// A tab left open across a release still runs the old build, and the old build's lazy chunks are gone
// from the server the moment the new one is deployed: its first lazy import (the download dialog, a
// module it had not opened yet) answers 404, and the whole app fell to the crash screen. Measured on
// prod 2026-09-19 — `GET /assets/DownloadDialog-….js 404` at 00:59, an hour after 0.89 went out.
//
// Vite announces such a failure as `vite:preloadError` (for the import itself as well as for its
// preloaded dependencies); cancelling it and reloading puts the tab on the current build. Once per
// minute at most: a chunk that is missing from the NEW build too would otherwise reload forever, so
// the second failure inside the minute is let through to the error boundary, which says so.
import { log } from "./log";

const KEY = "sf.staleBuildReload";
const WINDOW_MS = 60_000;

let reloading = false;

/** True once a reload for a newer build is under way: whatever fails to render in the meantime is
 *  that missing chunk, and the error boundary must not report it as a crash. */
export function reloadingForNewBuild(): boolean {
  return reloading;
}

function lastReload(): number {
  try {
    return Number(sessionStorage.getItem(KEY) ?? 0) || 0;
  } catch {
    return 0;
  }
}

/** Returns the uninstall (for tests: the listener is on `window`, which outlives a module reset). */
export function installStaleBuildReload(
  reload: () => void = () => window.location.reload(),
): () => void {
  const onError = (event: Event) => {
    if (reloading) {
      event.preventDefault();
      return;
    }
    if (Date.now() - lastReload() < WINDOW_MS) return;
    try {
      sessionStorage.setItem(KEY, String(Date.now()));
    } catch {
      // No storage (a private window): without the marker a reload could repeat, so don't.
      return;
    }
    event.preventDefault();
    reloading = true;
    log.warn("app.stale_build", "a chunk of this build is gone from the server; reloading");
    reload();
  };
  window.addEventListener("vite:preloadError", onError);
  return () => window.removeEventListener("vite:preloadError", onError);
}