/** Minimal async mutex: serializes critical sections (multi-row upserts) so Postgres never sees two overlapping lock orders. */
export class Mutex {
  private tail: Promise<void> = Promise.resolve();

  run<T>(fn: () => Promise<T>): Promise<T> {
    const next = this.tail.then(fn, fn);
    this.tail = next.then(
      () => undefined,
      () => undefined,
    );
    return next;
  }
}

/** One writer at a time for each hot table family. */
export const barsMutex = new Mutex();
export const quotesMutex = new Mutex();
export const masterMutex = new Mutex(); // instruments, companies, symbol_aliases