Import multiple protobuf files with common message names in a single proto file

I have several protobuf files that share some message names and for this reason I cannot have all the generated go files in one folder as I run into issues like "..re-declared in this block". I attached a minimal go project how to reproduce this issue. The structure of the project is like below:

protos/myMessages/Folder0/MyMessage0.proto

syntax = "proto2";

package myMessages.Folder0;

option go_package = "myMessages/Folder0";

message Message0 {
    optional uint32 payload = 1;
}

protos/myMessages/Folder1/MyMessage1.proto

syntax = "proto2";

package myMessages.Folder1;

option go_package = "myMessages/Folder1";

message Message0 {
    optional uint32 payload = 1;
}

protos/myMessages/helloworld.proto

syntax = "proto2";

package myMessages;

option go_package = "./;gen";

import "myMessages/Folder0/MyMessage0.proto";
import "myMessages/Folder1/MyMessage1.proto";

service Greeter {
  rpc SayHello1 (myMessages.Folder0.Message0) returns (myMessages.Folder0.Message0) {}
  rpc SayHello2 (myMessages.Folder1.Message0) returns (myMessages.Folder1.Message0) {}
}

main.go

package main

import (
	"context"
	"flag"
	"fmt"
	"log"
	"net"

	"google.golang.org/grpc"
	
	pb0 "main/build/myMessages/Folder0"
	pb1 "main/build/myMessages/Folder1"
	pb "main/build"
)

var (
	port = flag.Int("port", 50051, "The server port")
)

// server is used to implement helloworld.GreeterServer.
type server struct {
	pb.UnimplementedGreeterServer
}

func (s *server) SayHello1(ctx context.Context, in *pb0.Message0) (*pb0.Message0, error) {
	return &pb0.Message0{}, nil
}

func (s *server) SayHello2(ctx context.Context, in *pb1.Message0) (*pb1.Message0, error) {
	return &pb1.Message0{}, nil
}

func main() {
	flag.Parse()
	lis, err := net.Listen("tcp", fmt.Sprintf(":%d", *port))
	if err != nil {
		log.Fatalf("failed to listen: %v", err)
	}
	s := grpc.NewServer()
	pb.RegisterGreeterServer(s, &server{})
	log.Printf("server listening at %v", lis.Addr())
	if err := s.Serve(lis); err != nil {
		log.Fatalf("failed to serve: %v", err)
	}
}

Makefile

all: build protos

protos:
	protoc -Iprotos --go_out=build protos/myMessages/Folder0/MyMessage0.proto
	protoc -Iprotos --go_out=build protos/myMessages/Folder1/MyMessage1.proto
	protoc -Iprotos --go_out=build --go-grpc_out=build protos/helloworld.proto
	go build -o build/server main.go

build:
	mkdir -p build

After I run the make command, I get the error below:

mkdir -p build
protoc -Iprotos --go_out=build protos/myMessages/Folder0/MyMessage0.proto
protoc -Iprotos --go_out=build protos/myMessages/Folder1/MyMessage1.proto
protoc -Iprotos --go_out=build --go-grpc_out=build protos/helloworld.proto
go build -o build/server main.go
build/helloworld.pb.go:12:2: package myMessages/Folder0 is not in GOROOT (/home/andrei/env/gosdk/go1.17.2/src/myMessages/Folder0)
build/helloworld.pb.go:13:2: package myMessages/Folder1 is not in GOROOT (/home/andrei/env/gosdk/go1.17.2/src/myMessages/Folder1)
make: *** [Makefile:21: protos] Error 1

The generated build/helloworld.pb.go and build/helloworld_grpc.pb.go files have the following imports:

import (
	Folder0 "myMessages/Folder0"
	Folder1 "myMessages/Folder1"
)

But if I change these to the following then it works:

import (
	Folder0 "main/build/myMessages/Folder0"
	Folder1 "main/build/myMessages/Folder1"
)

What's the correct way to organise this code to avoid manually adding the main/build prefix to the imports in the generated files?

I was able to get this to work by doing the following:

1. Get your module name from go.mod. In this instance, I'll assume it's hello-world

module hello-world
...

2. Configure the protos to build as subpackages of your module

protos/myMessages/Folder0/MyMessage0.proto

...
option go_package = "hello-world/myMessages/Folder0";
...

protos/myMessages/Folder1/MyMessage1.proto

...
option go_package = "hello-world/myMessages/Folder1";
...

protos/myMessages/helloworld.proto

...
option go_package = "hello-world/myMessages";
...

3. Generate the proto code in your project directory using source_relative paths

protoc -Iprotos --go_out=. --go_opt=paths=source_relative protos/myMessages/Folder0/MyMessage0.proto
protoc -Iprotos --go_out=. --go_opt=paths=source_relative protos/myMessages/Folder1/MyMessage1.proto
protoc -Iprotos --go_out=. --go_opt=paths=source_relative --go-grpc_opt=paths=source_relative --go-grpc_out=. protos/myMessages/helloworld.proto

You could also configure these and the go_package paths to use the build directory: use --go_out=build in this step, and change the go package paths in the previous and following steps to "hello-world/build/myMessages/<something>".

However, I think build is better for build products (not generated code), and it isn't how they do it in the official example code: https://developers.google.com/protocol-buffers/docs/gotutorial

4. Import the myMessages submodules as necessary

main.go

package main

import (
	...

	pb "hello-world/myMessages"
	pb0 "hello-world/myMessages/Folder0"
	pb1 "hello-world/myMessages/Folder1"
)
...

I also suggest using proto3 instead of proto2 per: https://cloud.google.com/apis/design/proto3

> To simplify developer experience and improve runtime efficiency, gRPC APIs should use Protocol Buffers version 3 (proto3) for API definition.

I also suggest using buf tools and/or building with bazel instead of calling protoc directly to avoid chasing these problems around in the future.