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

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

#### Graceful Disconnection:
```go
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:
```go
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:
```go
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:
```go
// 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:
```bash
# 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