src/main/java/io/reactivex/internal/operators/observable/ObservableBlockingSubscribe.java
public static <T> void subscribe(ObservableSource<? extends T> o) {
        BlockingIgnoringReceiver callback = new BlockingIgnoringReceiver();
        LambdaObserver<T> ls = new LambdaObserver<T>(Functions.emptyConsumer(),
        callback, callback, Functions.emptyConsumer());

        o.subscribe(ls);

        BlockingHelper.awaitForComplete(callback, ls);
        Throwable e = callback.error;
        if (e != null) {
            throw ExceptionHelper.wrapOrThrow(e);
        }
    }
public static <T> void subscribe(ObservableSource<? extends T> o) {
        BlockingIgnoringReceiver callback = new BlockingIgnoringReceiver();
        LambdaObserver<T> ls = new LambdaObserver<T>(Functions.emptyConsumer(),
        callback, callback, Functions.emptyConsumer());

        o.subscribe(ls);

        BlockingHelper.awaitForComplete(callback, ls);
        Throwable e = callback.error;
        if (e != null) {
            throw ExceptionHelper.wrapOrThrow(e);
        }
    }
Runs the source observable to a terminal event, ignoring any values and rethrowing any exception.
@param o the source ObservableSource
@param <T> the value type