While moving jstests/sharding/merge_split_chunks_test.js to the core_sharding suite, I noticed that one mergeChunk command was failing for StaleConfig. From an example log, it seems that there is a cache refresh and then mergeChunk retry fails with StaleConfig repeatedly.

This only happened in a few suites, as shown in this Build Baron view of the failing patch.

When I removed the offending mergeChunk command (and the associated verifications), the test passed in all suites.

Related question; why does the original jstests/sharding/merge_split_chunks_test.js use a stale mongos? Is it just to ensure that refreshes take place and StaleConfig is not thrown to the client?

The work of this ticket is to answer the questions above and to restore the offending mergeChunk command to jstests/core_sharding/chunk_migration/merge_split_chunks.js.