Why is the check within func Mutex.Lock() needed after Txn committing?

[ch0ngsheng]

https://github.com/etcd-io/etcd/blob/be2929568f81080b20ef6812992f2e09c8dac91b/client/v3/concurrency/mutex.go#L78

After Txn is commited, getOwner should return exactly one KV and len(ownerKey)should equals to 1. Because if cmp is true, we will put one into it within the same txn.

So, in which case we will have len(ownerKey) equals to 0?

The relevant PR is https://github.com/etcd-io/etcd/pull/8107

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