Golang Parsing of ProtoBuf

Question

I'm new to Golang and I am trying to write an home automation framework in Golang, using the Micro framework and Protobuf framework.

I am currently having a hard time trying to implement a simple registry type service.

An example of the problem I am having is that I have the following, I want to be able to get a list of devices, provided a client does a GET request to http://localhost:8080/view/devices

I have the following protobuf definition:

syntax = "proto3";

service DRegistry {
    rpc View(ViewRequest) returns (DeviceRegistry) {}
} 

message DeviceRegistry {
    repeated Device devices = 1;
}

message ViewRequest {
    string Alias = 1;
}

message Device {
    string Alias = 1;
    string HWAddress = 2;
    string WakeUpMethod = 3;
    repeated string BoundServices = 4;
}

Then in my service defination I have the following:

package main

import (
    "log"

    micro "github.com/micro/go-micro"
    proto "github.com/srizzling/gotham/proto/device"

    "golang.org/x/net/context"
)

// DRegistry stands for Device Registry and is how devices register to Gotham.
type DRegistry struct{}

var devices map[string]proto.Device

func (g *DRegistry) View(ctx context.Context, req *proto.ViewRequest, rsp *proto.DeviceRegistry) error {
    filter := req.Alias
devices, err := filterDevices(filter)
rsp.Devices = devices
}

func filterDevices(filter string) (*[]proto.Device, error) {
    // Currently only supports listing a single service for now
    // TODO: expand filter to be more consise
    filteredDevices := make([]proto.Device, 0, len(devices))
    for _, e := range devices {
        for _, f := range e.BoundServices {
            if f == filter {
                filteredDevices = append(filteredDevices, e)
            }
        }
    }
    return &filteredDevices, nil
}

func main() {
    service := micro.NewService(
        micro.Name("DRegistry"),
    )
    proto.RegisterDRegistryHandler(service.Server(), new(DRegistry))

    if err := service.Run(); err != nil {
        log.Fatal(err)
    }
}

The problem I am having is that my IDE (Visual Studio Code) is complianing that I cannot use devices (type *[]device.Device) as type []*device.Device in assignment which is confusing.

TLDR: How do I assign a collection of proto.Devices to the proto.DeviceRegistry?

Hi! On what line do yo get the error message? *[]device.Device -- pointer to a slice of device.Device. []*device.Device -- slice of pointers to a device.Device. This read might be useful to you: divan.github.io/posts/avoid_gotchas — Stas Makarov
– Stas Makarov, Commented Mar 20, 2017 at 23:22

Alexander Kohler · Accepted Answer · 2017-03-21 00:19:10Z

func filterDevices(filter string) ([]*proto.Device, error) {
    // Currently only supports listing a single service for now
    // TODO: expand filter to be more consise
    filteredDevices := make([]*proto.Device, 0, len(devices))
    for _, e := range devices {
        for _, f := range e.BoundServices {
            if f == filter {
                filteredDevices = append(filteredDevices, &e)
            }
        }
    }
    return filteredDevices, nil
}

There is a difference between a slice of pointers ([]*) and a pointer to a slice (*[]). You are returning a pointer to slice, whereas what you want is a slice of pointers. We can solve this by:

Updating your filterDevices signature to return a slice of pointers
Updating your make call to make a slice of pointers
Taking the address of e in your call to append (we want a slice of pointers to devices)
Not returning the address of the slice

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