✨ Cache middleware: Store e2e headers.

As defined in RFC2616 - section-13.5.1, shared caches MUST
store end-to-end headers from backend response and MUST be
transmitted in any response formed from a cache entry.

This commit ensures a stronger consistency between responses
served from the handlers & from the cache middleware.

middleware/cache/cache.go

@@ -27,6 +27,19 @@ const (
 	cacheMiss        = "miss"
 )
 
+var ignoreHeaders = map[string]interface{}{
+	"Connection":          nil,
+	"Keep-Alive":          nil,
+	"Proxy-Authenticate":  nil,
+	"Proxy-Authorization": nil,
+	"TE":                  nil,
+	"Trailers":            nil,
+	"Transfer-Encoding":   nil,
+	"Upgrade":             nil,
+	"Content-Type":        nil, // already stored explicitely by the cache manager
+	"Content-Encoding":    nil, // already stored explicitely by the cache manager
+}
+
 // New creates a new middleware handler
 func New(config ...Config) fiber.Handler {
 	// Set default config
@@ -96,6 +109,9 @@ func New(config ...Config) fiber.Handler {
 			if len(e.cencoding) > 0 {
 				c.Response().Header.SetBytesV(fiber.HeaderContentEncoding, e.cencoding)
 			}
+			for k, v := range e.e2eHeaders {
+				c.Response().Header.AddBytesV(k, v)
+			}
 			// Set Cache-Control header if enabled
 			if cfg.CacheControl {
 				maxAge := strconv.FormatUint(e.exp-ts, 10)
@@ -133,6 +149,18 @@ func New(config ...Config) fiber.Handler {
 		e.status = c.Response().StatusCode()
 		e.ctype = utils.CopyBytes(c.Response().Header.ContentType())
 		e.cencoding = utils.CopyBytes(c.Response().Header.Peek(fiber.HeaderContentEncoding))
+		e.e2eHeaders = make(map[string][]byte)
+
+		// Store all end-to-end headers
+		// (more: https://datatracker.ietf.org/doc/html/rfc2616#section-13.5.1)
+		c.Response().Header.VisitAll(
+			func(key []byte, value []byte) {
+				keyS := string(key)
+				if _, isHopbyHop := ignoreHeaders[keyS]; !isHopbyHop {
+					e.e2eHeaders[keyS] = value
+				}
+			},
+		)
 
 		// default cache expiration
 		expiration := uint64(cfg.Expiration.Seconds())

middleware/cache/cache_test.go

@@ -302,6 +302,26 @@ func Test_CustomExpiration(t *testing.T) {
 	utils.AssertEqual(t, 6000, newCacheTime)
 }
 
+func Test_AdditionalE2EResponseHeaders(t *testing.T) {
+	app := fiber.New()
+	app.Use(New())
+
+	app.Get("/", func(c *fiber.Ctx) error {
+		c.Response().Header.Add("X-Foobar", "foobar")
+		return c.SendString("hi")
+	})
+
+	req := httptest.NewRequest("GET", "/", nil)
+	resp, err := app.Test(req)
+	utils.AssertEqual(t, nil, err)
+	utils.AssertEqual(t, "foobar", resp.Header.Get("X-Foobar"))
+
+	req = httptest.NewRequest("GET", "/", nil)
+	resp, err = app.Test(req)
+	utils.AssertEqual(t, nil, err)
+	utils.AssertEqual(t, "foobar", resp.Header.Get("X-Foobar"))
+}
+
 func Test_CacheHeader(t *testing.T) {
 	app := fiber.New()
 

middleware/cache/manager.go

@@ -13,11 +13,12 @@ import (
 // don't forget to replace the msgp import path to:
 // "github.com/gofiber/fiber/v2/internal/msgp"
 type item struct {
-	body      []byte
-	ctype     []byte
-	cencoding []byte
-	status    int
-	exp       uint64
+	body       []byte
+	ctype      []byte
+	cencoding  []byte
+	status     int
+	exp        uint64
+	e2eHeaders map[string][]byte
 }
 
 //msgp:ignore manager
@@ -61,6 +62,7 @@ func (m *manager) release(e *item) {
 	e.ctype = nil
 	e.status = 0
 	e.exp = 0
+	e.e2eHeaders = nil
 	m.pool.Put(e)
 }