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Summary: Historically, `BlobFileReader` has returned the blob(s) read from the file in the `PinnableSlice` provided by the client. This interface was preserved when caching was implemented for blobs, which meant that the blob data was copied multiple times when caching was in use: first, into the client-provided `PinnableSlice` (by `BlobFileReader::SaveValue`), and then, into the object stored in the cache (by `BlobSource::PutBlobIntoCache`). The patch eliminates these copies and the related allocations by changing `BlobFileReader` so it returns its results in the form of heap-allocated `BlobContents` objects that can be directly inserted into the cache. The allocations backing these `BlobContents` objects are made using the blob cache's allocator if the blobs are to be inserted into the cache (i.e. if a cache is configured and the `fill_cache` read option is set). Note: this PR focuses on the common case when blobs are compressed; some further small optimizations are possible for uncompressed blobs. Pull Request resolved: https://github.com/facebook/rocksdb/pull/10647 Test Plan: `make check` Reviewed By: riversand963 Differential Revision: D39335185 Pulled By: ltamasi fbshipit-source-id: 464503d60a5520d654c8273ffb8efd5d1bcd7b36