使用go-sqlmock测试gorm数据库迁移时出现空指针运行时错误

我尝试搭配使用go-sqlmock与gorm编写测试用例，目标是测试数据库初始化迁移逻辑，但运行时触发了panic: runtime error: invalid memory address or nil pointer dereference错误，暂未定位到根因。从错误栈判断，触发崩溃的是db.AutoMigrate(&models.User{})这行代码：目前排查确认到该行执行时db实例已正常初始化，传入参数models.User也已正确定义实例化，怀疑问题出在mocks.NewDatabase函数实现中，但暂未找到问题点。

希望大家帮忙查看下述相关代码定位问题，代码中已标注崩溃发生的位置（位于最后两个代码块），如果需要更多上下文可以告知我。

project/src/models/models.go

package models

import (
    "time"

    "github.com/google/uuid"
    "gorm.io/gorm"
)

type Base struct {
    ID        uuid.UUID      `json:"-" gorm:"primaryKey;type:uuid;not null"`
    CreatedAt time.Time      `json:"-" gorm:"autoCreateTime"`
    UpdatedAt time.Time      `json:"-" gorm:"autoUpdateTime"`
    DeletedAt gorm.DeletedAt `json:"-" gorm:"index"`
}

type User struct {
    Base
    Name     string `json:"-"`
    Email    string `json:"-" gorm:"unique_index:user_email_index"`
    Password string `json:"-" gorm:"size:72"`
}

project/src/mocks/database.go

package mocks

import (
    "project/src/models"
    "log"

    "github.com/DATA-DOG/go-sqlmock"
    "gorm.io/driver/mysql"
    "gorm.io/gorm"
)

func NewDatabase() (*gorm.DB, sqlmock.Sqlmock) {

    // get db and mock
    sqlDB, mock, err := sqlmock.New(
        sqlmock.QueryMatcherOption(sqlmock.QueryMatcherRegexp),
    )
    if err != nil {
        log.Fatalf("[sqlmock new] %s", err)
    }
    defer sqlDB.Close()

    // create dialector
    dialector := mysql.New(mysql.Config{
        Conn: sqlDB,
        DriverName: "mysql",
    })

    // a SELECT VERSION() query will be run when gorm opens the database
    // so we need to expect that here
    columns := []string{"version"}
    mock.ExpectQuery("SELECT VERSION()").WithArgs().WillReturnRows(
        mock.NewRows(columns).FromCSVString("1"),
    )

    // open the database
    db, err := gorm.Open(dialector, &gorm.Config{ PrepareStmt: true })
    if err != nil {
        log.Fatalf("[gorm open] %s", err)
    }

    return db, mock
}

project/src/database/init.go

package database

import (
    "project/src/models"

    "gorm.io/gorm"
)

// Init auto-migrates the DB.
func Init(db *gorm.DB) {
    // Migrate the schema
    // 此处触发panic
    // panic: runtime error: invalid memory address or nil pointer dereference
    // 此处User已正确定义并实例化
    db.AutoMigrate(&models.User{})
}

project/src/database/init_test.go（测试用例）

package database

import (
    "project/src/mocks"
    "testing"
)

func TestInitMigratesDB(t *testing.T) {
    db, mock := mocks.NewDatabase()
    mock.ExpectExec("CREATE TABLE users(.*)")
    mock.ExpectCommit()
    
    // 此处执行失败
    Init(db)
}

Running tool: /usr/local/go/bin/go test -timeout 30s -run ^TestInitMigratesDB$ project/src/database

--- FAIL: TestInitMigratesDB (0.00s)
panic: runtime error: invalid memory address or nil pointer dereference
    panic: runtime error: invalid memory address or nil pointer dereference [recovered]
    panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x0 pc=0x11ce22e]

goroutine 35 [running]:
testing.tRunner.func1.2({0x1505320, 0x19bfb00})
    /usr/local/go/src/testing/testing.go:1209 +0x24e
testing.tRunner.func1()
    /usr/local/go/src/testing/testing.go:1212 +0x218
panic({0x1505320, 0x19bfb00})
    /usr/local/go/src/runtime/panic.go:1038 +0x215
database/sql.(*Rows).close(0x0, {0x0, 0x0})
    /usr/local/go/src/database/sql/sql.go:3267 +0x8e
database/sql.(*Rows).Close(0x1e)
    /usr/local/go/src/database/sql/sql.go:3263 +0x1d
panic({0x1505320, 0x19bfb00})
    /usr/local/go/src/runtime/panic.go:1038 +0x215
database/sql.(*Rows).Next(0x0)
    /usr/local/go/src/database/sql/sql.go:2944 +0x27
database/sql.(*Row).Scan(0xc0000afbd8, {0xc0000efb38, 0x11, 0x1})
    /usr/local/go/src/database/sql/sql.go:3333 +0xb4
gorm.io/gorm/migrator.Migrator.CurrentDatabase({{0x0, 0xc000483350, {0x1659c58, 0xc00041a0f0}}})
    /go/pkg/mod/gorm.io/gorm@v1.21.15/migrator/migrator.go:673 +0x8d
gorm.io/gorm/migrator.Migrator.HasTable.func1(0xc0000f8380)
    /go/pkg/mod/gorm.io/gorm@v1.21.15/migrator/migrator.go:265 +0x51
gorm.io/gorm/migrator.Migrator.RunWithValue({{0x80, 0xc000483260, {0x1659c58, 0xc00041a0f0}}}, {0x1512320, 0xc0004fe2a0}, 0xc0000efcb8)
    /go/pkg/mod/gorm.io/gorm@v1.21.15/migrator/migrator.go:50 +0x126
gorm.io/gorm/migrator.Migrator.HasTable({{0x0, 0xc000483260, {0x1659c58, 0xc00041a0f0}}}, {0x1512320, 0xc0004fe2a0})
    /go/pkg/mod/gorm.io/gorm@v1.21.15/migrator/migrator.go:264 +0xe8
gorm.io/gorm/migrator.Migrator.AutoMigrate({{0x0, 0xc000426f90, {0x1659c58, 0xc00041a0f0}}}, {0xc00040f690, 0x0, 0x0})
    /go/pkg/mod/gorm.io/gorm@v1.21.15/migrator/migrator.go:92 +0x127
gorm.io/gorm.(*DB).AutoMigrate(0x151a800, {0xc00040f690, 0x1, 0x1})
    /go/pkg/mod/gorm.io/gorm@v1.21.15/migrator.go:26 +0x43
project/src/database.Init(0xc00041c230)
    /Projects/project/src/database/init.go:12 +0x7b
project/src/database.TestInitMigratesDB(0x0)
    /Projects/project/src/database/init_test.go:12 +0x5a
testing.tRunner(0xc0003a21a0, 0x15b5328)
    /usr/local/go/src/testing/testing.go:1259 +0x102
created by testing.(*T).Run
    /usr/local/go/src/testing/testing.go:1306 +0x35a
FAIL    project/src/database    0.276s
FAIL

根因 #

1. 核心错误是mocks/database.go中错误添加了defer sqlDB.Close()逻辑：defer会在NewDatabase函数返回前执行，导致返回给测试用例的gorm DB实例底层持有的sql连接已经被提前关闭，后续AutoMigrate访问已关闭的连接时触发空指针panic。
2. 次要问题是mock的SQL预期不全：AutoMigrate执行过程中除了SELECT VERSION()、建表语句外，还会执行查询当前数据库名、判断users表是否存在等SQL，现有测试只声明了建表的预期，会导致mock匹配失败。

修复步骤 #

1. 调整mocks/database.go #

删除defer sqlDB.Close()行，调整函数返回值把底层sqlDB实例返回，供测试用例结束后释放资源：

import "database/sql" // 新增引入

func NewDatabase() (*sql.DB, *gorm.DB, sqlmock.Sqlmock) {
    sqlDB, mock, err := sqlmock.New(
        sqlmock.QueryMatcherOption(sqlmock.QueryMatcherRegexp),
    )
    if err != nil {
        log.Fatalf("[sqlmock new] %s", err)
    }
    // 删除 defer sqlDB.Close() 这行

    dialector := mysql.New(mysql.Config{
        Conn: sqlDB,
        DriverName: "mysql",
    })

    columns := []string{"version"}
    mock.ExpectQuery("SELECT VERSION()").WithArgs().WillReturnRows(
        mock.NewRows(columns).FromCSVString("1"),
    )

    db, err := gorm.Open(dialector, &gorm.Config{ PrepareStmt: true })
    if err != nil {
        log.Fatalf("[gorm open] %s", err)
    }
    return sqlDB, db, mock
}

2. 调整测试用例init_test.go，补全mock预期和资源释放逻辑 #

func TestInitMigratesDB(t *testing.T) {
    sqlDB, db, mock := mocks.NewDatabase()
    // 测试结束后关闭底层连接
    defer sqlDB.Close()

    // 补全AutoMigrate所需的SQL预期
    // 预期查询当前数据库名
    mock.ExpectQuery("SELECT DATABASE()").WillReturnRows(sqlmock.NewRows([]string{"database"}).AddRow("test"))
    // 预期查询users表是否存在
    mock.ExpectQuery("SHOW TABLES LIKE 'users'").WillReturnRows(sqlmock.NewRows([]string{"tables_in_test"}))
    // 预期建表语句
    mock.ExpectExec("CREATE TABLE `users`.*").WillReturnResult(sqlmock.NewResult(1, 0))

    Init(db)

    // 校验所有预期SQL都被执行
    if err := mock.ExpectationsWereMet(); err != nil {
        t.Errorf("mock预期未全部满足: %s", err)
    }
}

如果不想调整NewDatabase的返回值，也可以直接删除defer sqlDB.Close()后，在测试用例中通过sqlDB, err := db.DB()获取底层sql实例再关闭。

内容的提问来源于stack exchange，提问作者LeviJames