[XdsClient] do async hop when unreffing gRPC transport (#34197)

Possible fix for deadlock described in b/297601540.

src/core/ext/xds/xds_transport_grpc.cc

@@ -28,6 +28,7 @@
 
 #include <grpc/byte_buffer.h>
 #include <grpc/byte_buffer_reader.h>
+#include <grpc/event_engine/event_engine.h>
 #include <grpc/grpc.h>
 #include <grpc/impl/channel_arg_names.h>
 #include <grpc/impl/connectivity_state.h>
@@ -42,11 +43,13 @@
 #include "src/core/lib/channel/channel_fwd.h"
 #include "src/core/lib/channel/channel_stack.h"
 #include "src/core/lib/config/core_configuration.h"
+#include "src/core/lib/event_engine/default_event_engine.h"
 #include "src/core/lib/gprpp/debug_location.h"
 #include "src/core/lib/gprpp/orphanable.h"
 #include "src/core/lib/gprpp/ref_counted_ptr.h"
 #include "src/core/lib/gprpp/time.h"
 #include "src/core/lib/iomgr/closure.h"
+#include "src/core/lib/iomgr/exec_ctx.h"
 #include "src/core/lib/iomgr/pollset_set.h"
 #include "src/core/lib/security/credentials/channel_creds_registry.h"
 #include "src/core/lib/security/credentials/credentials.h"
@@ -302,7 +305,14 @@ void GrpcXdsTransportFactory::GrpcXdsTransport::Orphan() {
     GPR_ASSERT(client_channel != nullptr);
     client_channel->RemoveConnectivityWatcher(watcher_);
   }
-  Unref();
+  // Do an async hop before unreffing.  This avoids a deadlock upon
+  // shutdown in the case where the xDS channel is itself an xDS channel
+  // (e.g., when using one control plane to find another control plane).
+  grpc_event_engine::experimental::GetDefaultEventEngine()->Run([this]() {
+    ApplicationCallbackExecCtx application_exec_ctx;
+    ExecCtx exec_ctx;
+    Unref();
+  });
 }
 
 OrphanablePtr<XdsTransportFactory::XdsTransport::StreamingCall>