Fix RemoteLogDownloaderTest#testPrefetchNum flaky error

[Samrat002]

Purpose

Linked issue: close #1677

Fix the flaky test RemoteLogDownloaderTest#testPrefetchNum

Brief change log

Root Cause Analysis

The RemoteLogDownloader was acquiring a semaphore permit before polling for a download task. If the queue was empty, it would hold the permit while idle, causing a transient dip in the available permit count. This created a race condition that made the testPrefetchNum test fail randomly.

Approach

I've flipped the logic in RemoteLogDownloader.fetchOnce(). Poll for a request first. Only then, try to acquire a permit.

If no permit is available, the request is re-queued, ensuring work isn't lost and priority is maintained. This change prevents the downloader from reserving capacity when it has no work to do.

Tests

ensure RemoteLogDownloaderTest#testDownloadLogInParallelAndInPriority passes multiple times

API and Format

N/A

Documentation

N/A

[Samrat002]

@wuchong PTAL whenever time