[v24.1.x] [CORE-2581] cst: move chunk downloads to remote segment bg loop #18269
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Remote segment background loop now downloads all enqueued chunks. This download was previous done by chunk API from any fiber. The downloads are only performed at the end of a loop iteration, provided that the index is materialized and we are not operating in legacy mode, where full segments are downloaded. The chunk download requests are kept in a fragmented vector. Only one download is expected per chunk start offset. The first request for a chunk is added to the vector, the chunk API is expected to enqueue the subsequent callers into a waiter list. Since chunk consumers use a shared ptr to access the chunk file handle, and the file handle is evicted when there are no consumers attached to it, we maintain a single entry for a chunk download request within remote to ensure that we do not return multiple ss::file objects for the same chunk file. (cherry picked from commit e2dd582)
The chunk API now uses the download_chunk method, and removes unused code. The downloads are now enqueued in the remote segment bg-loop. (cherry picked from commit cc1d48c)
The ducktape test for chunk read now deletes chunk files randomly while the random and seq consumer are running. This increases the chances of the chunk materialization failing and hitting the code path where chunk has to be rehydrated again. (cherry picked from commit 305118d)
(cherry picked from commit 5419116)
abhijat
approved these changes
May 6, 2024
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Backport of PR #18093