Summary:
A `BlockBasedTable` with `TieredSecondaryCache` containing a NVM cache inserts blocks into the compressed cache and the corresponding compressed block into the NVM cache. The `BlockFetcher` is used to get the uncompressed and compressed blocks by calling `ReadBlockContents()` and `GetUncompressedBlock()` respectively. If the file system supports FSBuffer (i.e returning a FS allocated buffer rather than caller provided), that buffer gets freed between the two calls. This PR fixes it by making the FSBuffer unique pointer a member rather than local variable.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/12712
Test Plan:
1. Add a unit test
2. Release validation stress test
Reviewed By: jaykorean
Differential Revision: D57974026
Pulled By: anand1976
fbshipit-source-id: cfa895914e74b4f628413b40e6e39d8d8e5286bd
6c60c8f3f3