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/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")
+}