etcdctl/ctlv3: do not modify db file on "status"

[gyuho]

This can break production rollback/downgrade process:

For example,

1. start etcd v3.3.10
2. write some data
3. use etcdctl v3.3.10 to save snapshot
4. somehow, upgrading Kubernetes fails, thus rolling back to previous version etcd v3.2.24
5. run etcdctl v3.2.24 snapshot status against the snapshot file saved from v3.3.10 server
6. run etcdctl v3.2.24 snapshot restore fails with "expected sha256 [12..."

/cc @wenjiaswe @jingyih

[jingyih]

Trying to understand this issue. So it is step 5 that is causing this issue?

[gyuho]

@jingyih Yes, etcdctl snapshot status without this change modifies the DB file. We disable freelist sync in v3.3, but when we open the DB file with write mode using etcdctl v3.2, we enable freelist sync again, thus writing the freelist to the DB file in snapshot status command.

[jingyih]

LGTM

Thanks. snapshot status should not modify the db file.