Support for `std::sync::Arc` as indirection

[segeljakt]

Hi, I wonder, would be possible to allow prost-types to be wrapped inside std::sync::Arc? The reason is that I want my messages to be sendable across both processes and threads. When sent across processes, my messages should be serialized/deserialized with prost. When sent across threads, I would like to use reference counting.

#[derive(prost::Message)]
struct MyMessage {
    #[prost(string, tag = "1")]
    label: Arc<String>,
    #[prost(message, tag = "2")]
    payload: Arc<Payload>,
}

#[derive(prost::Message)]
struct Payload {
    #[prost(i32, repeated, tag = "1")]
    stuff: Vec<i32>,
}

Thanks for the great crate.

[wchargin]

This would also be nice in conjunction with Tonic servers. If we had something analogous to bytes or btree_map, like:

Config::new()
    .arc(&[".my_service.FooRequest.my_big_field"])
    .compile_protos(...)

…then we could get away with fewer wasted clones in RPC handlers.

I think that this might be nice, but am not strongly lobbying for it, and wouldn’t be too upset if it were wontfixed.

[danburkert]

Sounds like a good addition! We should probably also support Box and Rc similarly.

[bjchambers]

This could also be helpful (I think) for cases where a large object is received in the request and needs to outlive it. Allowing that object to be an Arc would allow it to be taken outside the request/response lifetime without copying.

[name1e5s]

🤔Seems it's possible to implement the feature, I'll try this.

[The-Mr-L]

will this be on the road map?

[ldm0]

Sounds like a good addition! We should probably also support Box and Rc similarly.

Rc cannot be supported since prost::Message requires Send + Sync

[jasongoodwin]

Yeah I can here just to comment/upvote this. I'm trying to use a watch with tonic and I need to clone the message that hits each receiver because arc isn't supported. I can probably modify some of the generated code.

[AshleySchaeffer]

I can probably modify some of the generated code.

@jasongoodwin do you happen to have an implementation for this? I'm in a similar position, only the data I'm sending can get quite large.

[morrisonlevi]

Rc cannot be supported since prost::Message requires Send + Sync

Honestly... why does Message require Send + Sync? I peeked at the associated functions and there didn't seem any reason for it.

It's actually blocking me for another reason: I tried implementing Message for Cow<str>. I can logically encode and decode a Cow<str> into proto string:

1. Encoding is easy, it can borrow basically all of string encoding infrastructure.
2. Decoding is not bad either:
   • If it's an empty string, then decode it as Cow::Borrowed("").
   • If it's not an empty string, then decode it as Cow::Owned String.

But due to the requirement of Message to be Send + Sync, it doesn't matter.

The requirement of Send is particularly odd, because I would think serializing to protobuf is one way you can send non-Send types to other threads...