src/main/java/io/reactivex/plugins/RxJavaPlugins.java
public static boolean onBeforeBlocking() {
        BooleanSupplier f = onBeforeBlocking;
        if (f != null) {
            try {
                return f.getAsBoolean();
            } catch (Throwable ex) {
                throw ExceptionHelper.wrapOrThrow(ex);
            }
        }
        return false;
    }
public static boolean onBeforeBlocking() {
        BooleanSupplier f = onBeforeBlocking;
        if (f != null) {
            try {
                return f.getAsBoolean();
            } catch (Throwable ex) {
                throw ExceptionHelper.wrapOrThrow(ex);
            }
        }
        return false;
    }
Called before an operator attempts a blocking operation
such as awaiting a condition or signal
and should return true to indicate the operator
should not block but throw an IllegalArgumentException.
<p>History: 2.0.5 - experimental
@return true if the blocking should be prevented
@see #setFailOnNonBlockingScheduler(boolean)
@since 2.1