Golang微服务出现nil指针解引用panic问题排查求助

问题背景 #

开发的Golang gRPC微服务在调用SaveUser接口时触发panic，错误信息为：
panic: runtime error: invalid memory address or nil pointer dereference

相关代码如下：

type DBserver struct {
    protoc.UnimplementedUserManagementServer
    i.DBService
}

func NewDBserver(service i.DBService) *DBserver {
    return &DBserver{
        DBService: service,
    }
}

func (ds *DBserver) Listen(address string) {
    lis, err := net.Listen("tcp", address)
    if err != nil {
        //TODO: log properly (main logger file)
        log.Fatalf("could not listen to tcp conn port :%s : %s", address, err)
    }
    ServerRegistrar := grpc.NewServer()

    protoc.RegisterUserManagementServer(ServerRegistrar, &DBserver{})
    if err := ServerRegistrar.Serve(lis); err != nil {
        //TODO: log properly (main logger file)
        log.Fatalf("could not serve grpc :%s", err)
    }
}

func (ds *DBserver) SaveUser(ctx context.Context, u *protoc.User) (*protoc.DBresponse, error) {

    User := modals.User{
        Fname:     u.Fname,
        Mname:     u.Mname,
        Lname:     u.Lname,
        Username:  u.Username,
        Password:  u.Password,
        Contact:   int(u.Contact),
        Email:     u.Email,
        Sex:       u.Sex,
        Role:      int(u.Role),
        CreatedBy: u.CreatedBy,
    }
    log.Printf("calling DB to create user:\n %v", User)

    createdID, err := ds.DBService.Users("create", User)
    if err != nil {
        return &protoc.DBresponse{
            Success: false,
            ID:      "",
        }, err
    }
    return &protoc.DBresponse{
        Success: true,
        ID:      createdID,
    }, nil
}

运行时错误栈：

panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x18 pc=0x7b89be]

goroutine 9 [running]:
github.com/tazafrosoul/school-system/db-manager/transport/grpc.(*DBserver).SaveUser(0xc000022bf0, {0xc0001ee0f0?, 0x4f2966?}, 0x0?)
        /home/taz/projects/go/src/github.com/tazafrosoul/school-system/db-manager/transport/grpc/dbserver.go:56 +0x11e
github.com/tazafrosoul/school-system/common/grpc/protoc._UserManagement_SaveUser_Handler({0x8328e0?, 0xc000022bf0}, {0x947788, 0xc0001d6b70}, 0xc0001cc580, 0x0)
        /home/taz/projects/go/src/github.com/tazafrosoul/school-system/common/grpc/protoc/dbmgr_grpc.pb.go:79 +0x169
google.golang.org/grpc.(*Server).processUnaryRPC(0xc0001f8200, {0x947788, 0xc0001d6ae0}, {0x94a738, 0xc000228000}, 0xc0001b3200, 0xc0001d64e0, 0xc2c3c0, 0x0)
        /home/taz/go/pkg/mod/google.golang.org/grpc@v1.60.1/server.go:1372 +0xe03
google.golang.org/grpc.(*Server).handleStream(0xc0001f8200, {0x94a738, 0xc000228000}, 0xc0001b3200)
        /home/taz/go/pkg/mod/google.golang.org/grpc@v1.60.1/server.go:1783 +0xfec
google.golang.org/grpc.(*Server).serveStreams.func2.1()
        /home/taz/go/pkg/mod/google.golang.org/grpc@v1.60.1/server.go:1016 +0x59
created by google.golang.org/grpc.(*Server).serveStreams.func2 in goroutine 8
        /home/taz/go/pkg/mod/google.golang.org/grpc@v1.60.1/server.go:1027 +0x115
exit status 2

移除第56行createdID, err := ds.DBService.Users("create", User)并模拟变量后，代码可正常运行，需排查问题原因。

问题原因 #

问题出在Listen方法的服务注册环节：

protoc.RegisterUserManagementServer(ServerRegistrar, &DBserver{})

这里直接新建了一个空的DBserver实例，没有初始化它的DBService字段——而你通过NewDBserver方法创建的、已经正确注入DBService的实例根本没被用到。

当gRPC请求触发SaveUser方法时，当前实例的ds.DBService是nil，调用ds.DBService.Users自然会触发nil指针解引用panic。

修复方案 #

修改Listen方法，使用当前DBserver实例（即方法接收者ds）来注册服务，而不是新建空实例：

func (ds *DBserver) Listen(address string) {
    lis, err := net.Listen("tcp", address)
    if err != nil {
        log.Fatalf("could not listen to tcp conn port :%s : %s", address, err)
    }
    ServerRegistrar := grpc.NewServer()

    // 使用已初始化DBService的当前实例ds
    protoc.RegisterUserManagementServer(ServerRegistrar, ds)
    if err := ServerRegistrar.Serve(lis); err != nil {
        log.Fatalf("could not serve grpc :%s", err)
    }
}

这样注册的服务实例已经通过NewDBserver注入了有效的DBService，调用SaveUser时就不会出现nil指针问题了。

内容的提问来源于stack exchange，提问作者TaZ AfroSoul