Are interface values in Go safe to use concurrently?

Interface values are not inherently goroutine-safe. An interface value is a two-word struct (type pointer + data pointer). Assigning to an interface variable is not atomic — a goroutine reading the interface while another is writing it can observe a partially written state (mismatched type and data words).

// RACE CONDITION: assigning interface value from multiple goroutines
var handler http.Handler

// goroutine 1:
go func() { handler = &MyHandlerA{} }()

// goroutine 2:
go func() { handler = &MyHandlerB{} }()

// goroutine 3 (reader):
go func() { handler.ServeHTTP(w, r) }() // may see inconsistent type+data words

// Detection: go test -race finds this immediately

// Fix 1: protect with sync.Mutex
var mu sync.Mutex
var safeHandler http.Handler

setHandler := func(h http.Handler) {
    mu.Lock()
    safeHandler = h
    mu.Unlock()
}
getHandler := func() http.Handler {
    mu.Lock()
    defer mu.Unlock()
    return safeHandler
}

// Fix 2: atomic.Value (stores interface{} / any atomically)
var atomicHandler atomic.Value
atomicHandler.Store(http.Handler(&MyHandlerA{}))
h := atomicHandler.Load().(http.Handler)   // atomic read
h.ServeHTTP(w, r)

// Note: atomic.Value requires all stored values to be the same concrete type
// or the same interface type across calls

atomic.Value (since Go 1.4) is the idiomatic way to atomically swap an interface value: it is faster than a mutex for read-heavy scenarios (like a config that is rarely updated but read on every request). All Store calls must store values of the same concrete type.

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