HTTP Handler Not Goroutine-Safe

Why This Happens

Go's net/http server handles each request in its own goroutine. If your handler reads or writes shared variables like an in-memory cache or counter, you must synchronize access. This is easily missed because the code looks sequential in each handler function.

The Problem

var visitCount int

func handler(w http.ResponseWriter, r *http.Request) {
    visitCount++ // race condition
    fmt.Fprintf(w, "Visits: %d", visitCount)
}

The Fix

var visitCount int64

func handler(w http.ResponseWriter, r *http.Request) {
    count := atomic.AddInt64(&visitCount, 1)
    fmt.Fprintf(w, "Visits: %d", count)
}

Step-by-Step Fix

1. 1

   Identify shared state

   Find package-level or struct-level variables accessed by HTTP handlers without synchronization.

2. 2

   Choose a sync mechanism

   Use sync/atomic for simple counters, sync.Mutex for complex state, or sync.Map for concurrent maps.

3. 3

   Test with -race flag

   Run go test -race to verify no data races remain in your handlers.