/**
 * Multi-producer single-consumer unit-signal channel used to wake the
 * {@link PromisesExecutor} when any external progress source (input read or
 * run-closure completion) has made progress.
 *
 * Producers call {@link signal} to emit a progress event.
 * The single consumer calls {@link awaitNext} to await the next signal.
 *
 * Signals emitted with no consumer waiting are buffered, so no progress signal
 * is ever lost. At most one {@link awaitNext} may be pending at a time; a
 * concurrent second call rejects.
 */
export declare class ExternalProgressChannel {
    private pending;
    private waiter;
    signal(): void;
    awaitNext(): Promise<void>;
}
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