WriteConflictException was not being handled properly by the write command update executor. This caused them to erroneously appear to succeed.
The update_mms tests suddenly stopped scaling at all, where last week they did. This is something in the mongo tree, not the WT library as I keep that constant during my testing here and see very different results.
I spent a bit of time today narrowing it down and the issue happens between changeset c84d5ea and changeset 7f9e3f3 (FYI, c84d5ea is a parent of 7f9e3f3). The diff between these two changesets is very small and attached.
Here is the page of commits from GitHub:
Here are the results. I only show Update.MmsIncShallow1 although all show similar patterns.