Add a new plugin / extension mechanism called a resource provider along with retrofitting VFS on top of it.

A resource provider provides a mechanism for MuJoCo to read from filesystems other than the OS filesystem or the Virtual File System (VFS).

PiperOrigin-RevId: 525394983
Change-Id: I077ff5a7e2e76806b48b6defb531280aadc8b169

src/engine/engine_resource.h

@@ -0,0 +1,44 @@
+// Copyright 2022 DeepMind Technologies Limited
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#ifndef MUJOCO_SRC_ENGINE_ENGINE_RESOURCE_H_
+#define MUJOCO_SRC_ENGINE_ENGINE_RESOURCE_H_
+
+#include <mujoco/mjexport.h>
+#include "engine/engine_plugin.h"
+
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+// open the given resource; if the name doesn't have a prefix matching with a
+// resource provider, then the default_provider is used
+// if default_provider non-positive, then the OS filesystem is used
+MJAPI mjResource* mju_openResource(const char* name, int default_provider);
+
+// close the given resource; no-op if resource is NULL
+MJAPI void mju_closeResource(mjResource* resource);
+
+// set buffer to bytes read from the resource and return number of bytes in buffer;
+// return negative value if error
+MJAPI int mju_readResource(mjResource* resource, const void** buffer);
+
+// read file into memory buffer (allocated here with mju_malloc)
+void* mju_fileToMemory(const char* filename, int* filesize);
+
+#ifdef __cplusplus
+}
+#endif
+
+#endif  // MUJOCO_SRC_ENGINE_ENGINE_RESOURCE_H_