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Current test failures with the 2.5.x nightly build:
These are coming out of various tests in the suite, so it's likely some common problem in DocumentPersister.
Cross-referencing the "$set is empty" error to SERVER-12266.
There is also a problem with DBRef objects and $elemMatch queries in SERVER-12263.
Ensure $in argument is a real BSON array when loading references (#741)
Use replacement document for upserts with no modifiers (#741)
The "$set is empty" error (SERVER-12266) will require attempting either the pre-2.6 or 2.6+ method first, and then falling back on error. The server will not be removing the strictness check, nor can we rely on the pre-2.6 bug being backported (even if it was, it wouldn't help for versions prior to the 2.4.x bugfix). We can possibly check the server version up front.
The DBRef $elemMatch issue (SERVER-12263) should be fixed in a 2.6 release candidate.
SERVER-12263 was updated to support additional fields in $elemMatch DBRefs, but we're still missing support for querying with partial DBRef fields (e.g. just $id).
Handle _id-only upserts for MongoDB 2.6 and older versions (#741)
With #750 merged, we're just waiting on a server fix (hopefully in 2.6.0-rc0).
SERVER-12263 is resolved, so we'll have the fix in 2.6.0-rc0.