fix: prevent data race in balance strategy

[napallday]

For every BalanceStrategy, it may be shared across different consumer groups in one instance. Considering every built-in balance strategy has several fields in the struct as shown in the below snippet, data race issues may happen when several consumer group rebalances simultaneously.

type balanceStrategy struct {
	coreFn func(plan BalanceStrategyPlan, memberIDs []string, topic string, partitions []int32)
	name   string
}
type stickyBalanceStrategy struct {
	movements partitionMovements
}

Even worse, I think this issue could cause

• panic
• some topics/partitions are not subscribed by the corresponding consumers, because the corresponding fields in the struct may be modified by other consumer groups

To verify this issue, a unit test was written, which fails when -race detection is enabled.

func Test_stickyBalanceStrategy_Plan_data_race(t *testing.T) {
	for i := 0; i < 1000; i++ {
		go func(bs BalanceStrategy) {
			members := map[string]ConsumerGroupMemberMetadata{
				"m1": {
					Version: 3,
					Topics:  []string{"topic"},
				},
			}
			topics := map[string][]int32{
				"topic": {0, 1, 2},
			}
			_, _ = bs.Plan(members, topics)
		}(BalanceStrategySticky)
	}
}

To avoid this problem, it may be best not to use a singleton for each and every of BalanceStrategy. Instead, newly-added functions like NewBalanceStrategyRange() BalanceStrategy are recommended so that every time an exclusive variable is assigned to each consumer group.

Fixes #2422

[napallday]

just checked the failed integration tests, it shouldn't be caused by the code changes here.

ProducerTxnStateInError|ProducerTxnStateFatalError
    functional_producer_test.go:140: 
        	Error Trace:	/home/runner/work/sarama/sarama/functional_producer_test.go:140
        	Error:      	Received unexpected error:
        	            	kafka server: The broker is still loading offsets after a leader change for that offset's topic partition
        	Test:       	TestFuncTxnCommitNoMessages

[dnwe]

Thanks, the changes LGTM