[SERVER-69429] Missing checks in collMod for shard key and unique index - MongoDB Jira

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The following does not fail:

db.coll.createIndex({"a": 1}, {unique: false})

sh.shardCollection( "test.coll", { b: 1 }, false, { collation: { locale: "simple" } } )

db.runCommand({ collMod: "coll", index: { keyPattern: {"a": 1}, prepareUnique: true}})

db.runCommand({ collMod: "coll", index: { keyPattern: {"a": 1}, unique: true}})

Unlike trying to create the index on "a" with {unique: true} after the shardCollection which would give

cannot create unique index over {a: 1.0} with shard key pattern {b: 1.0}

I found this as part of the work on unlike topologies for cluster to cluster replication (Mongosync).

related to

SERVER-69874 Document or possibly mitigate scenario where shards end up with different prepareUnique and unique index settings