Summary:
Added DB::GetDbSessionId by using the same format and machinery as DB::GetDbIdentity.
The DB Session ID is generated (and therefore, updated) each time a DB object is opened. It is written to the LOG file right after the line of “DB SUMMARY”.
A test for the uniqueness, for different openings and during the same opening, is also added.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/6959
Test Plan: Passed make check
Reviewed By: zhichao-cao
Differential Revision: D21951721
Pulled By: gg814
fbshipit-source-id: 958a48a612db49a39998ea703cded45987d3fa8b