feat: defered execution of requests. Respond to client early, but continue handling the request

const testRouter = createTRPCRouter({
  deferedHello: publicProcedure.query(async ({ctx}) => {
      ctx.defer?.(sleep(2000).then(() => console.log('deferred: hello chad'))); //runs after client response send
    return 'Hello chad'
  }),
  hello: publicProcedure.query(() => 'Hello chad')
})

//Technical debt: tRPC only has 2 HTTP verbs - GET & POST. We don't expect our API to be public for now.
import { fetchRequestHandler } from '@trpc/server/adapters/fetch';
import { appRouter } from '@/server/root';
import { after } from 'next/server';

const handler = (req: Request) => {
 /** Hover over `defer` below for docs */
  const deferedTasks: Promise<unknown>[] = []
  const res = fetchRequestHandler({
    endpoint: '/v1/api/trpc',
    req,
    router: appRouter,
    createContext: async () => ({ defer: (p) => deferedTasks.push(p) })
  })

  after(res.then(() => Promise.allSettled(deferedTasks))) //Next.js will return to client, but continue handling the defered tasks.
  return res
};


export { handler as GET, handler as POST };

type TRPCContext = {
  /**
   * @description What if I want to return early to client, but continue handling the request?
   * I implemented 'defer()' as an abstraction for runtime, instead of calling 'after()' from next.js inside every procedure. If we change the server, we'd have to change every 'after()' call, while here we change the equivalent in 1 place.
   * And No, I can't use a middleware for this. Middleware must return before sending the request.
   * @warning ctx.defer() don't work in server-side calls
   * @example ctx.defer?.(await metrics.send()) // only in client -> server RPCs.
   * @example ctx.defer?.(await analytics.send()) // only in client -> server RPCs.
   * @example ctx.defer?.(await email.send()) // only in client -> server RPCs.
   */
  defer?: (task: Promise<unknown>) => unknown //return response early to client and continue handling the request. 
};

//
const t = initTRPC.context<TRPCContext>().create()