Clarify GetRebalanceProtocol() return value for KIP-848 / group.protocol=consumer

[harilsatra]

Context

We maintain a shared library used by multiple Kafka consumers. We want to migrate to the KIP-848 consumer protocol in an opt-in way, so during the migration we must support both:

• Classic protocol (group.protocol=classic, or default): rebalance callback uses Assign() / Unassign().
• New consumer protocol (group.protocol=consumer): rebalance callback uses IncrementalAssign() / IncrementalUnassign() only, as described in the KIP-848 migration guide.

We plan to branch in the rebalance callback based on the current protocol (two-way: classic vs new) and call the appropriate assign APIs.

Current behavior / documentation

We intended to use GetRebalanceProtocol() to detect which protocol is in use. The in-code comment currently says:

// GetRebalanceProtocol returns the current consumer group rebalance protocol,
// which is either "EAGER" or "COOPERATIVE".
// If the rebalance protocol is not known in the current state an empty string
// is returned.
// Should typically only be called during rebalancing.
func (c *Consumer) GetRebalanceProtocol() string {.}

That only describes the classic protocol (eager vs cooperative assignors). It does not say what is returned when group.protocol=consumer (KIP-848), where there is no client-side EAGER/COOPERATIVE choice and the protocol is fully incremental.

Question

What does GetRebalanceProtocol() return in this case? (e.g. "consumer", "COOPERATIVE", something else, or is it unspecified?)

Environment

• confluent-kafka-go: v2.13.0
• go: v1.23.5