context: add Push action

Signed-off-by: Tw <tw19881113@gmail.com>

caddyhttp/httpserver/context.go

@@ -29,6 +29,20 @@ type Context struct {
 	Req  *http.Request
 	URL  *url.URL
 	Args []interface{} // defined by arguments to .Include
+
+	// just used for adding preload links for server push
+	responseHeader http.Header
+}
+
+// NewContextWithHeader creates a context with given response header.
+//
+// To plugin developer:
+// The returned context's exported fileds remain empty,
+// you should then initialize them if you want.
+func NewContextWithHeader(rh http.Header) Context {
+	return Context{
+		responseHeader: rh,
+	}
 }
 
 // Include returns the contents of filename relative to the site root.
@@ -410,6 +424,15 @@ func (c Context) RandomString(minLen, maxLen int) string {
 	return string(result)
 }
 
+// Push adds a preload link in response header for server push
+func (c Context) Push(link string) string {
+	if c.responseHeader == nil {
+		return ""
+	}
+	c.responseHeader.Add("Link", "<"+link+">; rel=preload")
+	return ""
+}
+
 // buffer pool for .Include context actions
 var includeBufs = sync.Pool{
 	New: func() interface{} {

caddyhttp/httpserver/context_test.go

@@ -6,6 +6,7 @@ import (
 	"io/ioutil"
 	"net"
 	"net/http"
+	"net/http/httptest"
 	"net/url"
 	"os"
 	"path/filepath"
@@ -731,8 +732,9 @@ func initTestContext() (Context, error) {
 	if err != nil {
 		return Context{}, err
 	}
+	res := httptest.NewRecorder()
 
-	return Context{Root: http.Dir(os.TempDir()), Req: request}, nil
+	return Context{Root: http.Dir(os.TempDir()), responseHeader: res.Header(), Req: request}, nil
 }
 
 func getContextOrFail(t *testing.T) Context {
@@ -874,3 +876,35 @@ func TestFiles(t *testing.T) {
 		}
 	}
 }
+
+func TestPush(t *testing.T) {
+	for name, c := range map[string]struct {
+		input       string
+		expectLinks []string
+	}{
+		"oneLink": {
+			input:       `{{.Push "/test.css"}}`,
+			expectLinks: []string{"</test.css>; rel=preload"},
+		},
+		"multipleLinks": {
+			input:       `{{.Push "/test1.css"}} {{.Push "/test2.css"}}`,
+			expectLinks: []string{"</test1.css>; rel=preload", "</test2.css>; rel=preload"},
+		},
+	} {
+		c := c
+		t.Run(name, func(t *testing.T) {
+			ctx := getContextOrFail(t)
+			tmpl, err := template.New("").Parse(c.input)
+			if err != nil {
+				t.Fatal(err)
+			}
+			err = tmpl.Execute(ioutil.Discard, ctx)
+			if err != nil {
+				t.Fatal(err)
+			}
+			if got := ctx.responseHeader["Link"]; !reflect.DeepEqual(got, c.expectLinks) {
+				t.Errorf("Result not match: expect %v, but got %v", c.expectLinks, got)
+			}
+		})
+	}
+}