2.x: doOnSubscribe() does not propagate Exception to the onError()

[arazabishov]

There is a PublishProcessor which calls .doOnSubscribe(checkCondition) operator to perform certain check. checkCondition is intended to throw UnsupportedOperationException, which then has to be propagated to the subscriber down the stream to onError() method. Instead, UndeliverableException is being thrown and process crashes.

publishProcessor
    .filter(() -> { // predicate })
    .observeOn(scheduler)
    .doOnSubscribe(checkCondition)
    .to((sourceFlowable) -> new FancyFlowable(sourceFlowable)))
    .safeSubscribe(subscriber);

P.S. This is a cross-post from stackoverflow. According to the @akarnokd, this is issue of RxJava2's doOnSubscribe().

[arazabishov]

My apologies for being not very attentive, pull-request to fix this issue has already been submitted: #5103. Thanks @akarnokd.