feat: add durable client command spool

internal/client/spool/lock_unix.go

@@ -0,0 +1,47 @@
+//go:build !windows
+
+package spool
+
+import (
+	"errors"
+	"os"
+	"syscall"
+)
+
+func acquireInstanceLock(path string) (func() error, error) {
+	if err := ensurePrivateFile(path); err != nil {
+		return nil, err
+	}
+	file, err := os.OpenFile(path, os.O_RDWR, 0)
+	if err != nil {
+		return nil, err
+	}
+	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, ErrAlreadyOpen
+		}
+		return nil, err
+	}
+	return func() error {
+		unlockErr := syscall.Flock(int(file.Fd()), syscall.LOCK_UN)
+		closeErr := file.Close()
+		if unlockErr != nil {
+			return unlockErr
+		}
+		return closeErr
+	}, nil
+}
+
+func syncDirectory(path string) error {
+	directory, err := os.Open(path)
+	if err != nil {
+		return err
+	}
+	defer directory.Close()
+	return directory.Sync()
+}
+
+func replaceIdentityFile(temporaryPath, targetPath string) error {
+	return os.Rename(temporaryPath, targetPath)
+}