Summary:
**Summary:**
This is to sync an internal change of passing `Statistics*` to the GetContext constructor and collecting more stats as well as fix a bug this change created.
SstFileReader::MultiGet was passing nullptr for both `SystemClock*` and `Statistics*` to the GetContext constructor. After `Statistics*` was passed to the GetContext constructor (the internal change), this caused a segfault when a merge operation was triggered with statistics enabled, because the merge helper's StopWatchNano attempted to dereference the null clock pointer. Fix by passing `r->ioptions.clock` from the reader's options.
Additionally, add `assert(clock_)` guards to `StopWatchNano::Start()` and `ElapsedNanos()` to catch null clock bugs in debug builds. Can't do so in release build because it's on hot path.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/14393
Test Plan:
- `./sst_file_reader_test --gtest_filter='SstFileReaderTableMultiGetTest.Basic'` exercises merge with statistics enabled, previously segfaulted without the fix with the clock, now passes.
- `./sst_file_reader_test` - all tests pass.
Reviewed By: xingbowang
Differential Revision: D94599343
Pulled By: hx235
fbshipit-source-id: 0a748bb00ee27bb202d01d410b52657101c05de0
6aa939b288