Go map and concurrency security issues

kevin
Last updated on Jul 26, 2024 1 min read Golang
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It is unsafe for code in different goroutines to read and write to the same map simultaneously. The map values themselves may become inconsistent due to these operations, and related programs may experience unpredictable issues.

We can demonstrate the concurrency safety problem with map using the following code:

package main

func main() {
	m := make(map[int]int)

	go func() {
		for {
			_ = m[1]
		}
	}()

	go func() {
		for {
			m[2] = 2
		}
	}()

	select {}
}

The error message is: fatal error: concurrent map read and map write.

If you check the Go source code: hashmap_fast.go#L118, you’ll see that during reads, the hashWriting flag is checked, and if this flag is set, a concurrency error is reported.

The flag is set during writes: hashmap.go#L542

h.flags |= hashWriting

After setting, the flag is cleared: hashmap.go#L628

There are also several other checks for concurrent read and write issues in the code, such as checking for concurrent writes, similar issues when deleting keys, and concurrency problems during iteration.

Sometimes, concurrency issues with map are not easily detected; you can use the -race flag to check.

So, how can we ensure concurrency safety for map?

Click Read More for the full article.

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