Why Does `recover` Fail to Handle Concurrent Map Modification Crashes in Go?

Handling Concurrent Map Modifications: A Recover vs. Runtime Crash Dilemma

When dealing with concurrent map access, one might encounter a peculiar situation where recovering from a "concurrent map read and map write" panic seems futile. This is because the runtime's behavior in such cases is not a panic but a deliberate crash.

In Go 1.6, the runtime introduced a detection mechanism for concurrent misuse of maps. If multiple goroutines attempt to modify a map concurrently, the runtime triggers a crash, printing a diagnostic message. This behavior stems from the inherent danger of potential undefined behavior when maps are accessed concurrently for write operations.

Unfortunately, the typical approach of using defer with recover to handle panics is ineffective in this scenario. The recover function cannot intercept the crash initiated by the runtime. The recommended solution is to prevent the concurrent misuse of maps altogether.

In the provided example:

package main

import "time"

var m = make(map[string]string)

func main() {
    go func() {
        for {
            m["x"] = "foo"
        }
    }()
    go func() {
        for {
            m["x"] = "foo"
        }
    }()

    time.Sleep(1 * time.Second)
}

The concurrent writes to the map "m" will trigger the runtime crash. To prevent this, one would need to employ synchronization mechanisms, such as using a mutex or a channel, to ensure exclusive access to the map during write operations.

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