KAFKA-20738: Producer 2PC InitProducerId request must negotiate v6

[Shekharrajak]

Ref https://issues.apache.org/jira/browse/KAFKA-20738

A producer with transaction.two.phase.commit.enable=true fails initTransactions() with UnsupportedVersionException: Attempted to write a non-default enable2Pc at version 5.

Root cause: v6 is latestVersionUnstable, the builder caps latestAllowedVersion at 5, so NetworkClient can never negotiate v6.

Fix: When enable2Pc or keepPreparedTxn is set, the builder allows the unstable last version (v6) so v6 is negotiated against a capable broker. Plain producers are unaffected (still capped at v5). build() now throws a clear UnsupportedVersionException if the negotiated version is < 6 while a 2PC field is set, instead of leaking the serialization-internals message.

Tests: Added InitProducerIdRequestTest covering latestAllowedVersion == 6 when enable2Pc/keepPreparedTxn is set, and the v5 cap for non-2PC requests.

Note: this is the client-side half; broker-side keepPreparedTxn recovery is tracked in KAFKA-20739 #22704
Reviewers: Andrew Schofield aschofield@confluent.io

[Shekharrajak]

Ref #22704

[AndrewJSchofield]

I don't think this is a problem that needs a fix. It's a reflection of the partial implementation of KIP-939. The latest RPC version is marked as unstable so that production clusters do not accidentally support 2PC.

[Shekharrajak]

I don't think this is a problem that needs a fix. It's a reflection of the partial implementation of KIP-939. The latest RPC version is marked as unstable so that production clusters do not accidentally support 2PC.

Thanks @AndrewJSchofield for checking. Let me follow it closely https://issues.apache.org/jira/browse/KAFKA-15370