When we have only one shard key index, we cannot hide it.

This behaviour is implemented checking the name of the index:

assert.commandFailed(coll.hideIndex("skey_1")); // OK

But we can still hide the index using the key pattern (not correct):

assert.commandFailed(coll.hideIndex({skey: 1})); // KO

And we will jump the checks and hide it.

To be able to solve this issue we must get the indexName from the cmrIndex->idx and not from the local variable indexName in coll_mod.cpp.