process the CORS options once

The CORS middleware resulting from handlers.CORS would unnecessarily
re-process the CORS options every time it would be applied to a handler.

With this change, the CORS options are processed only once, when the
middleware is created.

Author: jub0bs

cors.go

@@ -10,7 +10,6 @@ import (
 type CORSOption func(*cors) error
 
 type cors struct {
-	h                      http.Handler
 	allowedHeaders         []string
 	allowedMethods         []string
 	allowedOrigins         []string
@@ -47,93 +46,95 @@ const (
 	corsOriginMatchAll         string = "*"
 )
 
-func (ch *cors) ServeHTTP(w http.ResponseWriter, r *http.Request) {
-	origin := r.Header.Get(corsOriginHeader)
-	if !ch.isOriginAllowed(origin) {
-		if r.Method != corsOptionMethod || ch.ignoreOptions {
-			ch.h.ServeHTTP(w, r)
-		}
-
-		return
-	}
-
-	if r.Method == corsOptionMethod {
-		if ch.ignoreOptions {
-			ch.h.ServeHTTP(w, r)
-			return
-		}
+func (ch *cors) wrap(h http.Handler) http.Handler {
+	return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		origin := r.Header.Get(corsOriginHeader)
+		if !ch.isOriginAllowed(origin) {
+			if r.Method != corsOptionMethod || ch.ignoreOptions {
+				h.ServeHTTP(w, r)
+			}
 
-		if _, ok := r.Header[corsRequestMethodHeader]; !ok {
-			w.WriteHeader(http.StatusBadRequest)
 			return
 		}
 
-		method := r.Header.Get(corsRequestMethodHeader)
-		if !ch.isMatch(method, ch.allowedMethods) {
-			w.WriteHeader(http.StatusMethodNotAllowed)
-			return
-		}
+		if r.Method == corsOptionMethod {
+			if ch.ignoreOptions {
+				h.ServeHTTP(w, r)
+				return
+			}
 
-		requestHeaders := strings.Split(r.Header.Get(corsRequestHeadersHeader), ",")
-		allowedHeaders := []string{}
-		for _, v := range requestHeaders {
-			canonicalHeader := http.CanonicalHeaderKey(strings.TrimSpace(v))
-			if canonicalHeader == "" || ch.isMatch(canonicalHeader, defaultCorsHeaders) {
-				continue
+			if _, ok := r.Header[corsRequestMethodHeader]; !ok {
+				w.WriteHeader(http.StatusBadRequest)
+				return
 			}
 
-			if !ch.isMatch(canonicalHeader, ch.allowedHeaders) {
-				w.WriteHeader(http.StatusForbidden)
+			method := r.Header.Get(corsRequestMethodHeader)
+			if !ch.isMatch(method, ch.allowedMethods) {
+				w.WriteHeader(http.StatusMethodNotAllowed)
 				return
 			}
 
-			allowedHeaders = append(allowedHeaders, canonicalHeader)
-		}
+			requestHeaders := strings.Split(r.Header.Get(corsRequestHeadersHeader), ",")
+			allowedHeaders := []string{}
+			for _, v := range requestHeaders {
+				canonicalHeader := http.CanonicalHeaderKey(strings.TrimSpace(v))
+				if canonicalHeader == "" || ch.isMatch(canonicalHeader, defaultCorsHeaders) {
+					continue
+				}
 
-		if len(allowedHeaders) > 0 {
-			w.Header().Set(corsAllowHeadersHeader, strings.Join(allowedHeaders, ","))
-		}
+				if !ch.isMatch(canonicalHeader, ch.allowedHeaders) {
+					w.WriteHeader(http.StatusForbidden)
+					return
+				}
 
-		if ch.maxAge > 0 {
-			w.Header().Set(corsMaxAgeHeader, strconv.Itoa(ch.maxAge))
-		}
+				allowedHeaders = append(allowedHeaders, canonicalHeader)
+			}
+
+			if len(allowedHeaders) > 0 {
+				w.Header().Set(corsAllowHeadersHeader, strings.Join(allowedHeaders, ","))
+			}
+
+			if ch.maxAge > 0 {
+				w.Header().Set(corsMaxAgeHeader, strconv.Itoa(ch.maxAge))
+			}
 
-		if !ch.isMatch(method, defaultCorsMethods) {
-			w.Header().Set(corsAllowMethodsHeader, method)
+			if !ch.isMatch(method, defaultCorsMethods) {
+				w.Header().Set(corsAllowMethodsHeader, method)
+			}
+		} else if len(ch.exposedHeaders) > 0 {
+			w.Header().Set(corsExposeHeadersHeader, strings.Join(ch.exposedHeaders, ","))
 		}
-	} else if len(ch.exposedHeaders) > 0 {
-		w.Header().Set(corsExposeHeadersHeader, strings.Join(ch.exposedHeaders, ","))
-	}
 
-	if ch.allowCredentials {
-		w.Header().Set(corsAllowCredentialsHeader, "true")
-	}
+		if ch.allowCredentials {
+			w.Header().Set(corsAllowCredentialsHeader, "true")
+		}
 
-	if len(ch.allowedOrigins) > 1 {
-		w.Header().Set(corsVaryHeader, corsOriginHeader)
-	}
+		if len(ch.allowedOrigins) > 1 {
+			w.Header().Set(corsVaryHeader, corsOriginHeader)
+		}
 
-	returnOrigin := origin
-	if ch.allowedOriginValidator == nil && len(ch.allowedOrigins) == 0 {
-		returnOrigin = "*"
-	} else {
-		for _, o := range ch.allowedOrigins {
-			// A configuration of * is different than explicitly setting an allowed
-			// origin. Returning arbitrary origin headers in an access control allow
-			// origin header is unsafe and is not required by any use case.
-			if o == corsOriginMatchAll {
-				returnOrigin = "*"
-				break
+		returnOrigin := origin
+		if ch.allowedOriginValidator == nil && len(ch.allowedOrigins) == 0 {
+			returnOrigin = "*"
+		} else {
+			for _, o := range ch.allowedOrigins {
+				// A configuration of * is different than explicitly setting an allowed
+				// origin. Returning arbitrary origin headers in an access control allow
+				// origin header is unsafe and is not required by any use case.
+				if o == corsOriginMatchAll {
+					returnOrigin = "*"
+					break
+				}
 			}
 		}
-	}
-	w.Header().Set(corsAllowOriginHeader, returnOrigin)
+		w.Header().Set(corsAllowOriginHeader, returnOrigin)
 
-	if r.Method == corsOptionMethod {
-		w.WriteHeader(ch.optionStatusCode)
-		return
-	}
-	ch.h.ServeHTTP(w, r)
+		if r.Method == corsOptionMethod {
+			w.WriteHeader(ch.optionStatusCode)
+			return
+		}
+		h.ServeHTTP(w, r)
+	})
 }
 
 // CORS provides Cross-Origin Resource Sharing middleware.
@@ -155,11 +156,7 @@ func (ch *cors) ServeHTTP(w http.ResponseWriter, r *http.Request) {
 //	    http.ListenAndServe(":8000", handlers.CORS()(r))
 //	}
 func CORS(opts ...CORSOption) func(http.Handler) http.Handler {
-	return func(h http.Handler) http.Handler {
-		ch := parseCORSOptions(opts...)
-		ch.h = h
-		return ch
-	}
+	return parseCORSOptions(opts...).wrap
 }
 
 func parseCORSOptions(opts ...CORSOption) *cors {