cache MediaType.toString

[fwbrasil]

Problem

On each request, Tapir uses the toString in MediaType to produce the header. In my benchmark, the instance of MediaType is always the same but the string has to be produced again on each request.

Solution

Make toString a lazy val so the value can be reused.

Notes

• This is part of Reduce allocation rate tapir#3552
• Are MediaType meant to be singletons? If yes, we could make toString a val to avoid the lazy val volatile read. It's likely a quite small overhead, though.

[adamw]

Are MediaType meant to be singletons? If yes, we could make toString a val to avoid the lazy val volatile read. It's likely a quite small overhead, though.

There's a number of pre-defined instances, but during parsing in server interpreters, a new instance is created. We could do pooling of the instances (so that these are true singletons - I think akka-http does it), but I have no idea if that would have any significant value. If in your tests lazy val removes this bottleneck, let's go with that.

[fwbrasil]

Thanks for the quick review! Yes, lazy val is fine with me. The overhead of the volatile read is very small, especially on x86 due to TSO.