Simultaneous acquisition of kv session lock and atomic update succeeds regardless of modifyIndex

[tcollinsworth]

PUT /v1/kv/someKey?cas=11789&acquire=b2d5847c-08c2-e8e6-d5b4-18d419e1d8e5

Always updates the value and returns true even if the the modifyIndex was incremented by another change (i.e., via the UI) after reading the modifyIndex and before writing with the old lower modifyIndex. When the acquire=sessionId is removed, the same code properly fails to update the value and receives false.

Can you not combine query string modifiers? Didn't see this in the documentation.

Was trying to use this for leadership management. Ideally it would all succeed or all fail. Without an all succeed or all failure semantic, since the lock is advisory, it requires more round trips.

1 acquire lock get back sessionId
2 query key to validate sessionId is still current and to get current modifyIndex, stop if sessionId does not match
3 update with cas, if session was lost update will fail because it incremented modifyIndex

The value being updated is to set the current clientId into the value of the lock Key so the leader will be able to detect when it successfully consecutively reacquires a lock after a lock loss due to false positive failure detection.

[armon]

The ?cas= and ?acquire= operations are distinct and cannot be composed in this manner. I'm not sure which of the two you are getting since it's not well defined, but you don't get the semantics of both of them.