feat(proxystate): add state file package for persistent proxy

internal/proxystate/state.go

@@ -0,0 +1,56 @@
+package proxystate
+
+import (
+	"encoding/json"
+	"fmt"
+	"os"
+	"path/filepath"
+	"syscall"
+)
+
+const stateFileName = "proxy-state.json"
+
+type State struct {
+	PID        int    `json:"pid"`
+	Addr       string `json:"addr"`
+	CACertPath string `json:"ca_cert_path"`
+}
+
+func StatePath(configDir string) string {
+	return filepath.Join(configDir, stateFileName)
+}
+
+func Write(path string, s State) error {
+	data, err := json.Marshal(s)
+	if err != nil {
+		return fmt.Errorf("marshal proxy state: %w", err)
+	}
+	return os.WriteFile(path, data, 0o600)
+}
+
+func Read(path string) (State, error) {
+	data, err := os.ReadFile(path)
+	if err != nil {
+		return State{}, fmt.Errorf("read proxy state: %w", err)
+	}
+	var s State
+	if err := json.Unmarshal(data, &s); err != nil {
+		return State{}, fmt.Errorf("unmarshal proxy state: %w", err)
+	}
+	return s, nil
+}
+
+func Remove(path string) error {
+	return os.Remove(path)
+}
+
+func (s State) IsRunning() bool {
+	if s.PID <= 0 {
+		return false
+	}
+	proc, err := os.FindProcess(s.PID)
+	if err != nil {
+		return false
+	}
+	return proc.Signal(syscall.Signal(0)) == nil
+}