Deadlock on YdbRepositoryTransaction#executeScanQuery

[std000]

The problem lies in the following code:

session.executeScanQuery(
    yql, sdkParams, settings,
    rs -> new ResultSetConverter(rs).stream(statement::readResult).forEach(spliterator::onNext)
).whenComplete(spliterator::onSupplierThreadComplete);

If the CompletableFuture<Status> completes before whenComplete(spliterator::onSupplierThreadComplete) is registered, then spliterator::onSupplierThreadComplete will be invoked on the main thread.

This method attempts to insert an element into an ArrayBlockingQueue<>(1). If the queue is not empty, it will lock itselft until the element is consumed. However, since the consumer (tryAdvance) runs on the same main thread, a deadlock occurs:

• onSupplierThreadComplete blocks, waiting for the queue to be drained.
• tryAdvance cannot proceed until onSupplierThreadComplete returns.

Eventually, onSupplierThreadComplete fails due to a timeout, and tryAdvance never receives the signal that data has ended — so it also fails.

[nvamelichev]

@lavrukov A brute force solution is to submit whenComplete() to a temporary thread when the handler's Thread.currentThread() == <reference to the thread that called YdbRepositoryTransaction#executeScanQuery()>. See: https://stackoverflow.com/questions/46034531/how-to-prevent-completablefuturewhencomplete-execution-in-context-thread

We might even use a throwaway virtual thread for this, especially if we upgrade YOJ to use Java 21 (which has virtual threads as a non-preview feature) instead of Java 17