[hal] Implement stub mockdata on RoboRIO (#2575)

This makes it much more user-friendly to use simulation classes without needing
to ifdef for C++ to avoid linker errors or be very careful about construction
to avoid runtime errors in Java.

hal/src/main/native/cpp/jni/simulation/BufferCallbackStore.h

@@ -0,0 +1,47 @@
+/*----------------------------------------------------------------------------*/
+/* Copyright (c) 2018-2020 FIRST. All Rights Reserved.                        */
+/* Open Source Software - may be modified and shared by FRC teams. The code   */
+/* must be accompanied by the FIRST BSD license file in the root directory of */
+/* the project.                                                               */
+/*----------------------------------------------------------------------------*/
+
+#pragma once
+
+#include <jni.h>
+
+#include <wpi/jni_util.h>
+
+#include "SimulatorJNI.h"
+#include "hal/Types.h"
+#include "hal/Value.h"
+#include "hal/handles/UnlimitedHandleResource.h"
+#include "hal/simulation/NotifyListener.h"
+
+namespace hal {
+namespace sim {
+class BufferCallbackStore {
+ public:
+  void create(JNIEnv* env, jobject obj);
+  void performCallback(const char* name, uint8_t* buffer, uint32_t length);
+  void free(JNIEnv* env);
+  void setCallbackId(int32_t id) { callbackId = id; }
+  int32_t getCallbackId() { return callbackId; }
+
+ private:
+  wpi::java::JGlobal<jobject> m_call;
+  int32_t callbackId;
+};
+
+void InitializeBufferStore();
+
+typedef int32_t (*RegisterBufferCallbackFunc)(int32_t index,
+                                              HAL_BufferCallback callback,
+                                              void* param);
+typedef void (*FreeBufferCallbackFunc)(int32_t index, int32_t uid);
+
+SIM_JniHandle AllocateBufferCallback(JNIEnv* env, jint index, jobject callback,
+                                     RegisterBufferCallbackFunc createCallback);
+void FreeBufferCallback(JNIEnv* env, SIM_JniHandle handle, jint index,
+                        FreeBufferCallbackFunc freeCallback);
+}  // namespace sim
+}  // namespace hal