techircd/FREEZING_ISSUE_FIXES.md

TechIRCd Freezing Issue - Fixes Applied

Issue Description

TechIRCd was experiencing freezing/hanging issues where the server would become unresponsive after running for a while, preventing new user connections and causing existing users to disconnect.

Root Causes Identified

1. Resource Leaks in Client Handling

• Problem: Disconnected clients weren't being properly cleaned up, leading to memory leaks and resource exhaustion
• Symptoms: Server gradually becoming slower and eventually unresponsive

2. Goroutine Leaks

• Problem: Client handler goroutines weren't exiting properly when connections were broken
• Symptoms: Increasing number of goroutines over time, eventually exhausting system resources

3. Lock Contention in Ping/Health Checks

• Problem: Server-wide ping and health checks were holding locks too long and running synchronously
• Symptoms: Server becoming unresponsive during health checks, client operations timing out

4. Inefficient Connection State Management

• Problem: Inconsistent tracking of client connection state, leading to operations on dead connections
• Symptoms: Hanging writes, blocked goroutines, server freezing

5. Deadlocked Shutdown Process

• Problem: Shutdown process could deadlock when trying to notify clients while holding locks
• Symptoms: Ctrl+C not working, server becoming completely unresponsive, requiring SIGKILL

Fixes Applied

1. Enhanced Client Cleanup (client.go)

Before:

func (c *Client) cleanup() {
    // Basic cleanup with potential blocking operations
    if c.conn != nil {
        c.conn.Close()  // Could hang
    }
    // Synchronous channel cleanup
    // Synchronous server removal
}

After:

func (c *Client) cleanup() {
    // Non-blocking cleanup with timeouts
    if c.conn != nil {
        c.conn.SetDeadline(time.Now().Add(5 * time.Second))
        c.conn.Close()
    }
    // Asynchronous channel cleanup in goroutine
    // Asynchronous server removal in goroutine
}

Benefits:

• Prevents cleanup operations from blocking other clients
• Uses timeouts to force close hanging connections
• Prevents deadlocks during shutdown

2. Improved Connection Handling

Connection Timeout Management:

func (c *Client) handleMessageRead(...) bool {
    // Set read deadline with timeout to prevent hanging
    readTimeout := 30 * time.Second
    if c.IsRegistered() {
        readTimeout = 5 * time.Minute
    }
    
    // Use goroutine with timeout for non-blocking reads
    scanChan := make(chan bool, 1)
    go func() {
        // Scanner in separate goroutine
        scanChan <- scanner.Scan()
    }()
    
    select {
    case result := <-scanChan:
        // Process result
    case <-time.After(readTimeout):
        // Handle timeout
        return false
    }
}

Benefits:

• Prevents infinite blocking on read operations
• Detects and handles dead connections quickly
• Provides graceful timeout handling

3. Non-blocking Ping and Health Checks (server.go)

Before:

func (s *Server) pingRoutine() {
    ticker := time.NewTicker(30 * time.Second)
    for {
        select {
        case <-ticker.C:
            s.performPingCheck()  // Blocking operation
        }
    }
}

After:

func (s *Server) pingRoutine() {
    ticker := time.NewTicker(60 * time.Second)  // Less frequent
    for {
        select {
        case <-ticker.C:
            go s.performPingCheck()  // Non-blocking
        }
    }
}

Enhanced Ping Check:

func (s *Server) performPingCheck() {
    // Get snapshot of client IDs without holding lock
    s.mu.RLock()
    clientIDs := make([]string, 0, len(s.clients))
    for clientID := range s.clients {
        clientIDs = append(clientIDs, clientID)
    }
    s.mu.RUnlock()
    
    // Process clients individually to prevent blocking
    for _, clientID := range clientIDs {
        // Non-blocking ping sending
        go func(id string) {
            // Send ping in separate goroutine
        }(clientID)
    }
}

Benefits:

• Eliminates blocking during server-wide operations
• Reduces lock contention
• Prevents cascade failures

4. Batched Health Checks

Before:

func (s *Server) performHealthCheck() {
    // Hold lock for entire operation
    s.mu.RLock()
    clients := make([]*Client, 0, len(s.clients))
    // ... process all clients synchronously
    s.mu.RUnlock()
}

After:

func (s *Server) performHealthCheck() {
    // Process clients in batches of 50
    batchSize := 50
    for i := 0; i < len(clientIDs); i += batchSize {
        batch := clientIDs[i:end]
        for _, clientID := range batch {
            // Process each client individually
        }
        // Small delay between batches
        time.Sleep(10 * time.Millisecond)
    }
}

Benefits:

• Prevents overwhelming the system during health checks
• Allows other operations to proceed between batches
• Reduces memory usage during large client counts

5. Enhanced Error Recovery

Panic Recovery:

defer func() {
    if r := recover(); r != nil {
        log.Printf("Panic in client handler for %s: %v", c.getClientInfo(), r)
    }
    c.cleanup()
}()

Graceful Disconnection:

func (c *Client) ForceDisconnect(reason string) {
    log.Printf("Force disconnecting client %s: %s", c.getClientInfo(), reason)
    
    c.mu.Lock()
    c.disconnected = true
    c.mu.Unlock()
    
    if c.conn != nil {
        c.SendMessage(fmt.Sprintf("ERROR :%s", reason))
    }
}

5. Robust Shutdown Process (server.go & main.go)

Before:

func (s *Server) Shutdown() {
    // Could deadlock holding locks
    s.mu.RLock()
    for _, client := range s.clients {
        client.SendMessage("ERROR :Server shutting down")
    }
    s.mu.RUnlock()
    close(s.shutdown)
}

After:

func (s *Server) Shutdown() {
    // Non-blocking shutdown with timeout protection
    
    // Close listeners immediately
    go func() { /* close listeners */ }()
    
    // Signal shutdown first
    close(s.shutdown)
    
    // Disconnect clients in batches asynchronously
    go func() {
        // Process clients in batches of 10
        // Each client disconnection in separate goroutine
        // Timeout protection for each operation
    }()
    
    // Force shutdown after reasonable timeout
    time.Sleep(2 * time.Second)
}

Signal Handling with Force Option:

// Double Ctrl+C for immediate shutdown
if shutdownInProgress {
    log.Println("Forcing immediate shutdown...")
    os.Exit(1)
}

// Timeout protection for graceful shutdown
select {
case <-shutdownComplete:
    log.Println("Graceful shutdown completed")
case <-time.After(10 * time.Second):
    log.Println("Shutdown timeout, forcing exit...")
}

Benefits:

• Prevents deadlocks during shutdown
• Allows double Ctrl+C for immediate force shutdown
• Timeout protection prevents hanging shutdown
• Asynchronous operations prevent blocking

Configuration Optimizations

Timing Adjustments:

• Ping Interval: Increased from 30s to 60s to reduce overhead
• Health Check Interval: Increased from 60s to 5 minutes
• Read Timeouts: More conservative timeouts for better stability
• Registration Timeout: Better enforcement to prevent hanging registrations

Resource Limits:

• Batch Processing: Health checks limited to 50 clients per batch
• Connection Limits: Better enforcement of max client limits
• Memory Management: Proactive cleanup of disconnected clients

Expected Results

1. Stability: Server should remain responsive under normal load
2. Resource Usage: More predictable memory and goroutine usage
3. Connection Handling: Faster detection and cleanup of dead connections
4. Performance: Reduced lock contention and blocking operations
5. Monitoring: Better logging and health monitoring

Monitoring

The server now provides better logging for:

• Client connection/disconnection events
• Health check results and statistics
• Resource usage patterns
• Error conditions and recovery actions

Testing

I've created test scripts to verify the fixes:

1. Shutdown Test (test_shutdown.sh)

• Tests graceful shutdown behavior
• Verifies server responds to SIGTERM
• Confirms shutdown completes within reasonable time

2. Stress Test (test_stress.sh)

• Simulates conditions that previously caused freezing
• Creates multiple stable and unstable connections
• Tests rapid connection/disconnection patterns
• Monitors server responsiveness during stress
• Verifies shutdown works after stress conditions

Usage:

# Test shutdown behavior
./test_shutdown.sh

# Test stability under stress
./test_stress.sh

Future Improvements

1. Metrics Endpoint: Add HTTP endpoint for real-time metrics
2. Connection Pooling: Implement connection pooling for better resource management
3. Circuit Breakers: Add circuit breakers for failing operations
4. Rate Limiting: Enhanced rate limiting per IP/user