findAndModify on mongoS upserts to the wrong shard

This is simplified jstests/sharding/find_and_modify_after_multi_write.js var st = new ShardingTest({ shards: 2, mongos: 2 });   var testDB = st.s.getDB('test'); testDB.dropDatabase();   testDB.adminCommand({ enableSharding: 'test' }); st.ensurePrimaryShard('test', 'shard0000'); testDB.adminCommand({ shardCollection: 'test.user', key: { x: 1 }}); testDB.adminCommand({ split: 'test.user', middle: { x: 0 }}); testDB.adminCommand({ moveChunk: 'test.user', find: { x: 0 }, to: 'shard0001' });   var testDB2 = st.s1.getDB('test'); testDB2.user.insert({ x: 123456 });   // Move chunk to bump version on a different mongos. testDB.adminCommand({ moveChunk: 'test.user', find: { x: 0 }, to: 'shard0000' });   testDB2.user.update({}, { $inc: { y: 987654 }}, false, true);   // Issue a targetted findAndModify and check that it was upserted to the right shard. testDB2.runCommand({ findAndModify: 'user', query: { x: 100 }, update: { $set: { y: 1 }}, upsert: true });   st.d0.getDB('test').user.findOne({ x: 100 }); The last line should return a record, but it does not because findAndModify updates the wrong shard. The query st.d1.getDB('test').user.findOne({ x: 100 }); finds the record.