Is Concurrent Iteration Over Go Maps with `range` Safe, and How Can It Be Achieved?

Concurrent Access to Maps with Range in Go

The concept of concurrent access to maps using the range keyword in Go is often misunderstood. To clarify this matter, let's explore the intricacies of accessing maps concurrently and the implications of using range.

Evaluation of Range Expression

The range expression in a for statement is evaluated only once before the loop begins, excluding certain specific scenarios. In the case of a map, the range expression simply points to the map variable.

Iteration with Range

During iteration with range, the key and value variables (k and v in most cases) hold the values from the map during each iteration. However, map access does not occur within the for block while executing an iteration. It happens when the k and v values are set for the next iteration.

Safe Concurrent Iteration

Understanding these concepts, it is safe to perform concurrent iteration over a map by using a lock to protect the map during the iteration. However, unlocking within the iteration should be done as a deferred statement to handle early returns.

var testMap = make(map[int]int)
var testMapLock = &sync.RWMutex{}

func IterateMapKeys(iteratorChannel chan int) error {
    testMapLock.RLock()
    defer testMapLock.RUnlock()
    for k, v := range testMap {
        testMapLock.RUnlock()
        someFunc()
        testMapLock.RLock()
        if someCond {
            return someErr
        }
    }
    return nil
}

Concurrent Modifications

It's crucial to note that this type of locking scheme only secures against concurrent access. It does not prevent concurrent modifications by other goroutines. However, if modifications are guarded with write locks, they remain safe, but the for loop won't necessarily iterate over the new pair.

Unsafe Practices

Unlocking the read lock within the for block is unsafe, as it allows other goroutines to modify the map and potentially cause a race condition.

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