dedupe Alt-Svc header values (#3461)

http3/server.go

@@ -325,10 +325,16 @@ func (s *Server) generateAltSvcHeader() {
 	if s.QuicConfig != nil && len(s.QuicConfig.Versions) > 0 {
 		supportedVersions = s.QuicConfig.Versions
 	}
+
+	// keep track of which have been seen so we don't yield duplicate values
+	seen := make(map[string]struct{}, len(supportedVersions))
 	var versionStrings []string
 	for _, version := range supportedVersions {
 		if v := versionToALPN(version); len(v) > 0 {
-			versionStrings = append(versionStrings, v)
+			if _, ok := seen[v]; !ok {
+				versionStrings = append(versionStrings, v)
+				seen[v] = struct{}{}
+			}
 		}
 	}
 

http3/server_test.go

@@ -893,6 +893,14 @@ var _ = Describe("Server", func() {
 			removeListener(&ln2)
 			checkSetHeaderError()
 		})
+
+		It("doesn't duplicate Alt-Svc values", func() {
+			s.QuicConfig.Versions = []quic.VersionNumber{quic.Version1, quic.Version1}
+			addListener(":443", &ln1)
+			checkSetHeaders(Equal(http.Header{"Alt-Svc": {`h3=":443"; ma=2592000`}}))
+			removeListener(&ln1)
+			checkSetHeaderError()
+		})
 	})
 
 	It("errors when ListenAndServe is called with s.TLSConfig nil", func() {