//go:build linux || darwin

/*
 * MIT License
 *
 * Copyright (c) 2025 Nicolas JUHEL
 *
 * Permission is hereby granted, free of charge, to any person obtaining a copy
 * of this software and associated documentation files (the "Software"), to deal
 * in the Software without restriction, including without limitation the rights
 * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
 * copies of the Software, and to permit persons to whom the Software is
 * furnished to do so, subject to the following conditions:
 *
 * The above copyright notice and this permission notice shall be included in all
 * copies or substantial portions of the Software.
 *
 * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
 * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
 * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
 * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
 * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
 * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
 * SOFTWARE.
 *
 */

// Package unix provides a robust, production-ready Unix domain socket server implementation
// for Go applications. It's designed to handle local inter-process communication (IPC)
// with a focus on reliability, performance, and ease of use.
//
// # Overview
//
// This package implements a high-performance Unix domain socket server that supports:
//   - SOCK_STREAM (connection-oriented) Unix sockets for reliable IPC
//   - File permissions and group ownership control for access management
//   - Graceful shutdown with connection draining and timeout management
//   - Context-aware operations with cancellation propagation
//   - Configurable idle timeouts for inactive connections
//   - Thread-safe concurrent connection handling (goroutine-per-connection)
//   - Connection counting and tracking
//   - Customizable connection configuration via UpdateConn callback
//   - Comprehensive event callbacks for monitoring and logging
//
// # Architecture
//
// ## Component Diagram
//
// The server follows a layered architecture with clear separation of concerns:
//
//	┌─────────────────────────────────────────────────────┐
//	│                  Unix Socket Server                 │
//	├─────────────────────────────────────────────────────┤
//	│                                                     │
//	│  ┌──────────────┐       ┌───────────────────┐       │
//	│  │   Listener   │       │  Context Manager  │       │
//	│  │  (Listen)    │       │  (ctx tracking)   │       │
//	│  └──────┬───────┘       └─────────┬─────────┘       │
//	│         │                         │                 │
//	│         ▼                         ▼                 │
//	│  ┌──────────────────────────────────────────┐       │
//	│  │          Connection Acceptor             │       │
//	│  │     (Accept loop + file setup)           │       │
//	│  └──────────────┬───────────────────────────┘       │
//	│                 │                                   │
//	│                 ▼                                   │
//	│         Per-Connection Goroutine                    │
//	│         ┌─────────────────────┐                     │
//	│         │  Connection Context │                     │
//	│         │   - sCtx (I/O wrap) │                     │
//	│         │   - Idle timeout    │                     │
//	│         │   - State tracking  │                     │
//	│         └──────────┬──────────┘                     │
//	│                    │                                │
//	│                    ▼                                │
//	│         ┌─────────────────────┐                     │
//	│         │   User Handler Func │                     │
//	│         │   (custom logic)    │                     │
//	│         └─────────────────────┘                     │
//	│                                                     │
//	│  Socket File Management:                            │
//	│   - Creation with permissions                       │
//	│   - Group ownership assignment                      │
//	│   - Automatic cleanup on shutdown                   │
//	│                                                     │
//	│  Optional Callbacks:                                │
//	│   - UpdateConn: Connection configuration            │
//	│   - FuncError: Error reporting                      │
//	│   - FuncInfo: Connection state changes              │
//	│   - FuncInfoSrv: Server lifecycle events            │
//	│                                                     │
//	└─────────────────────────────────────────────────────┘
//
// ## Data Flow
//
//  1. Server.Listen() starts the accept loop
//  2. For each new connection:
//     a. net.Listener.Accept() receives the connection
//     b. Connection counter incremented atomically
//     c. UpdateConn callback invoked (if registered)
//     d. Connection wrapped in sCtx (context + I/O)
//     e. Handler goroutine spawned
//     f. Idle timeout monitoring started (if configured)
//  3. Handler processes the connection
//  4. On close:
//     a. Connection closed
//     b. Context cancelled
//     c. Counter decremented
//     d. Goroutine terminates
//
// ## Lifecycle States
//
// The server maintains two atomic state flags:
//
//   - IsRunning: Server is accepting new connections
//
//   - false → true: Listen() called successfully
//
//   - true → false: Shutdown/Close initiated
//
//   - IsGone: Server is draining existing connections
//
//   - false → true: Shutdown() called
//
//   - Used to signal accept loop to stop
//
// ## Thread Safety Model
//
// Synchronization primitives used:
//   - atomic.Bool: run, gon (server state)
//   - atomic.Int64: nc (connection counter)
//   - libatm.Value: fe, fi, fs, ad (atomic storage)
//   - No mutexes: All state changes are lock-free
//
// Concurrency guarantees:
//   - All exported methods are safe for concurrent use
//   - Connection handlers run in isolated goroutines
//   - No shared mutable state between connections
//   - Atomic counters prevent race conditions
//
// # Features
//
// ## Unix Socket Benefits
//   - Zero network overhead: Communication within the same host
//   - File system permissions for access control
//   - No TCP/IP stack overhead: Lower latency than TCP loopback
//   - Higher throughput than TCP for local communication
//   - Process credentials passing on Linux (SCM_CREDENTIALS)
//   - File descriptor passing capability (SCM_RIGHTS)
//
// ## Security
//   - File system permissions for access control (chmod)
//   - Group ownership for fine-grained access (chown)
//   - No network exposure: Not accessible over the network
//   - Automatic socket file cleanup on shutdown
//   - Configurable file permissions (0600, 0660, 0666, etc.)
//
// ## Reliability
//   - Graceful shutdown with configurable timeouts
//   - Connection draining during shutdown (wait for active connections)
//   - Automatic resource reclamation (goroutines, memory, file descriptors)
//   - Idle connection timeout with automatic cleanup
//   - Context-aware operations with deadline and cancellation support
//   - Error recovery and propagation
//
// ## Monitoring & Observability
//   - Connection state change callbacks (new, read, write, close)
//   - Error reporting through callback functions
//   - Server lifecycle notifications
//   - Real-time connection counting (OpenConnections)
//   - Server state queries (IsRunning, IsGone)
//
// ## Performance
//   - Goroutine-per-connection model (suitable for 100s-1000s of connections)
//   - Lock-free atomic operations for state management
//   - Zero-copy I/O where possible
//   - Minimal memory overhead per connection (~10KB)
//   - Efficient connection tracking without locks
//   - Lower latency than TCP loopback (typically 2-5x faster)
//
// # Usage Example
//
// Basic echo server:
//
//	import (
//		"context"
//		"io"
//		"github.com/nabbar/golib/socket"
//		"github.com/nabbar/golib/socket/config"
//		"github.com/nabbar/golib/socket/server/unix"
//		"github.com/nabbar/golib/file/perm"
//	)
//
//	func main() {
//		handler := func(c socket.Context) {
//			defer c.Close()
//			io.Copy(c, c) // Echo back received data
//		}
//
//		cfg := config.Server{
//			Network:  protocol.NetworkUnix,
//			Address:  "/tmp/myapp.sock",
//			PermFile: perm.New(0660),
//			GroupPerm: -1, // Use default group
//		}
//
//		srv, err := unix.New(nil, handler, cfg)
//		if err != nil {
//			panic(err)
//		}
//
//		// Start the server
//		if err := srv.Listen(context.Background()); err != nil {
//			panic(err)
//		}
//	}
//
// # Concurrency Model
//
// ## Goroutine-Per-Connection
//
// The server uses a goroutine-per-connection model, where each accepted
// connection spawns a dedicated goroutine to handle it. This model is
// well-suited for:
//
//   - Low to medium concurrent connections (100s to low 1000s)
//   - Long-lived connections (persistent IPC, service communication)
//   - Applications requiring per-connection state and context
//   - Connections with varying processing times
//   - Connections requiring blocking I/O operations
//
// ## Scalability Characteristics
//
// Typical performance profile:
//
//	┌─────────────────┬──────────────┬────────────────┬──────────────┐
//	│  Connections    │  Goroutines  │  Memory Usage  │  Throughput  │
//	├─────────────────┼──────────────┼────────────────┼──────────────┤
//	│  10             │  ~12         │  ~100 KB       │  Excellent   │
//	│  100            │  ~102        │  ~1 MB         │  Excellent   │
//	│  1,000          │  ~1,002      │  ~10 MB        │  Good        │
//	│  10,000         │  ~10,002     │  ~100 MB       │  Fair*       │
//	│  100,000+       │  ~100,002+   │  ~1 GB+        │  Not advised │
//	└─────────────────┴──────────────┴────────────────┴──────────────┘
//
//	  * At 10K+ connections, consider profiling and potentially switching to
//	    an event-driven model or worker pool architecture.
//
// ## Memory Overhead
//
// Per-connection memory allocation:
//
//	Base overhead:           ~8 KB  (goroutine stack)
//	Connection context:      ~1 KB  (sCtx structure)
//	Buffers (handler):       Variable (depends on implementation)
//	─────────────────────────────────
//	Total minimum:           ~10 KB per connection
//
// Example calculation for 1000 connections:
//
//	1000 connections × 10 KB = ~10 MB base
//	+ application buffers (e.g., 4KB read buffer × 1000 = 4 MB)
//	= ~14 MB total for connections
//
// # Performance Considerations
//
// ## Throughput
//
// Unix sockets typically outperform TCP loopback for local IPC:
//
//   - Unix socket:    ~2-5x faster than TCP loopback
//   - Lower latency:  ~50% less than TCP loopback
//   - Higher bandwidth: No TCP/IP stack overhead
//
// The server's throughput is primarily limited by:
//
//  1. Handler function complexity (CPU-bound operations)
//  2. Disk I/O if using file-based operations
//  3. System limits: File descriptors, kernel buffers
//
// Typical throughput (echo handler on localhost):
//
//   - Unix socket: ~1M requests/sec (small payloads)
//   - TCP loopback: ~500K requests/sec (same conditions)
//
// ## Latency
//
// Expected latency profile:
//
//	┌──────────────────────┬─────────────────┐
//	│  Operation           │  Typical Time   │
//	├──────────────────────┼─────────────────┤
//	│  Connection accept   │  <500 µs        │
//	│  Handler spawn       │  <100 µs        │
//	│  Context creation    │  <10 µs         │
//	│  Read/Write syscall  │  <50 µs         │
//	│  Graceful shutdown   │  100 ms - 1 s   │
//	└──────────────────────┴─────────────────┘
//
// ## Resource Limits
//
// System-level limits to consider:
//
//  1. File Descriptors:
//     - Each connection uses 1 file descriptor
//     - Check: ulimit -n (default often 1024 on Linux)
//     - Increase: ulimit -n 65536 or via /etc/security/limits.conf
//
//  2. Socket Buffer Memory:
//     - Per-connection send/receive buffers (typically smaller than TCP)
//     - Tune: sysctl net.unix.max_dgram_qlen (for datagram sockets)
//
//  3. Filesystem:
//     - Socket file must be in a writable directory
//     - Path length limited (typically 108 characters on Linux)
//     - Cleanup required if process crashes
//
// # Limitations
//
// ## Known Limitations
//
//  1. No built-in rate limiting or connection throttling
//  2. No support for connection pooling or multiplexing
//  3. Goroutine-per-connection model limits scalability >10K connections
//  4. No built-in protocol framing (implement in handler)
//  5. No built-in metrics export (Prometheus, etc.)
//  6. Platform support: Linux and macOS only (not Windows)
//
// ## Not Suitable For
//
//   - Remote connections (use TCP instead)
//   - Ultra-high concurrency scenarios (>50K simultaneous connections)
//   - Low-latency HFT applications (<10µs response time)
//   - Systems requiring protocol multiplexing (use gRPC)
//   - Short-lived connections at very high rates (>100K conn/sec)
//   - Windows platforms (Unix sockets not supported)
//
// ## Comparison with Alternatives
//
//	┌──────────────────┬────────────────┬──────────────────┬──────────────┐
//	│  Feature         │  Unix Socket   │  TCP Loopback    │  Named Pipe  │
//	├──────────────────┼────────────────┼──────────────────┼──────────────┤
//	│  Overhead        │  Minimal       │  TCP/IP stack    │  Minimal     │
//	│  Throughput      │  Very High     │  High            │  High        │
//	│  Latency         │  Very Low      │  Low             │  Low         │
//	│  Permissions     │  Filesystem    │  Firewall        │  Filesystem  │
//	│  Network Access  │  No            │  Yes (loopback)  │  No          │
//	│  Platform        │  Unix/Linux    │  All platforms   │  Windows     │
//	│  Best For        │  Local IPC     │  Network compat  │  Windows IPC │
//	└──────────────────┴────────────────┴──────────────────┴──────────────┘
//
// # Best Practices
//
// ## Resource Management
//
//  1. Always use defer for cleanup:
//
//     defer srv.Close()  // Server
//     defer ctx.Close()  // Connection (in handler)
//
//  2. Implement graceful shutdown:
//
//     sigChan := make(chan os.Signal, 1)
//     signal.Notify(sigChan, os.Interrupt, syscall.SIGTERM)
//
//     <-sigChan
//     log.Println("Shutting down...")
//
//     shutdownCtx, cancel := context.WithTimeout(
//     context.Background(), 30*time.Second)
//     defer cancel()
//
//     if err := srv.Shutdown(shutdownCtx); err != nil {
//     log.Printf("Shutdown error: %v", err)
//     }
//
//  3. Monitor connection count:
//
//     go func() {
//     ticker := time.NewTicker(10 * time.Second)
//     defer ticker.Stop()
//
//     for range ticker.C {
//     count := srv.OpenConnections()
//     if count > warnThreshold {
//     log.Printf("WARNING: High connection count: %d", count)
//     }
//     }
//     }()
//
// ## Security
//
//  1. Set restrictive file permissions:
//
//     cfg.PermFile = perm.New(0600)  // Owner only
//     cfg.PermFile = perm.New(0660)  // Owner + group
//
//  2. Use group ownership for access control:
//
//     cfg.GroupPerm = getGidForService()
//
//  3. Configure idle timeouts to prevent resource exhaustion:
//
//     cfg.ConIdleTimeout = 5 * time.Minute
//
//  4. Validate input in handlers (prevent injection, DoS, etc.)
//
//  5. Clean up socket files on unexpected termination
//
// ## Testing
//
//  1. Test with concurrent connections:
//
//     for i := 0; i < numClients; i++ {
//     go func() {
//     conn, _ := net.Dial("unix", socketPath)
//     defer conn.Close()
//     // Test logic...
//     }()
//     }
//
//  2. Test graceful shutdown under load
//
//  3. Test with slow/misbehaving clients
//
//  4. Run with race detector: go test -race
//
// # Platform Support
//
// This package is only available on:
//   - Linux (all architectures)
//   - macOS/Darwin (all architectures)
//
// For Windows or other platforms, the package provides stub implementations
// that return nil. See ignore.go for unsupported platform handling.
//
// # Related Packages
//
//   - github.com/nabbar/golib/socket: Base interfaces and types
//   - github.com/nabbar/golib/socket/config: Server configuration
//   - github.com/nabbar/golib/file/perm: File permission management
//   - github.com/nabbar/golib/network/protocol: Protocol constants
//
// See the example_test.go file for runnable examples covering common use cases.
package unix