fix: use sentinel hash for empty Vary in cache key

This commit ensures a sentinel value is appended to the URL key when no Vary headers are present. This prevents collisions between the base URL key and the vary key, avoiding self-referential cache entries.

Author: bartventer

internal/normalization.go

@@ -324,9 +324,11 @@ func (f VaryKeyerFunc) VaryKey(urlKey string, varyHeaders map[string]string) str
 
 func NewVaryKeyer() VaryKeyer { return VaryKeyerFunc(makeVaryKey) }
 
+const noVaryHash = "0"
+
 func makeVaryKey(urlKey string, varyHeaders map[string]string) string {
 	if len(varyHeaders) == 0 {
-		return urlKey // No vary headers means no variation
+		return urlKey + "#" + noVaryHash // No Vary headers, so no variations
 	}
 	varyHash := makeVaryHash(varyHeaders)
 	return urlKey + "#" + strconv.FormatUint(varyHash, 10)

internal/normalization_test.go

@@ -101,19 +101,12 @@ func Test_makeVaryKey(t *testing.T) {
 		wantPrefix string
 		wantHash   func() string
 	}{
-		{
-			name:       "no vary headers (empty map)",
-			urlKey:     urlKey,
-			vary:       map[string]string{},
-			wantPrefix: urlKey,
-			wantHash:   func() string { return "" },
-		},
 		{
 			name:       "no vary headers (nil map)",
 			urlKey:     urlKey,
 			vary:       nil,
-			wantPrefix: urlKey,
-			wantHash:   func() string { return "" },
+			wantPrefix: urlKey + "#",
+			wantHash:   func() string { return noVaryHash },
 		},
 		{
 			name:       "single header",