Concurrent Map Read and Map Write

Why This Happens

Go maps are not safe for concurrent use. If one goroutine is writing to a map while another is reading or writing, the runtime detects this and crashes with a fatal error. Unlike data races on other types, concurrent map access is detected at runtime and causes an immediate crash.

The Problem

func main() {
    m := make(map[string]int)
    go func() {
        for i := 0; i < 1000; i++ {
            m["key"] = i
        }
    }()
    go func() {
        for i := 0; i < 1000; i++ {
            _ = m["key"]
        }
    }()
    time.Sleep(time.Second)
}

The Fix

func main() {
    var mu sync.RWMutex
    m := make(map[string]int)
    go func() {
        for i := 0; i < 1000; i++ {
            mu.Lock()
            m["key"] = i
            mu.Unlock()
        }
    }()
    go func() {
        for i := 0; i < 1000; i++ {
            mu.RLock()
            _ = m["key"]
            mu.RUnlock()
        }
    }()
    time.Sleep(time.Second)
}

Step-by-Step Fix

1. 1

   Identify the concurrent access

   The crash output shows which goroutines are accessing the map. Find the map variable in question.

2. 2

   Determine the access pattern

   Check if most accesses are reads with occasional writes (use RWMutex) or if you need a simpler sync.Map.

3. 3

   Add synchronization

   Wrap all map reads with RLock/RUnlock and writes with Lock/Unlock, or replace with sync.Map.