Go 报错 invalid memory address or nil pointer dereference 怎么解决？空指针 panic 排查与 5 种修复姿势

panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x0 pc=0x...]

goroutine 1 [running]:
main.(*User).SayHi(...)
        /app/main.go:10
main.main()
        /app/main.go:15 +0x...

package main

import "fmt"

type User struct{ Name string }

func (u *User) SayHi() string { return "hi, " + u.Name }

func findUser(id int) *User { return nil } // 模拟「没找到」

func main() {
	u := findUser(1)
	var i interface{} = u
	if i != nil {
		fmt.Println("接口不是 nil！") // 会打印：动态类型 *User 非空
	}
	// 类型断言拿出来还是 nil 指针，一调方法就 panic
	v := i.(*User)
	fmt.Println(v.SayHi()) // panic: invalid memory address or nil pointer dereference
}

defer func() {
	if err := recover(); err != nil {
		log.Printf("panic: %v\n%s", err, debug.Stack())
	}
}()

log.Printf("a=%#v, b=%#v", a, a.b)

dlv debug .
(dlv) break main.go:15
(dlv) continue
(dlv) print u    # 会显示 (*main.User)(0x0)

go vet ./...                 # 官方自带，必跑
staticcheck ./...            # SA5011 专门检查「可能为 nil 的指针被解引用」

func Greet(u *User) string {
	if u == nil {
		return "hi, anonymous" // 约定：nil 走默认值分支
	}
	return "hi, " + u.Name
}

u, err := findUser(1)
if err != nil {
	log.Println("find user failed:", err)
	return // ← 漏掉这行，下面的 u 就是 nil
}
fmt.Println(u.SayHi())

func (u *User) SayHi() string {
	if u == nil {
		return "hi, anonymous"
	}
	return "hi, " + u.Name
}

func findUser(id int) *User {
	// 找不到时返回 &User{} 或带 Valid 字段，避免调用方踩 nil
	return &User{Name: ""}
}

func withRecover(next http.Handler) http.Handler {
	return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
		defer func() {
			if err := recover(); err != nil {
				log.Printf("panic: %v\n%s", err, debug.Stack())
				http.Error(w, `{"code":500,"msg":"internal error"}`,
					http.StatusInternalServerError)
			}
		}()
		next.ServeHTTP(w, r)
	})
}