Provide clarification on reusing serialization buffers

[GiedriusS]

Documentation says:

MarshalToVT() ... This function is useful e.g. when using memory pooling to re-use serialization buffers.

Could you please provide clarification on how to use it? I'm asking because protobufs are typically used with gRPC, and grpc-go's SendMsg() returns before the buffer gets put on the wire. Hence, you cannot reuse it. Here is a relevant issue: grpc/grpc-go#2159. Someone has even attempted this http://www.golangdevops.com/2019/12/31/autopool/ but it's not a solution and the finalizers have poor performance. You could find even more details here thanos-io/thanos#4609.

Are there any examples of the usage of this function? I couldn't find anything.

If I am correct then the recommendation in the README seems dangerous.

[GiedriusS]

Related: grpc/grpc-go#4715.

[nearmeng]

Same question, do you have the solution now?

[GiedriusS]

No, I think you should not use gRPC or the gRPC Go implementation needs to be fixed if you care about pooling on the serialization side. There's some movement here: grpc/grpc-go#6619.

[vmg]

Hey, sorry I've never gotten to this issue before. I think @GiedriusS has already figured this out on his own, but for those reading: the recommendation in the README is unsafe if you're using pooling for the marshal side of protobuf (and I'm going to update it to clarify this). This is a known issue in the way GRPC is implemented in Go, because it keeps ownership of the buffers after they're passed to the RPC call. The tracking issue in grpc/grpc-go#6619 seems to be stalled again, which is slightly disappointing.

I can't think of any good ways to work around this issue until we get upstream changes in GRPC Go.