Let MojoSetDataPipeConsumerOptions() take null options.

mojo/edk/system/data_pipe_consumer_dispatcher.cc

 // Copyright 2013 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "mojo/edk/system/data_pipe_consumer_dispatcher.h"
 
 #include <utility>
 
 #include "base/logging.h"
 #include "mojo/edk/system/data_pipe.h"
@@ skipping 61 matching lines @@
 
   auto dispatcher = DataPipeConsumerDispatcher::Create();
   dispatcher->Init(std::move(data_pipe_));
   return dispatcher;
 }
 
 MojoResult DataPipeConsumerDispatcher::SetDataPipeConsumerOptionsImplNoLock(
     UserPointer<const MojoDataPipeConsumerOptions> options) {
   mutex().AssertHeld();
 
-  UserOptionsReader<MojoDataPipeConsumerOptions> reader(options);
-  if (!reader.is_valid())
-    return MOJO_RESULT_INVALID_ARGUMENT;
+  // The default of 0 means 1 element (however big that is).
+  uint32_t read_threshold_num_bytes = 0;
+  if (!options.IsNull()) {
+    UserOptionsReader<MojoDataPipeConsumerOptions> reader(options);
+    if (!reader.is_valid())
+      return MOJO_RESULT_INVALID_ARGUMENT;
 
-  if (!OPTIONS_STRUCT_HAS_MEMBER(MojoDataPipeConsumerOptions,
-                                 read_threshold_num_bytes, reader))
-    return MOJO_RESULT_OK;
+    if (!OPTIONS_STRUCT_HAS_MEMBER(MojoDataPipeConsumerOptions,
+                                   read_threshold_num_bytes, reader))
+      return MOJO_RESULT_OK;
 
-  return data_pipe_->ConsumerSetOptions(
-      reader.options().read_threshold_num_bytes);
+    read_threshold_num_bytes = reader.options().read_threshold_num_bytes;
+  }
+
+  return data_pipe_->ConsumerSetOptions(read_threshold_num_bytes);
 }
 
 MojoResult DataPipeConsumerDispatcher::GetDataPipeConsumerOptionsImplNoLock(
     UserPointer<MojoDataPipeConsumerOptions> options,
     uint32_t options_num_bytes) {
   mutex().AssertHeld();
 
   // Note: If/when |MojoDataPipeConsumerOptions| is extended beyond its initial
   // definition, more work will be necessary. (See the definition of
   // |MojoGetDataPipeConsumerOptions()| in mojo/public/c/system/data_pipe.h.)
@@ skipping 110 matching lines @@
   return rv;
 }
 
 bool DataPipeConsumerDispatcher::IsBusyNoLock() const {
   mutex().AssertHeld();
   return data_pipe_->ConsumerIsBusy();
 }
 
 }  // namespace system
 }  // namespace mojo