Since a very old commit we've had this code in the __wt_txn_id_alloc function:

        id = __wt_atomic_addv64(&txn_global->current, 1) - 1;

The comment above the code reads:

so we want post-increment semantics. Our atomic add primitive does pre-increment, so adjust the result here. 

It seems like at the time the fetch_add primitive did not exist in WiredTiger. Which means we can simplify this line of code to:

       id = __wt_atomic_fetch_addv64(&txn_global->current, 1); 

This will return the value pre-increment, and be the correct way of doing what the code aims to atomically.

Scope:

• Replace both occurrences with fetch_add.
• Run stress testing on the change.