Code reviewMediumcr-g231

Subject Concurrency bugsLevel Mid–Senior~25 minCommon in Concurrency interviewsIndustries Software development

Question

Review this Go code (built with Go 1.21) that launches per-shard workers.

There are two distinct concurrency bugs. Identify both.

What a strong answer looks like

Separate real bugs from style. Rank issues by severity, point at the root cause rather than the symptom, and suggest a concrete fix — specific and kind.

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Code to reviewgo

1func fanOut(shards []*Shard, results map[int]int) {2    var wg sync.WaitGroup3    for i := 0; i < len(shards); i++ {4        wg.Add(1)5        go func() {6            defer wg.Done()7            n := shards[i].Count()      // (1)8            results[i] = n              // (2)9        }()10    }11    wg.Wait()12}

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