system.js

/*
 * Copyright (c) 2021 RethinkDNS and its authors.
 *
 * This Source Code Form is subject to the terms of the Mozilla Public
 * License, v. 2.0. If a copy of the MPL was not distributed with this
 * file, You can obtain one at http://mozilla.org/MPL/2.0/.
 */
import * as util from "./commons/util.js";

// Evaluate if EventTarget APIs can replace this hand-rolled impl
// developers.cloudflare.com/workers/platform/changelog#2021-09-24
// once emitted, they stick; firing off new listeners forever, just the once.
const stickyEvents = new Set([
  // when process bring-up is done
  "prepare",
  // when env setup is done
  "ready",
  // when svc setup is done
  "steady",
  // when all systems are a-go
  "go",
]);

const events = new Set();

const listeners = new Map();
const waitGroup = new Map();

(() => {
  for (const e of events) {
    listeners.set(e, new Set());
    waitGroup.set(e, new Set());
  }

  for (const se of stickyEvents) {
    listeners.set(se, new Set());
    waitGroup.set(se, new Set());
  }
})();

// fires an event
export function pub(event, parcel = undefined) {
  awaiters(event, parcel);
  callbacks(event, parcel);
}

// invokes cb when event is fired
export function sub(event, cb) {
  const eventCallbacks = listeners.get(event);

  if (!eventCallbacks) {
    // if event is sticky, fire off the listener at once
    if (stickyEvents.has(event)) {
      util.microtaskBox(cb);
      return true;
    }
    return false;
  }

  eventCallbacks.add(cb);

  return true;
}

// waits till event fires or timesout
export function when(event, timeout = 0) {
  const wg = waitGroup.get(event);

  if (!wg) {
    // if stick event, fulfill promise right away
    if (stickyEvents.has(event)) {
      return Promise.resolve(event);
    }
    // no such event
    return Promise.reject(new Error(event + " missing"));
  }

  return new Promise((accept, reject) => {
    const tid =
      timeout > 0
        ? util.timeout(timeout, () => {
            reject(new Error(event + " elapsed " + timeout));
          })
        : -2;
    const fulfiller = function (parcel) {
      if (tid >= 0) clearTimeout(tid);
      accept(parcel, event);
    };
    wg.add(fulfiller);
  });
}

function awaiters(event, parcel) {
  const g = waitGroup.get(event);

  if (!g) return;

  // listeners valid just the once for stickyEvents
  if (stickyEvents.has(event)) {
    waitGroup.delete(event);
  }

  util.safeBox(g, parcel);
}

function callbacks(event, parcel) {
  const cbs = listeners.get(event);

  if (!cbs) return;

  // listeners valid just the once for stickyEvents
  if (stickyEvents.has(event)) {
    listeners.delete(event);
  }

  // callbacks are queued async and don't block the caller. On Workers,
  // where IOs or timers require event-context aka network-context,
  // which is only available when fns are invoked in response to an
  // incoming request (through the fetch event handler), such callbacks
  // may not even fire. Instead use: awaiters and not callbacks.
  util.microtaskBox(cbs, parcel);
}