Populate GOAWAY response body

lib/kube/proxy/forwarder.go

@@ -732,8 +732,21 @@ func (f *Forwarder) formatStatusResponseError(rw http.ResponseWriter, respErr er
 	// should retry the request themselves.
 	if errString := respErr.Error(); strings.HasSuffix(errString, `after Request.Body was written; define Request.GetBody to avoid this error`) &&
 		strings.Contains(errString, `http2: Transport: cannot retry err`) {
+
+		data, err := runtime.Encode(globalKubeCodecs.LegacyCodec(), &kubeerrors.NewTooManyRequests("Connection closed by upstream Kubernetes server", 1).ErrStatus)
+		if err != nil {
+			f.log.WarnContext(f.ctx, "Failed encoding error into kube Status object", "error", err)
+			trace.WriteError(rw, respErr)
+			return
+		}
+
 		rw.Header().Set("Retry-After", "1")
+		rw.Header().Set(responsewriters.ContentTypeHeader, "application/json")
 		rw.WriteHeader(http.StatusTooManyRequests)
+
+		if _, err := rw.Write(data); err != nil && !utils.IsOKNetworkError(err) {
+			f.log.WarnContext(f.ctx, "Failed writing kube error response body", "error", err)
+		}
 		return
 	}
 
@@ -759,7 +772,7 @@ func (f *Forwarder) formatStatusResponseError(rw http.ResponseWriter, respErr er
 	// `Error from server (InternalError): an error on the server ("unknown")
 	// has prevented the request from succeeding`` instead of the correct reason.
 	rw.WriteHeader(trace.ErrorToCode(respErr))
-	if _, err := rw.Write(data); err != nil {
+	if _, err := rw.Write(data); err != nil && !utils.IsOKNetworkError(err) {
 		f.log.WarnContext(f.ctx, "Failed writing kube error response body", "error", err)
 	}
 }

lib/kube/proxy/forwarder_test.go

@@ -24,6 +24,7 @@ import (
 	"crypto/tls"
 	"crypto/x509"
 	"crypto/x509/pkix"
+	"encoding/json"
 	"errors"
 	"fmt"
 	"io"
@@ -44,6 +45,7 @@ import (
 	"github.com/gravitational/trace"
 	"github.com/jonboulle/clockwork"
 	"github.com/julienschmidt/httprouter"
+	"github.com/stretchr/testify/assert"
 	"github.com/stretchr/testify/require"
 	"go.opentelemetry.io/otel"
 	"golang.org/x/net/http2"
@@ -1771,9 +1773,15 @@ func TestGOAWAYHandling(t *testing.T) {
 	require.NoError(t, err)
 	resp, err := forwarderServer.Client().Do(req)
 	require.NoError(t, err)
+
+	t.Cleanup(func() { assert.NoError(t, resp.Body.Close()) })
 	require.Equal(t, http.StatusTooManyRequests, resp.StatusCode)
 	require.Equal(t, "1", resp.Header.Get("Retry-After"))
-	require.NoError(t, resp.Body.Close())
+
+	var status metav1.Status
+	err = json.NewDecoder(resp.Body).Decode(&status)
+	require.NoError(t, err)
+	require.Equal(t, metav1.StatusReasonTooManyRequests, status.Reason)
 }
 
 // goawayServer is a fake [http2.Server] that terminates all received client