web/mw/cors.go

package mw

import "net/http"

const (
	allowedMethods = "GET, HEAD, PUT, PATCH, POST, DELETE, OPTIONS"
	allowedHeaders = "Content-Type, Authorization, X-Request-ID"
)

// CORS sets cross-origin resource sharing headers for the provided origins.
// Returns 204 No Content for OPTIONS preflight requests.
// Pass the outermost origins first; an empty slice is a no-op.
func CORS(origins []string) func(http.Handler) http.Handler {
	// "*" is a silent no-op here (exact-match only) — reject it loudly at
	// construction so a misconfigured service fails to boot instead of quietly
	// blocking every browser. For allow-all, call CORSAllowAll (development only).
	for _, o := range origins {
		if o == "*" {
			panic(`mw.CORS: "*" is not a valid origin — list explicit origins, or use CORSAllowAll() for allow-all`)
		}
	}

	originSet := make(map[string]struct{}, len(origins))
	for _, o := range origins {
		originSet[o] = struct{}{}
	}

	return func(next http.Handler) http.Handler {
		return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
			origin := r.Header.Get("Origin")
			if origin != "" {
				if _, allowed := originSet[origin]; allowed {
					w.Header().Set("Access-Control-Allow-Origin", origin)
					w.Header().Set("Access-Control-Allow-Methods", allowedMethods)
					w.Header().Set("Access-Control-Allow-Headers", allowedHeaders)
					w.Header().Set("Access-Control-Allow-Credentials", "true")
					w.Header().Set("Vary", "Origin")
				}
			}
			if r.Method == http.MethodOptions {
				w.WriteHeader(http.StatusNoContent)
				return
			}
			next.ServeHTTP(w, r)
		})
	}
}

// CORSAllowAll is a convenience wrapper that allows any origin.
// Use only in development — never in production.
func CORSAllowAll() func(http.Handler) http.Handler {
	return func(next http.Handler) http.Handler {
		return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
			origin := r.Header.Get("Origin")
			if origin == "" {
				origin = "*"
			}
			w.Header().Set("Access-Control-Allow-Origin", origin)
			w.Header().Set("Access-Control-Allow-Methods", allowedMethods)
			w.Header().Set("Access-Control-Allow-Headers", allowedHeaders)
			w.Header().Set("Vary", "Origin")
			if r.Method == http.MethodOptions {
				w.WriteHeader(http.StatusNoContent)
				return
			}
			next.ServeHTTP(w, r)
		})
	}
}