message: check grpc_testlib_server before start subprocess
diff: @@ -17,6 +17,7 @@ limitations under the License.
  #include "tensorflow/core/distributed_runtime/rpc/grpc_session.h"
  #include "tensorflow/core/lib/strings/str_util.h"
  +#include "tensorflow/core/platform/env.h"
  #include "tensorflow/core/util/device_name_utils.h"
  namespace tensorflow {
  @@ -50,9 +51,14 @@ Status TestCluster::MakeTestCluster(const SessionOptions& options, int n,
  }
  for (int i = 0; i < n; ++i) {
  + string server_file =
  + strings::StrCat(testing::TensorFlowSrcRoot(),
  + "/core/distributed_runtime/rpc/grpc_testlib_server");
  + if (!options.env->FileExists(server_file).ok()) {
  + return errors::Internal("Could not find grpc_testlib_server");
  + }
  const std::vector<string> argv(
  - {strings::StrCat(testing::TensorFlowSrcRoot(),
  - "/core/distributed_runtime/rpc/grpc_testlib_server"),
  + {server_file,
  /* see grpc_testlib_server.cc for flags */
  tf_jobs, "--tf_job=localhost", strings::StrCat("--tf_task=", i),
  strings::StrCat("--num_cpus=", num_cpus),
  