src/main/java/io/reactivex/plugins/RxJavaPlugins.java
public static void setOnCompletableSubscribe(
            @Nullable BiFunction<? super Completable, ? super CompletableObserver, ? extends CompletableObserver> onCompletableSubscribe) {
        if (lockdown) {
            throw new IllegalStateException("Plugins can't be changed anymore");
        }
        RxJavaPlugins.onCompletableSubscribe = onCompletableSubscribe;
    }
public static void setOnCompletableSubscribe(
            @Nullable BiFunction<? super Completable, ? super CompletableObserver, ? extends CompletableObserver> onCompletableSubscribe) {
        if (lockdown) {
            throw new IllegalStateException("Plugins can't be changed anymore");
        }
        RxJavaPlugins.onCompletableSubscribe = onCompletableSubscribe;
    }
Sets the specific hook function.
@param onCompletableSubscribe the hook function to set, null allowed