Is Concurrent Access to Go Maps Safe During a Range Loop?

Concurrent Access to Maps with Range in Go

Despite the warning that maps are not safe for concurrent use, it's unclear how the range statement interacts with concurrent access. The question arises whether executing inside a range loop constitutes a read or a turnover phase of the loop.

According to the language specification, the range expression is evaluated only once before beginning the loop. This means that the map value is only evaluated once, and any subsequent modifications to the map will not be reflected in the iteration.

However, it's important to note that while the map is not accessed during the execution of the iteration block, it is accessed when setting the k and v values for the next iteration. Therefore, unlocking the read lock inside the for block allows concurrent modifications to the map, even if the loop itself is iterating over a static map value.

To prevent concurrent modifications, it's recommended to keep the read lock held throughout the for loop. This ensures that no modifications can occur while iterating over the map, and any modifications made in other goroutines are visible to the loop through the Go Memory Model's guarantees.

It is important to note that the iterator logic does not define how modifications made during iteration are handled. This means that while the language allows modifications to the map during iteration, the behavior of the loop may vary depending on the specific implementation.

In summary, while it is safe to iterate over a map concurrently with a range expression, it is crucial to maintain a read lock throughout the loop to prevent concurrent modifications. This ensures that all modifications are visible to the loop, and the iterator logic will handle them as if they had been made by the loop's own goroutine.

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