As per ben.shteinfeld@mongodb.com, path arrayness can apparently not distinguish between the case where the index became multikey from the case where the index was dropped.

As a result, for an index drop, without any change of multikeyneess, the following error is returned to the client:

[fsm_workload_test:join_plan_cache] [jsTest] } on connection: connection to localhost:20000 with errmsg: query plan killed :: non-array path became multikey during yield: namespace=test0_fsmdb0.customer, path=c_nationkey : aggregate failed" : 1,

This error is misleading. If it is not possible to distinguish the two cases, then the error should be changed to "index dropped or non-array path became multikey..." or something along those lines.

Note that this error message appears twice in the codebase, even though I was only able to hit the one in path_arrayness.cpp.