Update connection structs to avoid read only warning from server

[GauthierHacout]

Using a ZK server in version 3.4.9 I always have a warning when my app is connecting to it. I believe this is due to this bit of code in the ZK codebase:

        boolean readOnly = false;
        try {
            readOnly = bia.readBool("readOnly");
            cnxn.isOldClient = false;
        } catch (IOException e) {
            // this is ok -- just a packet from an old client which
            // doesn't contain readOnly field
            LOG.warn("Connection request from old client "
                    + cnxn.getRemoteSocketAddress()
                    + "; will be dropped if server is in r-o mode");
        }

ZK now allows the addition of a flag for "ReadOnly" mode, if the client doesn't give this flag it will trigger a warning. If the server is in read-only mode & there's no readOnly field, the connection will we dropped.

The goal of this PR is to avoid triggering this warning, by passing a readOnly always setted to false. I think this can be a first step to handling readOnly ZK server.

[pmazzini]

Are you sure this is the right place where to put this?

I think these structs should match the ones in https://github.com/apache/zookeeper/blob/master/zookeeper-jute/src/main/resources/zookeeper.jute#L62

[pmazzini]

left a comment

[GauthierHacout]

To be honest you're probably more knowledgeable than me on Zookeeper, if you believe it's not the right thing to do I trust you ! I will try to look at how other ZK clients do this when I have time :)

[GauthierHacout]

I updated the Connection struct so we could add a readOnly flag, to avoid modifying the connRequest struct I append a single byte to the []byte used to authenticate to the zkServer. I was inspired by other clients such as kazoo (python ZK client) tell me what you think of this solution

[pmazzini]

I was inspired by other clients such as kazoo (python ZK client)

Can you share the reference?

[GauthierHacout]

Here's the kazoo client, you can see that a read_only flag is passed at init:
https://github.com/python-zk/kazoo/blob/6337fd6f72b59fb20886f980f2e0d6d41525dc35/kazoo/client.py#L107

The read_only flag is after used to extend the byte array by a single 1 or 0, the byte array is the result of the serialization of the connection request
https://github.com/python-zk/kazoo/blob/6337fd6f72b59fb20886f980f2e0d6d41525dc35/kazoo/protocol/serialization.py#L103

[nemith]

Java client appends it as well. Not sure why they didn't extend the jute defintion to add it.

https://github.com/apache/zookeeper/blob/c583a6e79654359b5daad5093d1730e370d3b75b/zookeeper-server/src/main/java/org/apache/zookeeper/ClientCnxn.java#L331

[GauthierHacout]

Hello,

It's been more than a month since this PR had any new comments about the change. Do you know if there's any plan to merge it or if there's any improvement I could do ?

[linsite]

Hello,

It's been more than a month since this PR had any new comments about the change. Do you know if there's any plan to merge it or if there's any improvement I could do ?

I think what they mean is that it's better to serialize connectRequest, then appends the readOnly flag to the end of buffer just as how Java implements it, instead of changing the connectRequest struct. Because the connectRequest struct later will be automatically generated from the jute file, see [https://github.com//issues/28], changing it would be difficult for later refactoring.
@GauthierHacout

[linsite]

Also I suggest you compact the commits into 1, it will be cleaner for the git committing tree.