v0.7.0 cli client with readline added, directory structure updated

cmd/chess-server/pid.go

@@ -0,0 +1,106 @@
+// FILE: lixenwraith/chess/cmd/chess-server/pid.go
+package main
+
+import (
+	"errors"
+	"fmt"
+	"os"
+	"strconv"
+	"strings"
+	"syscall"
+)
+
+// managePIDFile creates and manages a PID file with optional locking
+// Returns a cleanup function that must be called on exit
+func managePIDFile(path string, lock bool) (func(), error) {
+	// Open/create PID file with exclusive create first attempt
+	file, err := os.OpenFile(path, os.O_CREATE|os.O_EXCL|os.O_WRONLY, 0644)
+	if err != nil {
+		if !os.IsExist(err) {
+			return nil, fmt.Errorf("cannot create PID file: %w", err)
+		}
+
+		// File exists - check if stale
+		if lock {
+			if err := checkStalePID(path); err != nil {
+				return nil, err
+			}
+		}
+
+		// Reopen for writing (truncate existing content)
+		file, err = os.OpenFile(path, os.O_CREATE|os.O_WRONLY|os.O_TRUNC, 0644)
+		if err != nil {
+			return nil, fmt.Errorf("cannot open PID file: %w", err)
+		}
+	}
+
+	// Acquire exclusive lock if requested
+	if lock {
+		if err = syscall.Flock(int(file.Fd()), syscall.LOCK_EX|syscall.LOCK_NB); err != nil {
+			file.Close()
+			if errors.Is(err, syscall.EWOULDBLOCK) {
+				return nil, fmt.Errorf("cannot acquire lock: another instance is running")
+			}
+			return nil, fmt.Errorf("lock failed: %w", err)
+		}
+	}
+
+	// Write current PID
+	pid := os.Getpid()
+	if _, err = fmt.Fprintf(file, "%d\n", pid); err != nil {
+		file.Close()
+		os.Remove(path)
+		return nil, fmt.Errorf("cannot write PID: %w", err)
+	}
+
+	// Sync to ensure PID is written
+	if err = file.Sync(); err != nil {
+		file.Close()
+		os.Remove(path)
+		return nil, fmt.Errorf("cannot sync PID file: %w", err)
+	}
+
+	// Return cleanup function
+	cleanup := func() {
+		if lock {
+			// Release lock explicitly, file close works too
+			syscall.Flock(int(file.Fd()), syscall.LOCK_UN)
+		}
+		file.Close()
+		os.Remove(path)
+	}
+
+	return cleanup, nil
+}
+
+// checkStalePID reads an existing PID file and checks if the process is running
+func checkStalePID(path string) error {
+	// Try to read existing PID
+	data, err := os.ReadFile(path)
+	if err != nil {
+		return fmt.Errorf("cannot read existing PID file: %w", err)
+	}
+
+	pidStr := string(data)
+	pid, err := strconv.Atoi(strings.TrimSpace(pidStr))
+	if err != nil {
+		// Corrupted PID file
+		return fmt.Errorf("corrupted PID file (contains: %q)", pidStr)
+	}
+
+	// Check if process exists using kill(0), never errors on Unix
+	proc, _ := os.FindProcess(pid)
+
+	// Send signal 0 to check if process exists
+	if err = proc.Signal(syscall.Signal(0)); err != nil {
+		// Process doesn't exist or we don't have permission
+		if errors.Is(err, os.ErrProcessDone) || errors.Is(err, syscall.ESRCH) {
+			return fmt.Errorf("stale PID file found for defunct process %d", pid)
+		}
+		// Process exists but we can't signal it (different user?)
+		return fmt.Errorf("process %d exists but cannot verify ownership: %v", pid, err)
+	}
+
+	// Process is running
+	return fmt.Errorf("stale PID file: process %d is running but not holding lock", pid)
+}