[doc] go down this rabbit hole of box::pin, recursive async function and write up a doc explaining it

[sonhmai]

          I'm going down this 🐰 hole! this ruins my morning 😆

_Originally posted by @sonhmai in https://github.com/neitomic/doh-server/pull/15#discussion_r1659520046_

        let recursive_response = Box::pin(recursive_lookup(socket, &new_ns_name, QueryType::A)).await?;

• go down this rabbit hole of box::pin, recursive async function
• write up a doc explaining it

[sonhmai]

refs

• https://rust-lang.github.io/async-book/07_workarounds/04_recursion.html
• https://blog.cloudflare.com/pin-and-unpin-in-rust
• Youtube - The Why, What, and How of Pinning in Rust
• https://rust-lang.github.io/async-book/04_pinning/01_chapter.html

Box::pin

Box

Box usage makes sense since

• recursive_lookup is async, rust creates a state machine type for it

  enum RecursiveLookup {
  First(RecursiveLookup)
  }

• the size of the enum is not known at compile time, so we need to allocate it on heap -> Box
• recursive function can potentially be indefinitely also -> what happens if we makes the heap full by doing it indefinitely 🤔 . In java or python it would be a stack overflow, how about in rust?

pin

• makes sure that a value won't be moved in memory after it's created. the value here is Future, it can't be moved once they've polled. -> this is needed for futures type that contain self-reference (recursive call here).
• is a wrapper for pointers to futures
• TODO not clear why it's needed yet