fix(ENGEN-3331): prevent handler deadlock on credentials channel in listener mode

service/runtime/handler.go

@@ -194,7 +194,14 @@ func (h *Handler) ServeHTTP(rw http.ResponseWriter, inReq *http.Request) {
 	if path == tokenEndpoint && requestBody != nil {
 		uc := h.parseAuthTokenBody(requestBody)
 		if !uc.Empty() && h.userCredentialsCh != nil {
-			h.userCredentialsCh <- uc
+			// Non-blocking: the channel has no reader when tnls.Start()
+			// exits early (e.g. TunnelIsReady fails), which would deadlock
+			// this goroutine and hang the already-completed HTTP response.
+			select {
+			case h.userCredentialsCh <- uc:
+			default:
+				ll.Log("webhook listener not ready, skipping credentials forwarding")
+			}
 		}
 	}
 }

service/runtime/handler_test.go

@@ -0,0 +1,108 @@
+/*
+Copyright © 2021 Upvest GmbH <support@upvest.co>
+
+Licensed under the Apache License, Version 2.0 (the "License");
+you may not use this file except in compliance with the License.
+You may obtain a copy of the License at
+
+    http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing, software
+distributed under the License is distributed on an "AS IS" BASIS,
+WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+See the License for the specific language governing permissions and
+limitations under the License.
+*/
+
+package runtime
+
+import (
+	"fmt"
+	"net/http"
+	"net/http/httptest"
+	"strings"
+	"testing"
+	"time"
+
+	"github.com/google/uuid"
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+	"github.com/upvestco/httpsignature-proxy/config"
+	"github.com/upvestco/httpsignature-proxy/service/logger"
+	"github.com/upvestco/httpsignature-proxy/service/signer"
+	"github.com/upvestco/httpsignature-proxy/service/tunnels"
+)
+
+func newTestHandler(t *testing.T, backendURL string, ch chan tunnels.UserCredentials) (*Handler, uuid.UUID) {
+	t.Helper()
+	clientID := uuid.New()
+	keyCfg := config.KeyConfig{
+		BaseConfig: config.BaseConfig{
+			BaseUrl:  backendURL,
+			Password: testPass,
+			KeyID:    testKeyID,
+		},
+		ClientID: clientID.String(),
+	}
+	builder, err := signer.NewLocalPrivateSchemeBuilderFromSeed(privateTestKey, &keyCfg)
+	require.NoError(t, err)
+
+	signerConfigs := map[string]SignerConfig{
+		clientID.String(): {SignBuilder: builder, KeyConfig: keyCfg.BaseConfig},
+	}
+	cfg := &config.Config{DefaultTimeout: 30 * time.Second}
+	return newHandler(cfg, signerConfigs, ch, logger.New(false)), clientID
+}
+
+func TestHandler_AuthToken_DoesNotBlockWithoutChannelReader(t *testing.T) {
+	backend := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		w.WriteHeader(http.StatusOK)
+	}))
+	defer backend.Close()
+
+	ch := make(chan tunnels.UserCredentials) // unbuffered, no reader
+	h, clientID := newTestHandler(t, backend.URL, ch)
+
+	body := fmt.Sprintf("client_id=%s&client_secret=secret", clientID)
+	req := httptest.NewRequest(http.MethodPost, "/auth/token", strings.NewReader(body))
+	rec := httptest.NewRecorder()
+
+	done := make(chan struct{})
+	go func() {
+		h.ServeHTTP(rec, req)
+		close(done)
+	}()
+
+	select {
+	case <-done:
+		assert.Equal(t, http.StatusOK, rec.Code)
+	case <-time.After(3 * time.Second):
+		t.Fatal("ServeHTTP blocked: channel send deadlock")
+	}
+}
+
+func TestHandler_AuthToken_ForwardsCredentialsToChannel(t *testing.T) {
+	backend := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		w.WriteHeader(http.StatusOK)
+	}))
+	defer backend.Close()
+
+	ch := make(chan tunnels.UserCredentials, 1)
+	h, clientID := newTestHandler(t, backend.URL, ch)
+
+	secret := "test-secret"
+	body := fmt.Sprintf("client_id=%s&client_secret=%s", clientID, secret)
+	req := httptest.NewRequest(http.MethodPost, "/auth/token", strings.NewReader(body))
+	rec := httptest.NewRecorder()
+
+	h.ServeHTTP(rec, req)
+
+	assert.Equal(t, http.StatusOK, rec.Code)
+	select {
+	case uc := <-ch:
+		assert.Equal(t, clientID.String(), uc.ClientID)
+		assert.Equal(t, secret, uc.ClientSecret)
+	default:
+		t.Fatal("expected credentials on channel")
+	}
+}