Use C++17 [[fallthrough]] in transaction_test.cc

[jaykorean]

Summary

(Copied from https://www.internalfb.com/diff/D46606060)

This diff makes its files safe for use with -Wimplicit-fallthrough. Now that we're using C+20 there's no reason not to use this C++17 feature to make our code safer.
It's currently possible to write code like this:

switch(x){
  case 1:
    foo1();
  case 2:
    foo2();
    break;
  case 3:
    foo3();
}

But that's scary because we don't know whether the fallthrough from case 1 was intentional or not.
The -Wimplicit-fallthrough flag will make this an error. The solution is to either fix the bug by inserting break or indicating intention by using [[fallthrough]]; (from C++17).

switch(x){
  case 1:
    foo1();
    [[fallthrough]]; // Solution if we intended to fallthrough
    break;           // Solution if we did not intend to fallthrough
  case 2:
    foo2();
    break;
  case 3:
    foo3();
}

Test Plan

Existing tests

[facebook-github-bot]

@jaykorean has imported this pull request. If you are a Meta employee, you can view this diff on Phabricator.

[jowlyzhang]

We have a macro for this too: https://github.com/facebook/rocksdb/blob/main/port/lang.h#L10 how about using that macro for now to be consistent with other code and migrate the macro later.

[facebook-github-bot]

@jaykorean has updated the pull request. You must reimport the pull request before landing.

[jowlyzhang]

LGTM, thanks for the PR!

[facebook-github-bot]

@jaykorean has imported this pull request. If you are a Meta employee, you can view this diff on Phabricator.

[facebook-github-bot]

@jaykorean merged this pull request in 9a2a6db.