On ordered operations, mongos will stop processing replies on the first error it sees. For unordered operations this is not the case. So mongos expects to get back a number of replies corresponding to the number of operations it sent in the case of unordered operations.

However, for certain kinds of errors such as StaleConfig errors, mongod stops processing operations right away (even for unordered operations) and this violates the contract with mongos. We can solve this by "filling up" the rest of the reply vector with copies of the last error in the case of StaleConfig, etc.

This is currently the model in use by batch write operations.

After this ticket is complete, the unit tests here will need to be altered - they are testing that after a single StaleConfig error, even in unordered operations there are no more reply items. However that will no longer be the case.