Yonik uncovered this with the TestRealTimeGet test: if a flush-all is
underway, it is possible for an incoming update to pick a DWPT that is
stale, ie, not yet pulled/marked for flushing, yet the DW has cutover
to a new deletes queue. If this happens, and the deleted term was
also updated in one of the non-stale DWPTs, then the wrong document is
deleted and the test fails by detecting the wrong value.
There's a 2nd failure mode that I haven't figured out yet, whereby 2
docs are returned when searching by id (there should only ever be 1
doc since the test uses updateDocument which is atomic wrt
Yonik verified the test passes pre-DWPT, so my guess is (but I
have yet to verify) this test also passes on 3.x. I'll backport
the test to 3.x to be sure.