Allow multiple values for an HTTP header and

add a test to ensure this works.

caddyhttp/header/header.go

@@ -24,8 +24,12 @@ func (h Headers) ServeHTTP(w http.ResponseWriter, r *http.Request) (int, error)
 	for _, rule := range h.Rules {
 		if httpserver.Path(r.URL.Path).Matches(rule.Path) {
 			for _, header := range rule.Headers {
+				// One can either delete a header, add multiple values to a header, or simply
+				// set a header.
 				if strings.HasPrefix(header.Name, "-") {
 					w.Header().Del(strings.TrimLeft(header.Name, "-"))
+				} else if strings.HasPrefix(header.Name, "+") {
+					w.Header().Add(strings.TrimLeft(header.Name, "+"), replacer.Replace(header.Value))
 				} else {
 					w.Header().Set(header.Name, replacer.Replace(header.Value))
 				}

caddyhttp/header/header_test.go

@@ -4,6 +4,8 @@ import (
 	"net/http"
 	"net/http/httptest"
 	"os"
+	"reflect"
+	"sort"
 	"testing"
 
 	"github.com/mholt/caddy/caddyhttp/httpserver"
@@ -55,3 +57,33 @@ func TestHeader(t *testing.T) {
 		}
 	}
 }
+
+func TestMultipleHeaders(t *testing.T) {
+	he := Headers{
+		Next: httpserver.HandlerFunc(func(w http.ResponseWriter, r *http.Request) (int, error) {
+			return 0, nil
+		}),
+		Rules: []Rule{
+			{Path: "/a", Headers: []Header{
+				{Name: "+Link", Value: "</images/image.png>; rel=preload"},
+				{Name: "+Link", Value: "</css/main.css>; rel=preload"},
+			}},
+		},
+	}
+
+	req, err := http.NewRequest("GET", "/a", nil)
+	if err != nil {
+		t.Fatalf("Could not create HTTP request: %v", err)
+	}
+
+	rec := httptest.NewRecorder()
+	he.ServeHTTP(rec, req)
+
+	desiredHeaders := []string{"</css/main.css>; rel=preload", "</images/image.png>; rel=preload"}
+	actualHeaders := rec.HeaderMap[http.CanonicalHeaderKey("Link")]
+	sort.Strings(actualHeaders)
+
+	if !reflect.DeepEqual(desiredHeaders, actualHeaders) {
+		t.Errorf("Expected header to contain: %v but got: %v", desiredHeaders, actualHeaders)
+	}
+}