broken.ts

const decoder = new TextDecoder();

async function handleRequest({ request, respondWith }: Deno.RequestEvent) {
  let resolve: (response: Response) => void;
  const p = new Promise<Response>((res) => {
    resolve = res;
  });
  const r = respondWith(p);
  let response: Response;
  if (request.body) {
    const body = decoder.decode(await request.arrayBuffer());
    console.log("body:", body);
    response = new Response("body\n");
  } else {
    response = new Response("no body\n");
  }
  resolve!(response);
  await r;
  console.log("done");
}

for await (const conn of Deno.listen({ port: 8000 })) {
  console.log("conn");
  (async () => {
    const httpConn = Deno.serveHttp(conn);
    for await (const requestEvent of httpConn) {
      console.log("request");
      handleRequest(requestEvent);
    }
  })();
}

working.ts

const decoder = new TextDecoder();

async function handleRequest({ request, respondWith }: Deno.RequestEvent) {
  console.log("handleRequest");
  let resolve: (response: Response) => void;
  const p = new Promise<Response>((res) => {
    resolve = res;
  });
  const r = respondWith(p);
  let response: Response;
  if (request.body) {
    const body = decoder.decode(await request.arrayBuffer());
    console.log("body:", body);
    response = new Response("body\n");
  } else {
    response = new Response("no body\n");
  }
  resolve!(response);
  console.log(" request done");
  return r;
}

const listener = Deno.listen({ port: 8000 });

for await (const conn of listener) {
  console.log("conn");
  const httpConn = Deno.serveHttp(conn);
  (async () => {
    for await (const requestEvent of httpConn) {
      await handleRequest(requestEvent);
    }
  })();
}