Summary:
Currently, manifest size is determined before getting min_log_num.

But between getting manifest size and getting min_log_num, concurrently, a flush might succeed, which will write new records to manifest to make some WALs become outdated, then min_log_num will be correspondingly increased, but the new records in manifest will not be copied into the checkpoint because the manifest's size is determined before them, then the newly outdated WALs will still exist in the checkpoint's manifest, but they are not linked/copied to the checkpoint because their log number is < min_log_num, so a corruption of missing WAL will be reported when restoring from the checkpoint.

Pull Request resolved: https://github.com/facebook/rocksdb/pull/7836

Test Plan: make crash_test

Reviewed By: ajkr

Differential Revision: D25788204

Pulled By: cheng-chang

fbshipit-source-id: a4e5acf30f08270b3c0a95304ff559a9e655252f