Fix failed sync controller blocking startup retries

internal/controller/syncmanager/controller.go

@@ -197,19 +197,34 @@ func (r *Reconciler) ensureSyncController(ctx context.Context, log *zap.SugaredL
 		return reconcile.Result{}, fmt.Errorf("failed to create sync controller: %w", err)
 	}
 
-	r.syncCancels[key] = ctrlCancel
-
 	// time to start the controller; this will spawn a new goroutine if
 	// successful; the new controller will be pre-seeded with all knonwn
 	// (engaged) clusters by the DMCM.
-	if err = r.dmcm.StartController(ctrlCtx, log.With("prkey", key), syncController); err != nil {
-		ctrlCancel(errors.New("failed to start sync controller"))
-		return reconcile.Result{}, fmt.Errorf("failed to start sync controller: %w", err)
+	err = r.startAndRecord(key, ctrlCancel, func() error {
+		return r.dmcm.StartController(ctrlCtx, log.With("prkey", key), syncController)
+	})
+	if err != nil {
+		return reconcile.Result{}, err
 	}
 
 	return reconcile.Result{}, nil
 }
 
+// startAndRecord starts a controller and only remembers its cancel function if the start
+// succeeded. Otherwise a failed start would block all later retries.
+func (r *Reconciler) startAndRecord(key string, cancel context.CancelCauseFunc, start func() error) error {
+	if err := start(); err != nil {
+		cancel(errors.New("failed to start sync controller"))
+		return fmt.Errorf("failed to start sync controller: %w", err)
+	}
+
+	r.syncCancelsLock.Lock()
+	r.syncCancels[key] = cancel
+	r.syncCancelsLock.Unlock()
+
+	return nil
+}
+
 func (r *Reconciler) cleanupController(log *zap.SugaredLogger, pubRes *syncagentv1alpha1.PublishedResource) error {
 	key := getPublishedResourceKey(pubRes)
 	log.Infow("Stopping sync controller…", "prkey", key)

internal/controller/syncmanager/controller_test.go

@@ -0,0 +1,52 @@
+/*
+Copyright 2025 The KCP Authors.
+
+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 syncmanager
+
+import (
+	"context"
+	"errors"
+	"testing"
+)
+
+func TestStartAndRecord(t *testing.T) {
+	r := &Reconciler{syncCancels: map[string]context.CancelCauseFunc{}}
+
+	ctx, cancel := context.WithCancelCause(context.Background())
+
+	if err := r.startAndRecord("key", cancel, func() error { return errors.New("boom") }); err == nil {
+		t.Fatal("expected an error")
+	}
+
+	if _, exists := r.syncCancels["key"]; exists {
+		t.Error("cancel function must not be recorded after a failed start")
+	}
+
+	if ctx.Err() == nil {
+		t.Error("context should be cancelled after a failed start")
+	}
+
+	// a retry must be able to succeed
+	_, cancel = context.WithCancelCause(context.Background())
+
+	if err := r.startAndRecord("key", cancel, func() error { return nil }); err != nil {
+		t.Fatalf("unexpected error: %v", err)
+	}
+
+	if _, exists := r.syncCancels["key"]; !exists {
+		t.Error("cancel function should be recorded after a successful start")
+	}
+}