When upgrading the FCV from v4.4 to v5.0, the internal format of config.chunks and config.collections  documents is updated following those steps:

1. Add uuid and timestamp fields to config.chunks documents
2. Create uuid-related indexes
3. Add timestamp to config.collection documents
4. Remove the nss field from config.chunks documents

Because of upgrade order, when working with chunk documents the recurring pattern to support both old and new chunks format is the following:

if(timestamp set on config.collections)
   queries target chunks via uuid
else
   queries target chunks via namespace

This is correct because the presence of a timestamp in config.collections (added at step 3) guarantees the existence of an index on the uuid field (created at step 2). For example, this pattern is implemented in the chunk merge commit path.

There is one specific snippet in the chunk split commit path that mistakenly implemented a slightly different pattern:

if(timestamp set on the config.chunks document to split)
    target chunk via uuid
else
    target chunk via namespace 

This is incorrect because the presence of a timestamp in config.chunks (added at step 1) does not guarantee the existence of an index on the uuid field (created at step 2).

The consequence is that when a chunk split operation commits between (1) and (3), it is highly probable for the change to result in a COLLSCAN.

Purpose of this ticket is to implement the right pattern also in the split commit path.