Wait before retrying a download whose cache write failed (0.3.1) - #21

When a write to the partial cache file fails, for example on a full disk, a retained download no longer requests the source again at once. It now waits 5 s before the next attempt, doubling up to a minute while attempts keep failing.

Changes

• HttpCacheStream.download() counts attempts in a row that ended on an error, meaning an onError reported after the last onPosition.
  • A retained stream waits _retryDelay(n) after such an attempt: 5, 10, 20, 40, then 60 s.
  • An attempt that ends cleanly resets the count.
  • The catch path keeps its 5 s delay and now continues, so it doesn't wait twice.
• CHANGELOG ## 0.3.1 Fixes entry, and the version goes to 0.3.1. 0.3.0 is already on pub.dev, so its section loses the (unreleased) label.

Why

BufferedIOSink.flush keeps its failed future, so CacheDownloader cancels. The Downloader loop then exits without its own 5 s delay, and download()'s while (true) loop started a new HTTP attempt immediately. Every attempt reported an error.

On a device whose storage is full, a stream kept requesting its source again and again for as long as it was retained. That costs mobile data, battery and a flood of reported errors.

Tests

• New test/e2e/write_failure_test.dart blocks the partial file with a directory where the file goes:
  • the failed write is reported, and the origin sees one request in 3 s. Without the fix it sees 1389;
  • once the blocker is removed, the attempt after the wait completes the download, for two requests in all.
• Full suite: 147 tests pass. dart analyze lib test is clean.