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This commit introduces revision of authStore. The revision number represents a version of authStore that is incremented by updating auth related information. The revision is required for avoiding TOCTOU problems. Currently there are two types of the TOCTOU problems in v3 auth. The first one is in ordinal linearizable requests with a sequence like below (): 1. Request from client CA is processed in follower FA. FA looks up the username (let it U) for the request from a token of the request. At this time, the request is authorized correctly. 2. Another request from client CB is processed in follower FB. CB is for changing U's password. 3. FB forwards the request from CB to the leader before FA. Now U's password is updated and the request from CA should be rejected. 4. However, the request from CA is processed by the leader because authentication is already done in FA. For avoiding the above sequence, this commit lets etcdserverpb.RequestHeader have a member revision. The member is initialized during authentication by followers and checked in a leader. If the revision in RequestHeader is lower than the leader's authStore revision, it means a sequence like above happened. In such a case, the state machine returns auth.ErrAuthRevisionObsolete. The error code lets nodes retry their requests. The second one, a case of serializable range and txn, is more subtle. Because these requests are processed in follower directly. The TOCTOU problem can be caused by a sequence like below: 1. Serializable request from client CA is processed in follower FA. At first, FA looks up the username (let it U) and its permission before actual access to KV. 2. Another request from client CB is processed in follower FB and forwarded to the leader. The cluster including FA now commits a log entry of the request from CB. Assume the request changed the permission or password of U. 3. Now the serializable request from CA is accessing to KV. Even if the access is allowed at the point of 1, now it can be invalid because of the change introduced in 2. For avoiding the above sequence, this commit lets the functions of serializable requests (EtcdServer.Range() and EtcdServer.Txn()) compare the revision in the request header with the latest revision of authStore after the actual access. If the saved revision is lower than the latest one, it means the permission can be changed. Although it would introduce false positives (e.g. changing other user's password), it prevents the TOCTOU problem. This idea is an implementation of Anthony's comment: https://github.com/coreos/etcd/pull/5739#issuecomment-228128254