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…odies Same os_image_hash was downloaded by every concurrent /verify without a lock. The finish path did exists()+remove_dir_all()+rename on the shared cache directory, which fails with ENOTEMPTY (os error 39) under QUEUE_CONCURRENCY=20. Downloads also failed closed on truncated HTTP bodies (end of file before message length reached) with no retry. Serialize same-hash downloads behind a per-hash async mutex with double-checked metadata.json, retry truncated bodies up to 3 times, and reject a complete stream whose byte count does not match Content-Length.
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Problem
Concurrent
POST /verifyfor the sameos_image_hashraced on the OS image cache.ensure_image_downloadedcheckedmetadata.jsonwith no lock, anddownload_imagefinished withexists()+remove_dir_all+renameon the shared destination. UnderQUEUE_CONCURRENCY=20that failed withDirectory not empty (os error 39).Truncated HTTP bodies also failed closed with no retry (
end of file before message length reached).Fix
Serialize same-hash downloads behind a per-hash async mutex and double-check
metadata.jsonso only one task downloads. Retry truncated bodies up to 3 times, and reject a complete stream whose byte count does not matchContent-Length.Verified with
cargo test -p dstack-verifier --all-features, includingconcurrent_same_hash_downloads_single_flightandtruncated_image_download_retries_after_content_length_mismatch.