store: copy old value when refresh + cas

[xiang90]

Fix #5651

/cc @heyitsanthony @mwitkow

[heyitsanthony]

should this return an error if value != "" in case someone expects this to use the given value like a refreshless CAS?

[heyitsanthony]

lgtm

[LLParse]

Hi @xiang90, I see this is labelled for backport. Which versions is this fix targeted for?

[heyitsanthony]

@LLParse this was backported into 2.3.7.

[LLParse]

@heyitsanthony Thanks...

For reference, I downloaded https://github.com/coreos/etcd/releases/download/v2.3.7/etcd-v2.3.7-darwin-amd64.zip on Sep 2 17:39 MST and it exhibited this behavior. I will build the tagged release from source for now.

[heyitsanthony]

@LLParse I fetched 2.3.7-darwin-amd64.zip and I'm seeing Git SHA: fd17c91 when I run etcd. Checking out the release-2.3 repo at that commit, I see 9ce0e8b right before it. The fix should be built into the binary. How are you testing this?

[LLParse]

My code must be the problem. Sorry. It occurs to me that my initial write could've been erroneous when I ran into this issue and I simply ran into this issue when looking for an answer. Alternatively, I'm using the API incorrectly.

This is the initial registration call.

const (
    nodeLockTTL = 30 * time.Second
)

    n.Id = uuid.NewV4().String()
    n.Info = &NodeInfo{n.Config.Hostname}
    nodeInfoJson, _ := json.Marshal(n.Info)
    n.InfoJson = string(nodeInfoJson)
    log.Printf("Creating node %s\n", n.Id)

    _, err := n.Keys.Set(
        context.Background(),
        fmt.Sprintf("/%s/nodes/%s", n.Config.ClusterName, n.Id),
        n.InfoJson,
        &etcd.SetOptions{TTL: nodeLockTTL})

This is the call that I perceived as setting the value to an empty string.

            _, err := n.Keys.Set(
                context.Background(),
                fmt.Sprintf("/%s/nodes/%s", n.Config.ClusterName, n.Id),
                "",
                &etcd.SetOptions{TTL: nodeLockTTL, Refresh: true})

[LLParse]

Whoooops, definitely my mistake. It is functioning properly for me now - I must've messed up the initial set. My apologies, please disregard this.