRateLimit,
        bufferSizeLimit: Math.round(this.options.bufferSizeLimit / 1024 / 1024) + 'MB'
      }
    });
    
    // Check for potential DoS conditions
    if (highVolumeIPs > 3) {
      SmtpLogger.warn(`Potential DoS detected: ${highVolumeIPs} IPs with high connection rates`);
    }
    
    // Assess memory usage trends
    if (memoryUsageMB.heapUsed > 500) { // Over 500MB heap used
      SmtpLogger.warn(`High memory usage detected: ${memoryUsageMB.heapUsed}MB heap used`);
    }
    
    // Clean up expired IP rate limits and validate resource tracking
    this.cleanupIpRateLimits();
  }
  
  /**
   * Clean up expired IP rate limits and perform additional resource monitoring
   */
  private cleanupIpRateLimits(): void {
    const now = Date.now();
    const windowThreshold = now - this.options.connectionRateWindow;
    let activeIps = 0;
    let removedEntries = 0;
    
    // Iterate through IP connections and manage entries
    for (const [ip, data] of this.ipConnections.entries()) {
      // If the last connection was before the window threshold + one extra window, remove the entry
      if (data.lastConnection < windowThreshold - this.options.connectionRateWindow) {
        // Remove stale entries to prevent memory growth
        this.ipConnections.delete(ip);
        removedEntries++;
      } 
      // If last connection was before the window threshold, reset the count
      else if (data.lastConnection < windowThreshold) {
        if (data.count > 0) {
          // Reset but keep the IP in the map with a zero count
          this.ipConnections.set(ip, { 
            count: 0, 
            firstConnection: now, 
            lastConnection: now 
          });
        }
      } else {
        // This IP is still active within the current window
        activeIps++;
      }
    }
    
    // Log cleanup activity if significant changes occurred
    if (removedEntries > 0) {
      SmtpLogger.debug(`IP rate limit cleanup: removed ${removedEntries} stale entries, ${this.ipConnections.size} remaining, ${activeIps} active in current window`);
    }
    
    // Check for memory leaks in connection tracking
    if (this.activeConnections.size > 0 && this.connectionStats.activeConnections !== this.activeConnections.size) {
      SmtpLogger.warn(`Connection tracking inconsistency detected: stats.active=${this.connectionStats.activeConnections}, actual=${this.activeConnections.size}`);
      // Fix the inconsistency
      this.connectionStats.activeConnections = this.activeConnections.size;
    }
    
    // Validate and clean leaked resources if needed
    this.validateResourceTracking();
  }
  
  /**
   * Validate and repair resource tracking to prevent leaks
   */
  private validateResourceTracking(): void {
    // Prepare a detailed report if inconsistencies are found
    const inconsistenciesFound = [];
    
    // 1. Check active connections count matches activeConnections set size
    if (this.connectionStats.activeConnections !== this.activeConnections.size) {
      inconsistenciesFound.push({
        issue: 'Active connection count mismatch',
        stats: this.connectionStats.activeConnections,
        actual: this.activeConnections.size,
        action: 'Auto-corrected'
      });
      this.connectionStats.activeConnections = this.activeConnections.size;
    }
    
    // 2. Check for destroyed sockets in active connections
    let destroyedSocketsCount = 0;
    for (const socket of this.activeConnections) {
      if (socket.destroyed) {
        destroyedSocketsCount++;
        // This should not happen - remove destroyed sockets from tracking
        this.activeConnections.delete(socket);
      }
    }
    
    if (destroyedSocketsCount > 0) {
      inconsistenciesFound.push({
        issue: 'Destroyed sockets in active list',
        count: destroyedSocketsCount,
        action: 'Removed from tracking'
      });
      // Update active connections count after cleanup
      this.connectionStats.activeConnections = this.activeConnections.size;
    }
    
    // 3. Check for sessions without corresponding active connections
    const sessionCount = this.smtpServer.getSessionManager().getSessionCount();
    if (sessionCount > this.activeConnections.size) {
      inconsistenciesFound.push({
        issue: 'Orphaned sessions',
        sessions: sessionCount,
        connections: this.activeConnections.size,
        action: 'Session cleanup recommended'
      });
    }
    
    // If any inconsistencies found, log a detailed report
    if (inconsistenciesFound.length > 0) {
      SmtpLogger.warn('Resource tracking inconsistencies detected and repaired', { inconsistencies: inconsistenciesFound });
    }
  }
  
  /**
   * Handle a new connection with resource management
   * @param socket - Client socket
   */
  public async handleNewConnection(socket: plugins.net.Socket): Promise<void> {
    // Update connection stats
    this.connectionStats.totalConnections++;
    this.connectionStats.activeConnections = this.activeConnections.size + 1;
    
    if (this.connectionStats.activeConnections > this.connectionStats.peakConnections) {
      this.connectionStats.peakConnections = this.connectionStats.activeConnections;
    }
    
    // Get client IP
    const remoteAddress = socket.remoteAddress || '0.0.0.0';
    
    // Check rate limits by IP
    if (this.isIPRateLimited(remoteAddress)) {
      this.rejectConnection(socket, 'Rate limit exceeded');
      this.connectionStats.rejectedConnections++;
      return;
    }
    
    // Check per-IP connection limit
    if (this.hasReachedIPConnectionLimit(remoteAddress)) {
      this.rejectConnection(socket, 'Too many connections from this IP');
      this.connectionStats.rejectedConnections++;
      return;
    }
    
    // Check if maximum global connections reached
    if (this.hasReachedMaxConnections()) {
      this.rejectConnection(socket, 'Too many connections');
      this.connectionStats.rejectedConnections++;
      return;
    }
    
    // Add socket to active connections
    this.activeConnections.add(socket);
    
    // Set up socket options
    socket.setKeepAlive(true);
    socket.setTimeout(this.options.socketTimeout);
    
    // Explicitly set socket buffer sizes to prevent memory issues
    socket.setNoDelay(true); // Disable Nagle's algorithm for better responsiveness
    
    // Set limits on socket buffer size if supported by Node.js version
    try {
      // Here we set reasonable buffer limits to prevent memory exhaustion attacks
      const highWaterMark = 64 * 1024; // 64 KB
      // Note: Socket high water mark methods can't be set directly in newer Node.js versions
      // These would need to be set during socket creation or with a different API
    } catch (error) {
      // Ignore errors from older Node.js versions that don't support these methods
      SmtpLogger.debug(`Could not set socket buffer limits: ${error instanceof Error ? error.message : String(error)}`);
    }
    
    // Track this IP connection
    this.trackIPConnection(remoteAddress);
    
    // Set up event handlers
    this.setupSocketEventHandlers(socket);
    
    // Create a session for this connection
    this.smtpServer.getSessionManager().createSession(socket, false);
    
    // Log the new connection using adaptive logger
    const socketDetails = getSocketDetails(socket);
    adaptiveLogger.logConnection(socket, 'connect');
    
    // Update adaptive logger with current connection count
    adaptiveLogger.updateConnectionCount(this.connectionStats.activeConnections);
    
    // Send greeting
    this.sendGreeting(socket);
  }
  
  /**
   * Check if an IP has exceeded the rate limit
   * @param ip - Client IP address
   * @returns True if rate limited
   */
  private isIPRateLimited(ip: string): boolean {
    const now = Date.now();
    const ipData = this.ipConnections.get(ip);
    
    if (!ipData) {
      return false; // No previous connections
    }
    
    // Check if we're within the rate window
    const isWithinWindow = now - ipData.firstConnection <= this.options.connectionRateWindow;
    
    // If within window and count exceeds limit, rate limit is applied
    if (isWithinWindow && ipData.count >= this.options.connectionRateLimit) {
      SmtpLogger.warn(`Rate limit exceeded for IP ${ip}: ${ipData.count} connections in ${Math.round((now - ipData.firstConnection) / 1000)}s`);
      return true;
    }
    
    return false;
  }
  
  /**
   * Track a new connection from an IP
   * @param ip - Client IP address
   */
  private trackIPConnection(ip: string): void {
    const now = Date.now();
    const ipData = this.ipConnections.get(ip);
    
    if (!ipData) {
      // First connection from this IP
      this.ipConnections.set(ip, {
        count: 1,
        firstConnection: now,
        lastConnection: now
      });
    } else {
      // Check if we need to reset the window
      if (now - ipData.lastConnection > this.options.connectionRateWindow) {
        // Reset the window
        this.ipConnections.set(ip, {
          count: 1,
          firstConnection: now,
          lastConnection: now
        });
      } else {
        // Increment within the current window
        this.ipConnections.set(ip, {
          count: ipData.count + 1,
          firstConnection: ipData.firstConnection,
          lastConnection: now
        });
      }
    }
  }
  
  /**
   * Check if an IP has reached its connection limit
   * @param ip - Client IP address
   * @returns True if limit reached
   */
  private hasReachedIPConnectionLimit(ip: string): boolean {
    let ipConnectionCount = 0;
    
    // Count active connections from this IP
    for (const socket of this.activeConnections) {
      if (socket.remoteAddress === ip) {
        ipConnectionCount++;
      }
    }
    
    return ipConnectionCount >= this.options.maxConnectionsPerIP;
  }
  
  /**
   * Handle a new secure TLS connection with resource management
   * @param socket - Client TLS socket
   */
  public async handleNewSecureConnection(socket: plugins.tls.TLSSocket): Promise<void> {
    // Update connection stats
    this.connectionStats.totalConnections++;
    this.connectionStats.activeConnections = this.activeConnections.size + 1;
    
    if (this.connectionStats.activeConnections > this.connectionStats.peakConnections) {
      this.connectionStats.peakConnections = this.connectionStats.activeConnections;
    }
    
    // Get client IP
    const remoteAddress = socket.remoteAddress || '0.0.0.0';
    
    // Check rate limits by IP
    if (this.isIPRateLimited(remoteAddress)) {
      this.rejectConnection(socket, 'Rate limit exceeded');
      this.connectionStats.rejectedConnections++;
      return;
    }
    
    // Check per-IP connection limit
    if (this.hasReachedIPConnectionLimit(remoteAddress)) {
      this.rejectConnection(socket, 'Too many connections from this IP');
      this.connectionStats.rejectedConnections++;
      return;
    }
    
    // Check if maximum global connections reached
    if (this.hasReachedMaxConnections()) {
      this.rejectConnection(socket, 'Too many connections');
      this.connectionStats.rejectedConnections++;
      return;
    }
    
    // Add socket to active connections
    this.activeConnections.add(socket);
    
    // Set up socket options
    socket.setKeepAlive(true);
    socket.setTimeout(this.options.socketTimeout);
    
    // Explicitly set socket buffer sizes to prevent memory issues
    socket.setNoDelay(true); // Disable Nagle's algorithm for better responsiveness
    
    // Set limits on socket buffer size if supported by Node.js version
    try {
      // Here we set reasonable buffer limits to prevent memory exhaustion attacks
      const highWaterMark = 64 * 1024; // 64 KB
      // Note: Socket high water mark methods can't be set directly in newer Node.js versions
      // These would need to be set during socket creation or with a different API
    } catch (error) {
      // Ignore errors from older Node.js versions that don't support these methods
      SmtpLogger.debug(`Could not set socket buffer limits: ${error instanceof Error ? error.message : String(error)}`);
    }
    
    // Track this IP connection
    this.trackIPConnection(remoteAddress);
    
    // Set up event handlers
    this.setupSocketEventHandlers(socket);
    
    // Create a session for this connection
    this.smtpServer.getSessionManager().createSession(socket, true);
    
    // Log the new secure connection using adaptive logger
    adaptiveLogger.logConnection(socket, 'connect');
    
    // Update adaptive logger with current connection count
    adaptiveLogger.updateConnectionCount(this.connectionStats.activeConnections);
    
    // Send greeting
    this.sendGreeting(socket);
  }
  
  /**
   * Set up event handlers for a socket with enhanced resource management
   * @param socket - Client socket
   */
  public setupSocketEventHandlers(socket: plugins.net.Socket | plugins.tls.TLSSocket): void {
    // Store existing socket event handlers before adding new ones
    const existingDataHandler = socket.listeners('data')[0] as (...args: any[]) => void;
    const existingCloseHandler = socket.listeners('close')[0] as (...args: any[]) => void;
    const existingErrorHandler = socket.listeners('error')[0] as (...args: any[]) => void;
    const existingTimeoutHandler = socket.listeners('timeout')[0] as (...args: any[]) => void;
    
    // Remove existing event handlers if they exist
    if (existingDataHandler) socket.removeListener('data', existingDataHandler);
    if (existingCloseHandler) socket.removeListener('close', existingCloseHandler);
    if (existingErrorHandler) socket.removeListener('error', existingErrorHandler);
    if (existingTimeoutHandler) socket.removeListener('timeout', existingTimeoutHandler);
    
    // Data event - process incoming data from the client with resource limits
    let buffer = '';
    let totalBytesReceived = 0;
    
    socket.on('data', async (data) => {
      try {
        // Get current session and update activity timestamp
        const session = this.smtpServer.getSessionManager().getSession(socket);
        if (session) {
          this.smtpServer.getSessionManager().updateSessionActivity(session);
        }
        
        // Check if we're in DATA receiving mode - handle differently
        if (session && session.state === SmtpState.DATA_RECEIVING) {
          // In DATA mode, pass raw chunks directly to command handler with special marker
          // Don't line-buffer large email content
          try {
            const dataString = data.toString('utf8');
            // Use a special prefix to indicate this is raw data, not a command line
            // CRITICAL FIX: Must await to prevent async pile-up
            await this.smtpServer.getCommandHandler().processCommand(socket, `__RAW_DATA__${dataString}`);
            return;
          } catch (dataError) {
            SmtpLogger.error(`Data handler error during DATA mode: ${dataError instanceof Error ? dataError.message : String(dataError)}`);
            socket.destroy();
            return;
          }
        }
        
        // For command mode, continue with line-buffered processing
        // Check buffer size limits to prevent memory attacks
        totalBytesReceived += data.length;
        
        if (buffer.length > this.options.bufferSizeLimit) {
          // Buffer is too large, reject the connection
          SmtpLogger.warn(`Buffer size limit exceeded: ${buffer.length} bytes for ${socket.remoteAddress}`);
          this.sendResponse(socket, `${SmtpResponseCode.EXCEEDED_STORAGE} Message too large, disconnecting`);
          socket.destroy();
          return;
        }
        
        // Impose a total transfer limit to prevent DoS
        if (totalBytesReceived > this.options.bufferSizeLimit * 2) {
          SmtpLogger.warn(`Total transfer limit exceeded: ${totalBytesReceived} bytes for ${socket.remoteAddress}`);
          this.sendResponse(socket, `${SmtpResponseCode.EXCEEDED_STORAGE} Transfer limit exceeded, disconnecting`);
          socket.destroy();
          return;
        }
        
        // Convert buffer to string safely with explicit encoding
        const dataString = data.toString('utf8');
        
        // Buffer incoming data
        buffer += dataString;
        
        // Process complete lines
        let lineEndPos;
        while ((lineEndPos = buffer.indexOf(SMTP_DEFAULTS.CRLF)) !== -1) {
          // Extract a complete line
          const line = buffer.substring(0, lineEndPos);
          buffer = buffer.substring(lineEndPos + 2); // +2 to skip CRLF
          
          // Check line length to prevent extremely long lines
          if (line.length > 4096) { // 4KB line limit is reasonable for SMTP
            SmtpLogger.warn(`Line length limit exceeded: ${line.length} bytes for ${socket.remoteAddress}`);
            this.sendResponse(socket, `${SmtpResponseCode.SYNTAX_ERROR} Line too long, disconnecting`);
            socket.destroy();
            return;
          }
          
          // Process non-empty lines
          if (line.length > 0) {
            try {
              // CRITICAL FIX: Must await processCommand to prevent async pile-up
              // This was causing the busy loop with high CPU usage when many empty lines were processed
              await this.smtpServer.getCommandHandler().processCommand(socket, line);
            } catch (error) {
              // Handle any errors in command processing
              SmtpLogger.error(`Command handler error: ${error instanceof Error ? error.message : String(error)}`);
              this.sendResponse(socket, `${SmtpResponseCode.LOCAL_ERROR} Internal server error`);
              
              // If there's a severe error, close the connection
              if (error instanceof Error && 
                  (error.message.includes('fatal') || error.message.includes('critical'))) {
                socket.destroy();
                return;
              }
            }
          }
        }
        
        // If buffer is getting too large without CRLF, it might be a DoS attempt
        if (buffer.length > 10240) { // 10KB is a reasonable limit for a line without CRLF
          SmtpLogger.warn(`Incomplete line too large: ${buffer.length} bytes for ${socket.remoteAddress}`);
          this.sendResponse(socket, `${SmtpResponseCode.SYNTAX_ERROR} Incomplete line too large, disconnecting`);
          socket.destroy();
        }
      } catch (error) {
        // Handle any unexpected errors during data processing
        SmtpLogger.error(`Data handler error: ${error instanceof Error ? error.message : String(error)}`);
        socket.destroy();
      }
    });
    
    // Add drain event handler to manage flow control
    socket.on('drain', () => {
      // Socket buffer has been emptied, resume data flow if needed
      if (socket.isPaused()) {
        socket.resume();
        SmtpLogger.debug(`Resumed socket for ${socket.remoteAddress} after drain`);
      }
    });
    
    // Close event - clean up when connection is closed
    socket.on('close', (hadError) => {
      this.handleSocketClose(socket, hadError);
    });
    
    // Error event - handle socket errors
    socket.on('error', (err) => {
      this.handleSocketError(socket, err);
    });
    
    // Timeout event - handle socket timeouts
    socket.on('timeout', () => {
      this.handleSocketTimeout(socket);
    });
  }
  
  /**
   * Get the current connection count
   * @returns Number of active connections
   */
  public getConnectionCount(): number {
    return this.activeConnections.size;
  }
  
  /**
   * Check if the server has reached the maximum number of connections
   * @returns True if max connections reached
   */
  public hasReachedMaxConnections(): boolean {
    return this.activeConnections.size >= this.options.maxConnections;
  }
  
  /**
   * Close all active connections
   */
  public closeAllConnections(): void {
    const connectionCount = this.activeConnections.size;
    if (connectionCount === 0) {
      return;
    }
    
    SmtpLogger.info(`Closing all connections (count: ${connectionCount})`);
    
    for (const socket of this.activeConnections) {
      try {
        // Send service closing notification
        this.sendServiceClosing(socket);
        
        // End the socket gracefully
        socket.end();
        
        // Force destroy after a short delay if not already destroyed
        const destroyTimer = setTimeout(() => {
          if (!socket.destroyed) {
            socket.destroy();
          }
          this.cleanupTimers.delete(destroyTimer);
        }, 100);
        this.cleanupTimers.add(destroyTimer);
      } catch (error) {
        SmtpLogger.error(`Error closing connection: ${error instanceof Error ? error.message : String(error)}`);
        // Force destroy on error
        try {
          socket.destroy();
        } catch (e) {
          // Ignore destroy errors
        }
      }
    }
    
    // Clear active connections
    this.activeConnections.clear();
    
    // Stop resource monitoring to prevent hanging timers
    if (this.resourceCheckInterval) {
      clearInterval(this.resourceCheckInterval);
      this.resourceCheckInterval = null;
    }
  }
  
  /**
   * Handle socket close event
   * @param socket - Client socket
   * @param hadError - Whether the socket was closed due to error
   */
  private handleSocketClose(socket: plugins.net.Socket | plugins.tls.TLSSocket, hadError: boolean): void {
    try {
      // Update connection statistics
      this.connectionStats.closedConnections++;
      this.connectionStats.activeConnections = this.activeConnections.size - 1;
      
      // Get socket details for logging
      const socketDetails = getSocketDetails(socket);
      const socketId = `${socketDetails.remoteAddress}:${socketDetails.remotePort}`;
      
      // Log with appropriate level based on whether there was an error
      if (hadError) {
        SmtpLogger.warn(`Socket closed with error: ${socketId}`);
      } else {
        SmtpLogger.debug(`Socket closed normally: ${socketId}`);
      }
      
      // Get the session before removing it
      const session = this.smtpServer.getSessionManager().getSession(socket);
      
      // Remove from active connections
      this.activeConnections.delete(socket);
      
      // Remove from session manager
      this.smtpServer.getSessionManager().removeSession(socket);
      
      // Cancel any timeout ID stored in the session
      if (session?.dataTimeoutId) {
        clearTimeout(session.dataTimeoutId);
      }
      
      // Log connection close with session details if available
      adaptiveLogger.logConnection(socket, 'close', session);
      
      // Update adaptive logger with new connection count
      adaptiveLogger.updateConnectionCount(this.connectionStats.activeConnections);
    } catch (error) {
      // Handle any unexpected errors during cleanup
      SmtpLogger.error(`Error in handleSocketClose: ${error instanceof Error ? error.message : String(error)}`);
      
      // Ensure socket is removed from active connections even if an error occurs
      this.activeConnections.delete(socket);
    }
  }
  
  /**
   * Handle socket error event
   * @param socket - Client socket
   * @param error - Error object
   */
  private handleSocketError(socket: plugins.net.Socket | plugins.tls.TLSSocket, error: Error): void {
    try {
      // Update connection statistics
      this.connectionStats.erroredConnections++;
      
      // Get socket details for context
      const socketDetails = getSocketDetails(socket);
      const socketId = `${socketDetails.remoteAddress}:${socketDetails.remotePort}`;
      
      // Get the session
      const session = this.smtpServer.getSessionManager().getSession(socket);
      
      // Detailed error logging with context information
      SmtpLogger.error(`Socket error for ${socketId}: ${error.message}`, {
        errorCode: (error as any).code,
        errorStack: error.stack,
        sessionId: session?.id,
        sessionState: session?.state,
        remoteAddress: socketDetails.remoteAddress,
        remotePort: socketDetails.remotePort
      });
      
      // Log the error for connection tracking using adaptive logger
      adaptiveLogger.logConnection(socket, 'error', session, error);
      
      // Cancel any timeout ID stored in the session
      if (session?.dataTimeoutId) {
        clearTimeout(session.dataTimeoutId);
      }
      
      // Close the socket if not already closed
      if (!socket.destroyed) {
        socket.destroy();
      }
      
      // Remove from active connections (cleanup after error)
      this.activeConnections.delete(socket);
      
      // Remove from session manager
      this.smtpServer.getSessionManager().removeSession(socket);
    } catch (handlerError) {
      // Meta-error handling (errors in the error handler)
      SmtpLogger.error(`Error in handleSocketError: ${handlerError instanceof Error ? handlerError.message : String(handlerError)}`);
      
      // Ensure socket is destroyed and removed from active connections
      if (!socket.destroyed) {
        socket.destroy();
      }
      this.activeConnections.delete(socket);
    }
  }
  
  /**
   * Handle socket timeout event
   * @param socket - Client socket
   */
  private handleSocketTimeout(socket: plugins.net.Socket | plugins.tls.TLSSocket): void {
    try {
      // Update connection statistics
      this.connectionStats.timedOutConnections++;
      
      // Get socket details for context
      const socketDetails = getSocketDetails(socket);
      const socketId = `${socketDetails.remoteAddress}:${socketDetails.remotePort}`;
      
      // Get the session
      const session = this.smtpServer.getSessionManager().getSession(socket);
      
      // Get timing information for better debugging
      const now = Date.now();
      const idleTime = session?.lastActivity ? now - session.lastActivity : 'unknown';
      
      if (session) {
        // Log the timeout with extended details
        SmtpLogger.warn(`Socket timeout from ${session.remoteAddress}`, {
          sessionId: session.id,
          remoteAddress: session.remoteAddress,
          state: session.state,
          timeout: this.options.socketTimeout,
          idleTime: idleTime,
          emailState: session.envelope?.mailFrom ? 'has-sender' : 'no-sender',
          recipientCount: session.envelope?.rcptTo?.length || 0
        });
        
        // Cancel any timeout ID stored in the session
        if (session.dataTimeoutId) {
          clearTimeout(session.dataTimeoutId);
        }
        
        // Send timeout notification to client
        this.sendResponse(socket, `${SmtpResponseCode.SERVICE_NOT_AVAILABLE} Connection timeout - closing connection`);
      } else {
        // Log timeout without session context
        SmtpLogger.warn(`Socket timeout without session from ${socketId}`);
      }
      
      // Close the socket gracefully
      try {
        socket.end();
        
        // Set a forced close timeout in case socket.end() doesn't close the connection
        const timeoutDestroyTimer = setTimeout(() => {
          if (!socket.destroyed) {
            SmtpLogger.warn(`Forcing destroy of timed out socket: ${socketId}`);
            socket.destroy();
          }
          this.cleanupTimers.delete(timeoutDestroyTimer);
        }, 5000); // 5 second grace period for socket to end properly
        this.cleanupTimers.add(timeoutDestroyTimer);
      } catch (error) {
        SmtpLogger.error(`Error ending timed out socket: ${error instanceof Error ? error.message : String(error)}`);
        
        // Ensure socket is destroyed even if end() fails
        if (!socket.destroyed) {
          socket.destroy();
        }
      }
      
      // Clean up resources
      this.activeConnections.delete(socket);
      this.smtpServer.getSessionManager().removeSession(socket);
    } catch (handlerError) {
      // Handle any unexpected errors during timeout handling
      SmtpLogger.error(`Error in handleSocketTimeout: ${handlerError instanceof Error ? handlerError.message : String(handlerError)}`);
      
      // Ensure socket is destroyed and removed from tracking
      if (!socket.destroyed) {
        socket.destroy();
      }
      this.activeConnections.delete(socket);
    }
  }
  
  /**
   * Reject a connection
   * @param socket - Client socket
   * @param reason - Reason for rejection
   */
  private rejectConnection(socket: plugins.net.Socket | plugins.tls.TLSSocket, reason: string): void {
    // Log the rejection
    const socketDetails = getSocketDetails(socket);
    SmtpLogger.warn(`Connection rejected from ${socketDetails.remoteAddress}:${socketDetails.remotePort}: ${reason}`);
    
    // Send rejection message
    this.sendResponse(socket, `${SmtpResponseCode.SERVICE_NOT_AVAILABLE} ${this.options.hostname} Service temporarily unavailable - ${reason}`);
    
    // Close the socket
    try {
      socket.end();
    } catch (error) {
      SmtpLogger.error(`Error ending rejected socket: ${error instanceof Error ? error.message : String(error)}`);
    }
  }
  
  /**
   * Send greeting message
   * @param socket - Client socket
   */
  private sendGreeting(socket: plugins.net.Socket | plugins.tls.TLSSocket): void {
    const greeting = `${SmtpResponseCode.SERVICE_READY} ${this.options.hostname} ESMTP service ready`;
    this.sendResponse(socket, greeting);
  }
  
  /**
   * Send service closing notification
   * @param socket - Client socket
   */
  private sendServiceClosing(socket: plugins.net.Socket | plugins.tls.TLSSocket): void {
    const message = `${SmtpResponseCode.SERVICE_CLOSING} ${this.options.hostname} Service closing transmission channel`;
    this.sendResponse(socket, message);
  }
  
  /**
   * Send response to client
   * @param socket - Client socket
   * @param response - Response to send
   */
  private sendResponse(socket: plugins.net.Socket | plugins.tls.TLSSocket, response: string): void {
    // Check if socket is still writable before attempting to write
    if (socket.destroyed || socket.readyState !== 'open' || !socket.writable) {
      SmtpLogger.debug(`Skipping response to closed/destroyed socket: ${response}`, {
        remoteAddress: socket.remoteAddress,
        remotePort: socket.remotePort,
        destroyed: socket.destroyed,
        readyState: socket.readyState,
        writable: socket.writable
      });
      return;
    }
    
    try {
      socket.write(`${response}${SMTP_DEFAULTS.CRLF}`);
      adaptiveLogger.logResponse(response, socket);
    } catch (error) {
      // Log error and destroy socket
      SmtpLogger.error(`Error sending response: ${error instanceof Error ? error.message : String(error)}`, {
        response,
        remoteAddress: socket.remoteAddress,
        remotePort: socket.remotePort,
        error: error instanceof Error ? error : new Error(String(error))
      });
      
      socket.destroy();
    }
  }
  
  /**
   * Handle a new connection (interface requirement)
   */
  public async handleConnection(socket: plugins.net.Socket | plugins.tls.TLSSocket, secure: boolean): Promise<void> {
    if (secure) {
      this.handleNewSecureConnection(socket as plugins.tls.TLSSocket);
    } else {
      this.handleNewConnection(socket as plugins.net.Socket);
    }
  }
  
  /**
   * Check if accepting new connections (interface requirement)
   */
  public canAcceptConnection(): boolean {
    return !this.hasReachedMaxConnections();
  }
  
  /**
   * Clean up resources
   */
  public destroy(): void {
    // Clear resource monitoring interval
    if (this.resourceCheckInterval) {
      clearInterval(this.resourceCheckInterval);
      this.resourceCheckInterval = null;
    }
    
    // Clear all cleanup timers
    for (const timer of this.cleanupTimers) {
      clearTimeout(timer);
    }
    this.cleanupTimers.clear();
    
    // Close all active connections
    this.closeAllConnections();
    
    // Clear maps
    this.activeConnections.clear();
    this.ipConnections.clear();
    
    // Reset connection stats
    this.connectionStats = {
      totalConnections: 0,
      activeConnections: 0,
      peakConnections: 0,
      rejectedConnections: 0,
      closedConnections: 0,
      erroredConnections: 0,
      timedOutConnections: 0
    };
    
    SmtpLogger.debug('ConnectionManager destroyed');
  }
}