[xds_client_fuzzer] fail bootstrap parsing if xds_servers is empty (#33119)

b/269022924

src/core/ext/xds/xds_bootstrap_grpc.cc

@@ -259,6 +259,13 @@ const JsonLoaderInterface* GrpcXdsBootstrap::JsonLoader(const JsonArgs&) {
 void GrpcXdsBootstrap::JsonPostLoad(const Json& /*json*/,
                                     const JsonArgs& /*args*/,
                                     ValidationErrors* errors) {
+  // Verify that there is at least one server present.
+  {
+    ValidationErrors::ScopedField field(errors, ".xds_servers");
+    if (servers_.empty() && !errors->FieldHasErrors()) {
+      errors->AddError("must be non-empty");
+    }
+  }
   // Verify that each authority has the right prefix in the
   // client_listener_resource_name_template field.
   {

test/core/xds/xds_bootstrap_test.cc

@@ -314,6 +314,19 @@ TEST(XdsBootstrapTest, MissingXdsServers) {
       << bootstrap.status();
 }
 
+TEST(XdsBootstrapTest, EmptyXdsServers) {
+  const char* json_str =
+      "{"
+      "  \"xds_servers\": ["
+      "  ]"
+      "}";
+  auto bootstrap = GrpcXdsBootstrap::Create(json_str);
+  EXPECT_EQ(
+      bootstrap.status().message(),
+      "errors validating JSON: [field:xds_servers error:must be non-empty]")
+      << bootstrap.status();
+}
+
 TEST(XdsBootstrapTest, TopFieldsWrongTypes) {
   const char* json_str =
       "{"

test/core/xds/xds_client_corpora/bootstrap_xds_servers_empty

@@ -0,0 +1,10 @@
+bootstrap: "{\"xds_servers\": []}"
+actions {
+  start_watch {
+    resource_type {
+      listener {
+      }
+    }
+    resource_name: "\003"
+  }
+}