http3: initialize trailer map with empty trailer entries when parsing the header (#4656)

* feat: pre-populate trailers on response with empty values

* fix: improve comment/func name

http3/client_test.go

@@ -585,7 +585,7 @@ var _ = Describe("Client", func() {
 
 			trailerBuf := &bytes.Buffer{}
 			enc := qpack.NewEncoder(trailerBuf)
-			Expect(enc.WriteField(qpack.HeaderField{Name: "Grpc-Status", Value: "0"})).To(Succeed())
+			Expect(enc.WriteField(qpack.HeaderField{Name: "This-Is-A-Trailer", Value: "0"})).To(Succeed())
 			Expect(enc.Close()).To(Succeed())
 			b := (&headersFrame{Length: uint64(trailerBuf.Len())}).Append(nil)
 			b = append(b, trailerBuf.Bytes()...)
@@ -603,7 +603,7 @@ var _ = Describe("Client", func() {
 			Expect(err).ToNot(HaveOccurred())
 			_, err = io.ReadAll(rsp.Body)
 			Expect(err).ToNot(HaveOccurred())
-			Expect(rsp.Trailer).To(Equal(http.Header{"Grpc-Status": []string{"0"}}))
+			Expect(rsp.Trailer).To(Equal(http.Header{"This-Is-A-Trailer": []string{"0"}}))
 			Expect(rsp.Proto).To(Equal("HTTP/3.0"))
 			Expect(rsp.ProtoMajor).To(Equal(3))
 			Expect(rsp.StatusCode).To(Equal(418))
@@ -637,7 +637,7 @@ var _ = Describe("Client", func() {
 			{
 				trailerBuf := &bytes.Buffer{}
 				enc := qpack.NewEncoder(trailerBuf)
-				Expect(enc.WriteField(qpack.HeaderField{Name: "Grpc-Status", Value: "0"})).To(Succeed())
+				Expect(enc.WriteField(qpack.HeaderField{Name: "This-Is-A-Trailer", Value: "0"})).To(Succeed())
 				Expect(enc.Close()).To(Succeed())
 				b := (&headersFrame{Length: uint64(trailerBuf.Len())}).Append(nil)
 				b = append(b, trailerBuf.Bytes()...)
@@ -647,7 +647,7 @@ var _ = Describe("Client", func() {
 			{
 				trailerBuf := &bytes.Buffer{}
 				enc := qpack.NewEncoder(trailerBuf)
-				Expect(enc.WriteField(qpack.HeaderField{Name: "Grpc-Status", Value: "1"})).To(Succeed())
+				Expect(enc.WriteField(qpack.HeaderField{Name: "This-Is-A-Trailer", Value: "1"})).To(Succeed())
 				Expect(enc.Close()).To(Succeed())
 				b := (&headersFrame{Length: uint64(trailerBuf.Len())}).Append(nil)
 				b = append(b, trailerBuf.Bytes()...)
@@ -666,7 +666,7 @@ var _ = Describe("Client", func() {
 			Expect(err).ToNot(HaveOccurred())
 			_, err = io.ReadAll(rsp.Body)
 			Expect(err).To(MatchError(errors.New("additional HEADERS frame received after trailers")))
-			Expect(rsp.Trailer).To(Equal(http.Header{"Grpc-Status": []string{"0"}}))
+			Expect(rsp.Trailer).To(Equal(http.Header{"This-Is-A-Trailer": []string{"0"}}))
 			Expect(rsp.Proto).To(Equal("HTTP/3.0"))
 			Expect(rsp.ProtoMajor).To(Equal(3))
 			Expect(rsp.StatusCode).To(Equal(418))
@@ -679,7 +679,7 @@ var _ = Describe("Client", func() {
 			{
 				trailerBuf := &bytes.Buffer{}
 				enc := qpack.NewEncoder(trailerBuf)
-				Expect(enc.WriteField(qpack.HeaderField{Name: "Grpc-Status", Value: "0"})).To(Succeed())
+				Expect(enc.WriteField(qpack.HeaderField{Name: "This-Is-A-Trailer", Value: "0"})).To(Succeed())
 				Expect(enc.Close()).To(Succeed())
 				b := (&headersFrame{Length: uint64(trailerBuf.Len())}).Append(nil)
 				b = append(b, trailerBuf.Bytes()...)
@@ -706,7 +706,7 @@ var _ = Describe("Client", func() {
 			Expect(err).ToNot(HaveOccurred())
 			_, err = io.ReadAll(rsp.Body)
 			Expect(err).To(MatchError(errors.New("DATA frame received after trailers")))
-			Expect(rsp.Trailer).To(Equal(http.Header{"Grpc-Status": []string{"0"}}))
+			Expect(rsp.Trailer).To(Equal(http.Header{"This-Is-A-Trailer": []string{"0"}}))
 			Expect(rsp.Proto).To(Equal("HTTP/3.0"))
 			Expect(rsp.ProtoMajor).To(Equal(3))
 			Expect(rsp.StatusCode).To(Equal(418))

http3/headers.go

@@ -4,6 +4,7 @@ import (
 	"errors"
 	"fmt"
 	"net/http"
+	"net/textproto"
 	"net/url"
 	"strconv"
 	"strings"
@@ -222,6 +223,7 @@ func updateResponseFromHeaders(rsp *http.Response, headerFields []qpack.HeaderFi
 	rsp.Proto = "HTTP/3.0"
 	rsp.ProtoMajor = 3
 	rsp.Header = hdr.Headers
+	processTrailers(rsp)
 	rsp.ContentLength = hdr.ContentLength
 
 	status, err := strconv.Atoi(hdr.Status)
@@ -232,3 +234,27 @@ func updateResponseFromHeaders(rsp *http.Response, headerFields []qpack.HeaderFi
 	rsp.Status = hdr.Status + " " + http.StatusText(status)
 	return nil
 }
+
+// processTrailers initializes the rsp.Trailer map, and adds keys for every announced header value.
+// The Trailer header is removed from the http.Response.Header map.
+// It handles both duplicate as well as comma-separated values for the Trailer header.
+// For example:
+//
+//	Trailer: Trailer1, Trailer2
+//	Trailer: Trailer3
+//
+// Will result in a http.Response.Trailer map containing the keys "Trailer1", "Trailer2", "Trailer3".
+func processTrailers(rsp *http.Response) {
+	rawTrailers, ok := rsp.Header["Trailer"]
+	if !ok {
+		return
+	}
+
+	rsp.Trailer = make(http.Header)
+	for _, rawVal := range rawTrailers {
+		for _, val := range strings.Split(rawVal, ",") {
+			rsp.Trailer[http.CanonicalHeaderKey(textproto.TrimString(val))] = nil
+		}
+	}
+	delete(rsp.Header, "Trailer")
+}

http3/headers_test.go

@@ -335,6 +335,23 @@ var _ = Describe("Response", func() {
 		Expect(rsp.Status).To(Equal("200 OK"))
 	})
 
+	It("parses trailer", func() {
+		headers := []qpack.HeaderField{
+			{Name: ":status", Value: "200"},
+			{Name: "trailer", Value: "Trailer1, Trailer2"},
+			{Name: "trailer", Value: "TRAILER3"},
+		}
+		rsp := &http.Response{}
+		err := updateResponseFromHeaders(rsp, headers)
+		Expect(err).NotTo(HaveOccurred())
+		Expect(rsp.Header).To(HaveLen(0))
+		Expect(rsp.Trailer).To(Equal(http.Header(map[string][]string{
+			"Trailer1": nil,
+			"Trailer2": nil,
+			"Trailer3": nil,
+		})))
+	})
+
 	It("rejects pseudo header fields after regular header fields", func() {
 		headers := []qpack.HeaderField{
 			{Name: "content-length", Value: "42"},

http3/response_writer.go

@@ -307,10 +307,10 @@ func (w *responseWriter) writeTrailers() error {
 	var b bytes.Buffer
 	enc := qpack.NewEncoder(&b)
 	for trailer := range w.trailers {
+		trailerName := strings.ToLower(strings.TrimPrefix(trailer, http.TrailerPrefix))
 		if vals, ok := w.header[trailer]; ok {
-			name := strings.TrimPrefix(trailer, http.TrailerPrefix)
 			for _, val := range vals {
-				if err := enc.WriteField(qpack.HeaderField{Name: strings.ToLower(name), Value: val}); err != nil {
+				if err := enc.WriteField(qpack.HeaderField{Name: trailerName, Value: val}); err != nil {
 					return err
 				}
 			}