21671 direct io + readahead exception fix - #5152
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…buffer The compressed read-ahead buffer was owned by the single scanReader field of a shared CompressedChunkReader, so one buffer was reused across every reader of the file and across threads. To make that shared object safe, its state lived in a static per-thread, per-path Block map. That cross-instance, cross-thread reuse caused the reinit and double-free faults on this path. Own the read-ahead buffer on the per-scan reader instead. forScan() now returns a per-reader view that owns a plain buffer, allocated on open and freed on close. Each scan reader is single-threaded, so no buffer is shared across threads and no thread-local state is needed. N scanners over N inputs give N buffers by construction. Remove the static Block map, the bufferSize == -1 reinit, and the aligned slice resolution in cleanBuffer; each buffer class now frees the buffer type it allocates. Rename ThreadLocalReadAheadBuffer to ReadAheadBuffer and DirectThreadLocalReadAheadBuffer to DirectReadAheadBuffer, since the names no longer describe thread-local state. Add a concurrent-scan test that reads one file from several threads at once, plus per-instance ownership and lifecycle tests.
CachingRebufferer.instantiateRebufferer(isScan) returned this, so it dropped the isScan flag. A scan with the chunk cache on therefore populated the cache with one-shot chunks and evicted hot data. Delegate to the source when isScan is true, so the scan bypasses the cache and uses its own read-ahead buffer. For an Mmap source this also bypasses the chunk cache, which is correct: mmap data already lives in the OS page cache, so the chunk cache only duplicates it. Add ChunkCacheScanBypassTest: a scan adds nothing to the cache while a point read populates it, and a scan under the cache still reads through the read-ahead buffer.
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Fixes direct io + read ahead exception, CASSANRA-21671