Summary
The detection for the non-overlapping case in WT_MODIFY application is prone to false negatives.

Motivation

• Does this affect any team outside of WT?
  In MongoDB time-series inserts, the support for using WT_MODIFY is disabled due to performance being worse than a simple full document. One of the reasons for doing that is that WiredTiger is materialising all intermediate modifies in a transaction once it is out of the fast path. The inserts are non-overlapping and don't contain padding so they should be taking the fast single-pass path instead of the slower one.

• Is this issue urgent?
  No

Suggested Solution
I've attached a patch for WT_MODIFY that improves the padding and overlap detection. It seems to pass the WT tests for WT_MODIFY application (test_cursor12.py).