go - 无效的内存地址或QueryRow上的nil指针取消引用

Question

我是新手，尝试创建登录功能，尝试从数据库查询行时出现此错误:runtime error: invalid memory address or nil pointer dere
ference
此行导致的崩溃:

    result := db.QueryRow("SELECT password FROM users WHERE email=$1", credentials.Email)

这是代码:

    type Credentials struct {
    Email    string `json:"email", db:"email"`
    Password string `json:"password", db:"password"`
}

func SignIn(w http.ResponseWriter, r *http.Request) {
    credentials := &Credentials{}
    err := json.NewDecoder(r.Body).Decode(credentials)
    if err != nil {
        // If there is something wrong with the request body, return a 400 status
        w.WriteHeader(http.StatusBadRequest)
        return
    }

    psqlInfo := fmt.Sprintf("host=%s port=%d user=%s "+"dbname=%s sslmode=disable", dbInfo.Host, dbInfo.Port, dbInfo.User, dbInfo.DBname)
    db, err := sql.Open("postgres", psqlInfo)

    println(credentials.Email)
    result := db.QueryRow("SELECT password FROM users WHERE email=$1", credentials.Email)
    defer db.Close()
    if err != nil {
        //If there is an issue with the database, return a 500 error.
        w.WriteHeader(http.StatusInternalServerError)
        return
    }

    //We create another instance of 'Credentials' t store the credentials we get from the database
    storedCreds := &Credentials{}

    // Store the obtained password in `storedCreds`
    err = result.Scan(&storedCreds.Password)
    if err != nil {
        // If an entry with the email does not exist, send an "Unauthorized"(401) status
        if err == sql.ErrNoRows {
            w.WriteHeader(http.StatusUnauthorized)
            return
        }

        //If the error is of any other type, send a 500 status
        w.WriteHeader(http.StatusInternalServerError)
        return
    }

    if credentials.Password != storedCreds.Password {
        //The two passwords does not match, return a 401 status
        w.WriteHeader(http.StatusUnauthorized)
    }
}

我检查了凭据，电子邮件不为空，并且我不明白是什么导致了此错误。

Answer 1

    db, err := sql.Open("postgres", psqlInfo)
    // ...
    result := db.QueryRow(...)

db可能是 nil，因为 sql.Open 失败。

您不会像在函数返回 err != nil类型时始终应该检查的那样检查 error。