Forgive the noob question, but why is it necessary to have a custom runtime when using a binary from a compiled language? It seems to me that golang support should just mean binary support, and then c++ and rust would be able to comply already, no?
The lambda team is now letting you write that runtime, and presumably providing more documentation on the responsibilities of the runtime.
Check out the example C++ and Rust runtimes to understand why each language needed to have it's own custom runtime.
The way Lambda manages to achieve the performance it does is because it bootstraps the application and then runs each new request through the loaded application.
This means Lambda needs some way to communicate with an already running service.
Initially, Lambda had language specific runtimes that took care of this (i.e. with Node they have their own bootstrapper that loads your top-level JS file and then calls the defined method each time it receives a new event).
With the release of the Go runtime, they built a framework that gets compiled into your service that runs a simplified HTTP server that Lambda then submits events to.
This latest generalised version eschews an embedded HTTP server for letting your app do something like a long-poll to a local event RPC source in the Lambda container. Basically, your app boots and attempts to pull a job off the queue, if there's a job, your Lambda runs, if there isn't, your Lambda service gets paused until there's something to send it.
I hope this runtime API is a simplification of the go rpc api