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30199552ac
The existing `download_source` implementation optimizes performance by keeping the connection to S3 open and draining data directly from the socket. While this eliminates the overhead (60-100ms) of repeatedly establishing new connections, it leads to rapid exhaustion of client- side connections. On a single shard, two `mx_readers` for load and stream are enough to trigger this issue. Since each client typically holds two connections, readers keeping index and data sources open can cause deadlocks where processes stall due to unavailable connections. Introduce `chunked_download_source`, a new S3 download method built on `download_source`, to dynamically manage connections: - Buffers data in 5MiB chunks using a producer-consumer model - Closes connections once buffers reach capacity, returning them to the pool for other clients - Uses a filling fiber that resumes fetching once buffers are consumed from the queue Performance remains comparable to `download_source`, achieving 95MiB/s for sequential 1GiB downloads from S3. However, preloading large chunks may cause read amplification. Fixes: https://github.com/scylladb/scylladb/issues/23785 Closes scylladb/scylladb#23880