Hold replay batches until Retry-After has passed

[ColtenOuO]

Summary

After a 429, the replay feed put the refused batch back at the front of the queue and recorded when the server's rate-limit window closes, but nothing read that value. The next flush was still scheduled a fixed second later, and sendQueuedBatch posted without checking the window, so a Retry-After: 30 was answered with a new request every second until the window passed.

Changes

• Add scheduleFlush(), which waits for whichever is later: the usual one second or the end of the rate-limit window. Both fixed one-second setTimeout calls now go through it.
• sendQueuedBatch no longer posts while the window is open; it reschedules the flush for the window's end.
• Behaviour change: a flush awaited while the window is still open, such as the interview's last one, now returns without posting instead of drawing another 429.
• Add tests/browser/replay-feed.test.js, which runs the feed against mocked timers and fetch. After a 429 with Retry-After: 30, nothing is posted for the next 29 seconds, and at 30 seconds the kept batch goes out together with the event queued behind it.

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Summary by cubic

Holds replay batches until the server's Retry-After window has passed. Previously, after a 429 the feed scheduled the next flush a fixed second later and posted without checking the recorded window, so a Retry-After: 30 drew new requests every second.

Bug Fixes

• Adds scheduleFlush(), which waits for the later of the usual one second or the end of the rate-limit window; both fixed one-second timers now use it.
• sendQueuedBatch no longer posts while the window is open and reschedules the flush for the window's end.
• A flush awaited while the window is still open now returns without posting instead of drawing another 429.
• An interview's final "ended" event is lost if the tab closes before the window passes.
• Adds tests/browser/replay-feed.test.js, which mocks timers and fetch to verify nothing posts for 29 seconds after a 429 and the kept batch goes out with the queued event at second 30.

^{Written for commit 0779fbd. Summary will update on new commits.}

[jserv]

Thank @ColtenOuO for contributing!