runtime/class_loader_context.cc

/*
 * Copyright (C) 2017 The Android Open Source Project
 *
 * 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.
 */

#include "class_loader_context.h"

#include "art_field-inl.h"
#include "base/dchecked_vector.h"
#include "base/stl_util.h"
#include "class_linker.h"
#include "class_loader_utils.h"
#include "dex_file.h"
#include "handle_scope-inl.h"
#include "jni_internal.h"
#include "oat_file_assistant.h"
#include "obj_ptr-inl.h"
#include "runtime.h"
#include "scoped_thread_state_change-inl.h"
#include "thread.h"
#include "well_known_classes.h"

namespace art {

static constexpr char kPathClassLoaderString[] = "PCL";
static constexpr char kDelegateLastClassLoaderString[] = "DLC";
static constexpr char kClassLoaderOpeningMark = '[';
static constexpr char kClassLoaderClosingMark = ']';
static constexpr char kClassLoaderSeparator = ';';
static constexpr char kClasspathSeparator = ':';
static constexpr char kDexFileChecksumSeparator = '*';

ClassLoaderContext::ClassLoaderContext()
    : special_shared_library_(false),
      dex_files_open_attempted_(false),
      dex_files_open_result_(false),
      owns_the_dex_files_(false) {}

ClassLoaderContext::ClassLoaderContext(bool owns_the_dex_files)
    : special_shared_library_(false),
      dex_files_open_attempted_(true),
      dex_files_open_result_(true),
      owns_the_dex_files_(owns_the_dex_files) {}

ClassLoaderContext::~ClassLoaderContext() {
  if (!owns_the_dex_files_) {
    // If the context does not own the dex/oat files release the unique pointers to
    // make sure we do not de-allocate them.
    for (ClassLoaderInfo& info : class_loader_chain_) {
      for (std::unique_ptr<OatFile>& oat_file : info.opened_oat_files) {
        oat_file.release();
      }
      for (std::unique_ptr<const DexFile>& dex_file : info.opened_dex_files) {
        dex_file.release();
      }
    }
  }
}

std::unique_ptr<ClassLoaderContext> ClassLoaderContext::Create(const std::string& spec) {
  std::unique_ptr<ClassLoaderContext> result(new ClassLoaderContext());
  if (result->Parse(spec)) {
    return result;
  } else {
    return nullptr;
  }
}

// The expected format is: "ClassLoaderType1[ClasspathElem1*Checksum1:ClasspathElem2*Checksum2...]".
// The checksum part of the format is expected only if parse_cheksums is true.
bool ClassLoaderContext::ParseClassLoaderSpec(const std::string& class_loader_spec,
                                              ClassLoaderType class_loader_type,
                                              bool parse_checksums) {
  const char* class_loader_type_str = GetClassLoaderTypeName(class_loader_type);
  size_t type_str_size = strlen(class_loader_type_str);

  CHECK_EQ(0, class_loader_spec.compare(0, type_str_size, class_loader_type_str));

  // Check the opening and closing markers.
  if (class_loader_spec[type_str_size] != kClassLoaderOpeningMark) {
    return false;
  }
  if (class_loader_spec[class_loader_spec.length() - 1] != kClassLoaderClosingMark) {
    return false;
  }

  // At this point we know the format is ok; continue and extract the classpath.
  // Note that class loaders with an empty class path are allowed.
  std::string classpath = class_loader_spec.substr(type_str_size + 1,
                                                   class_loader_spec.length() - type_str_size - 2);

  class_loader_chain_.push_back(ClassLoaderInfo(class_loader_type));

  if (!parse_checksums) {
    Split(classpath, kClasspathSeparator, &class_loader_chain_.back().classpath);
  } else {
    std::vector<std::string> classpath_elements;
    Split(classpath, kClasspathSeparator, &classpath_elements);
    for (const std::string& element : classpath_elements) {
      std::vector<std::string> dex_file_with_checksum;
      Split(element, kDexFileChecksumSeparator, &dex_file_with_checksum);
      if (dex_file_with_checksum.size() != 2) {
        return false;
      }
      uint32_t checksum = 0;
      if (!ParseInt(dex_file_with_checksum[1].c_str(), &checksum)) {
        return false;
      }
      class_loader_chain_.back().classpath.push_back(dex_file_with_checksum[0]);
      class_loader_chain_.back().checksums.push_back(checksum);
    }
  }

  return true;
}

// Extracts the class loader type from the given spec.
// Return ClassLoaderContext::kInvalidClassLoader if the class loader type is not
// recognized.
ClassLoaderContext::ClassLoaderType
ClassLoaderContext::ExtractClassLoaderType(const std::string& class_loader_spec) {
  const ClassLoaderType kValidTypes[] = {kPathClassLoader, kDelegateLastClassLoader};
  for (const ClassLoaderType& type : kValidTypes) {
    const char* type_str = GetClassLoaderTypeName(type);
    if (class_loader_spec.compare(0, strlen(type_str), type_str) == 0) {
      return type;
    }
  }
  return kInvalidClassLoader;
}

// The format: ClassLoaderType1[ClasspathElem1:ClasspathElem2...];ClassLoaderType2[...]...
// ClassLoaderType is either "PCL" (PathClassLoader) or "DLC" (DelegateLastClassLoader).
// ClasspathElem is the path of dex/jar/apk file.
bool ClassLoaderContext::Parse(const std::string& spec, bool parse_checksums) {
  if (spec.empty()) {
    return true;
  }

  // Stop early if we detect the special shared library, which may be passed as the classpath
  // for dex2oat when we want to skip the shared libraries check.
  if (spec == OatFile::kSpecialSharedLibrary) {
    LOG(INFO) << "The ClassLoaderContext is a special shared library.";
    special_shared_library_ = true;
    return true;
  }

  std::vector<std::string> class_loaders;
  Split(spec, kClassLoaderSeparator, &class_loaders);

  for (const std::string& class_loader : class_loaders) {
    ClassLoaderType type = ExtractClassLoaderType(class_loader);
    if (type == kInvalidClassLoader) {
      LOG(ERROR) << "Invalid class loader type: " << class_loader;
      return false;
    }
    if (!ParseClassLoaderSpec(class_loader, type, parse_checksums)) {
      LOG(ERROR) << "Invalid class loader spec: " << class_loader;
      return false;
    }
  }
  return true;
}

// Opens requested class path files and appends them to opened_dex_files. If the dex files have
// been stripped, this opens them from their oat files (which get added to opened_oat_files).
bool ClassLoaderContext::OpenDexFiles(InstructionSet isa, const std::string& classpath_dir) {
  CHECK(!dex_files_open_attempted_) << "OpenDexFiles should not be called twice";

  dex_files_open_attempted_ = true;
  // Assume we can open all dex files. If not, we will set this to false as we go.
  dex_files_open_result_ = true;

  if (special_shared_library_) {
    // Nothing to open if the context is a special shared library.
    return true;
  }

  // Note that we try to open all dex files even if some fail.
  // We may get resource-only apks which we cannot load.
  // TODO(calin): Refine the dex opening interface to be able to tell if an archive contains
  // no dex files. So that we can distinguish the real failures...
  for (ClassLoaderInfo& info : class_loader_chain_) {
    for (const std::string& cp_elem : info.classpath) {
      // If path is relative, append it to the provided base directory.
      std::string location = cp_elem;
      if (location[0] != '/') {
        location = classpath_dir + '/' + location;
      }
      std::string error_msg;
      // When opening the dex files from the context we expect their checksum to match their
      // contents. So pass true to verify_checksum.
      if (!DexFile::Open(location.c_str(),
                         location.c_str(),
                         /*verify_checksum*/ true,
                         &error_msg,
                         &info.opened_dex_files)) {
        // If we fail to open the dex file because it's been stripped, try to open the dex file
        // from its corresponding oat file.
        // This could happen when we need to recompile a pre-build whose dex code has been stripped.
        // (for example, if the pre-build is only quicken and we want to re-compile it
        // speed-profile).
        // TODO(calin): Use the vdex directly instead of going through the oat file.
        OatFileAssistant oat_file_assistant(location.c_str(), isa, false);
        std::unique_ptr<OatFile> oat_file(oat_file_assistant.GetBestOatFile());
        std::vector<std::unique_ptr<const DexFile>> oat_dex_files;
        if (oat_file != nullptr &&
            OatFileAssistant::LoadDexFiles(*oat_file, location, &oat_dex_files)) {
          info.opened_oat_files.push_back(std::move(oat_file));
          info.opened_dex_files.insert(info.opened_dex_files.end(),
                                       std::make_move_iterator(oat_dex_files.begin()),
                                       std::make_move_iterator(oat_dex_files.end()));
        } else {
          LOG(WARNING) << "Could not open dex files from location: " << location;
          dex_files_open_result_ = false;
        }
      }
    }
  }

  return dex_files_open_result_;
}

bool ClassLoaderContext::RemoveLocationsFromClassPaths(
    const dchecked_vector<std::string>& locations) {
  CHECK(!dex_files_open_attempted_)
      << "RemoveLocationsFromClasspaths cannot be call after OpenDexFiles";

  std::set<std::string> canonical_locations;
  for (const std::string& location : locations) {
    canonical_locations.insert(DexFile::GetDexCanonicalLocation(location.c_str()));
  }
  bool removed_locations = false;
  for (ClassLoaderInfo& info : class_loader_chain_) {
    size_t initial_size = info.classpath.size();
    auto kept_it = std::remove_if(
        info.classpath.begin(),
        info.classpath.end(),
        [canonical_locations](const std::string& location) {
            return ContainsElement(canonical_locations,
                                   DexFile::GetDexCanonicalLocation(location.c_str()));
        });
    info.classpath.erase(kept_it, info.classpath.end());
    if (initial_size != info.classpath.size()) {
      removed_locations = true;
    }
  }
  return removed_locations;
}

std::string ClassLoaderContext::EncodeContextForOatFile(const std::string& base_dir) const {
  CheckDexFilesOpened("EncodeContextForOatFile");
  if (special_shared_library_) {
    return OatFile::kSpecialSharedLibrary;
  }

  if (class_loader_chain_.empty()) {
    return "";
  }

  std::ostringstream out;

  for (size_t i = 0; i < class_loader_chain_.size(); i++) {
    const ClassLoaderInfo& info = class_loader_chain_[i];
    if (i > 0) {
      out << kClassLoaderSeparator;
    }
    out << GetClassLoaderTypeName(info.type);
    out << kClassLoaderOpeningMark;
    for (size_t k = 0; k < info.opened_dex_files.size(); k++) {
      const std::unique_ptr<const DexFile>& dex_file = info.opened_dex_files[k];
      const std::string& location = dex_file->GetLocation();
      if (k > 0) {
        out << kClasspathSeparator;
      }
      // Find paths that were relative and convert them back from absolute.
      if (!base_dir.empty() && location.substr(0, base_dir.length()) == base_dir) {
        out << location.substr(base_dir.length() + 1).c_str();
      } else {
        out << dex_file->GetLocation().c_str();
      }
      out << kDexFileChecksumSeparator;
      out << dex_file->GetLocationChecksum();
    }
    out << kClassLoaderClosingMark;
  }
  return out.str();
}

jobject ClassLoaderContext::CreateClassLoader(
    const std::vector<const DexFile*>& compilation_sources) const {
  CheckDexFilesOpened("CreateClassLoader");

  Thread* self = Thread::Current();
  ScopedObjectAccess soa(self);

  std::vector<const DexFile*> class_path_files;

  // TODO(calin): Transition period: assume we only have a classloader until
  // the oat file assistant implements the full class loader check.
  if (!class_loader_chain_.empty()) {
    CHECK_EQ(1u, class_loader_chain_.size());
    CHECK_EQ(kPathClassLoader, class_loader_chain_[0].type);
    class_path_files = MakeNonOwningPointerVector(class_loader_chain_[0].opened_dex_files);
  }

  // Classpath: first the class-path given; then the dex files we'll compile.
  // Thus we'll resolve the class-path first.
  class_path_files.insert(class_path_files.end(),
                          compilation_sources.begin(),
                          compilation_sources.end());

  ClassLinker* const class_linker = Runtime::Current()->GetClassLinker();
  return class_linker->CreatePathClassLoader(self, class_path_files);
}

std::vector<const DexFile*> ClassLoaderContext::FlattenOpenedDexFiles() const {
  CheckDexFilesOpened("FlattenOpenedDexFiles");

  std::vector<const DexFile*> result;
  for (const ClassLoaderInfo& info : class_loader_chain_) {
    for (const std::unique_ptr<const DexFile>& dex_file : info.opened_dex_files) {
      result.push_back(dex_file.get());
    }
  }
  return result;
}

const char* ClassLoaderContext::GetClassLoaderTypeName(ClassLoaderType type) {
  switch (type) {
    case kPathClassLoader: return kPathClassLoaderString;
    case kDelegateLastClassLoader: return kDelegateLastClassLoaderString;
    default:
      LOG(FATAL) << "Invalid class loader type " << type;
      UNREACHABLE();
  }
}

void ClassLoaderContext::CheckDexFilesOpened(const std::string& calling_method) const {
  CHECK(dex_files_open_attempted_)
      << "Dex files were not successfully opened before the call to " << calling_method
      << "attempt=" << dex_files_open_attempted_ << ", result=" << dex_files_open_result_;
}

bool ClassLoaderContext::DecodePathClassLoaderContextFromOatFileKey(
    const std::string& context_spec,
    std::vector<std::string>* out_classpath,
    std::vector<uint32_t>* out_checksums,
    bool* out_is_special_shared_library) {
  ClassLoaderContext context;
  if (!context.Parse(context_spec, /*parse_checksums*/ true)) {
    LOG(ERROR) << "Invalid class loader context: " << context_spec;
    return false;
  }

  *out_is_special_shared_library = context.special_shared_library_;
  if (context.special_shared_library_) {
    return true;
  }

  if (context.class_loader_chain_.empty()) {
    return true;
  }

  // TODO(calin): assert that we only have a PathClassLoader until the logic for
  // checking the context covers all case.
  CHECK_EQ(1u, context.class_loader_chain_.size());
  const ClassLoaderInfo& info = context.class_loader_chain_[0];
  CHECK_EQ(kPathClassLoader, info.type);
  DCHECK_EQ(info.classpath.size(), info.checksums.size());

  *out_classpath = info.classpath;
  *out_checksums = info.checksums;
  return true;
}

// Collects the dex files from the give Java dex_file object. Only the dex files with
// at least 1 class are collected. If a null java_dex_file is passed this method does nothing.
static bool CollectDexFilesFromJavaDexFile(ObjPtr<mirror::Object> java_dex_file,
                                           ArtField* const cookie_field,
                                           std::vector<const DexFile*>* out_dex_files)
      REQUIRES_SHARED(Locks::mutator_lock_) {
  if (java_dex_file == nullptr) {
    return true;
  }
  // On the Java side, the dex files are stored in the cookie field.
  mirror::LongArray* long_array = cookie_field->GetObject(java_dex_file)->AsLongArray();
  if (long_array == nullptr) {
    // This should never happen so log a warning.
    LOG(ERROR) << "Unexpected null cookie";
    return false;
  }
  int32_t long_array_size = long_array->GetLength();
  // Index 0 from the long array stores the oat file. The dex files start at index 1.
  for (int32_t j = 1; j < long_array_size; ++j) {
    const DexFile* cp_dex_file = reinterpret_cast<const DexFile*>(static_cast<uintptr_t>(
        long_array->GetWithoutChecks(j)));
    if (cp_dex_file != nullptr && cp_dex_file->NumClassDefs() > 0) {
      // TODO(calin): It's unclear why the dex files with no classes are skipped here and when
      // cp_dex_file can be null.
      out_dex_files->push_back(cp_dex_file);
    }
  }
  return true;
}

// Collects all the dex files loaded by the given class loader.
// Returns true for success or false if an unexpected state is discovered (e.g. a null dex cookie,
// a null list of dex elements or a null dex element).
static bool CollectDexFilesFromSupportedClassLoader(ScopedObjectAccessAlreadyRunnable& soa,
                                                    Handle<mirror::ClassLoader> class_loader,
                                                    std::vector<const DexFile*>* out_dex_files)
      REQUIRES_SHARED(Locks::mutator_lock_) {
  CHECK(IsPathOrDexClassLoader(soa, class_loader) || IsDelegateLastClassLoader(soa, class_loader));

  // All supported class loaders inherit from BaseDexClassLoader.
  // We need to get the DexPathList and loop through it.
  ArtField* const cookie_field =
      jni::DecodeArtField(WellKnownClasses::dalvik_system_DexFile_cookie);
  ArtField* const dex_file_field =
      jni::DecodeArtField(WellKnownClasses::dalvik_system_DexPathList__Element_dexFile);
  ObjPtr<mirror::Object> dex_path_list =
      jni::DecodeArtField(WellKnownClasses::dalvik_system_BaseDexClassLoader_pathList)->
          GetObject(class_loader.Get());
  CHECK(cookie_field != nullptr);
  CHECK(dex_file_field != nullptr);
  if (dex_path_list == nullptr) {
    // This may be null if the current class loader is under construction and it does not
    // have its fields setup yet.
    return true;
  }
  // DexPathList has an array dexElements of Elements[] which each contain a dex file.
  ObjPtr<mirror::Object> dex_elements_obj =
      jni::DecodeArtField(WellKnownClasses::dalvik_system_DexPathList_dexElements)->
          GetObject(dex_path_list);
  // Loop through each dalvik.system.DexPathList$Element's dalvik.system.DexFile and look
  // at the mCookie which is a DexFile vector.
  if (dex_elements_obj == nullptr) {
    // TODO(calin): It's unclear if we should just assert here. For now be prepared for the worse
    // and assume we have no elements.
    return true;
  } else {
    StackHandleScope<1> hs(soa.Self());
    Handle<mirror::ObjectArray<mirror::Object>> dex_elements(
        hs.NewHandle(dex_elements_obj->AsObjectArray<mirror::Object>()));
    for (int32_t i = 0; i < dex_elements->GetLength(); ++i) {
      mirror::Object* element = dex_elements->GetWithoutChecks(i);
      if (element == nullptr) {
        // Should never happen, log an error and break.
        // TODO(calin): It's unclear if we should just assert here.
        // This code was propagated to oat_file_manager from the class linker where it would
        // throw a NPE. For now, return false which will mark this class loader as unsupported.
        LOG(ERROR) << "Unexpected null in the dex element list";
        return false;
      }
      ObjPtr<mirror::Object> dex_file = dex_file_field->GetObject(element);
      if (!CollectDexFilesFromJavaDexFile(dex_file, cookie_field, out_dex_files)) {
        return false;
      }
    }
  }

  return true;
}

static bool GetDexFilesFromDexElementsArray(
    ScopedObjectAccessAlreadyRunnable& soa,
    Handle<mirror::ObjectArray<mirror::Object>> dex_elements,
    std::vector<const DexFile*>* out_dex_files) REQUIRES_SHARED(Locks::mutator_lock_) {
  DCHECK(dex_elements != nullptr);

  ArtField* const cookie_field =
      jni::DecodeArtField(WellKnownClasses::dalvik_system_DexFile_cookie);
  ArtField* const dex_file_field =
      jni::DecodeArtField(WellKnownClasses::dalvik_system_DexPathList__Element_dexFile);
  ObjPtr<mirror::Class> const element_class = soa.Decode<mirror::Class>(
      WellKnownClasses::dalvik_system_DexPathList__Element);
  ObjPtr<mirror::Class> const dexfile_class = soa.Decode<mirror::Class>(
      WellKnownClasses::dalvik_system_DexFile);

  for (int32_t i = 0; i < dex_elements->GetLength(); ++i) {
    mirror::Object* element = dex_elements->GetWithoutChecks(i);
    // We can hit a null element here because this is invoked with a partially filled dex_elements
    // array from DexPathList. DexPathList will open each dex sequentially, each time passing the
    // list of dex files which were opened before.
    if (element == nullptr) {
      continue;
    }

    // We support this being dalvik.system.DexPathList$Element and dalvik.system.DexFile.
    // TODO(calin): Code caried over oat_file_manager: supporting both classes seem to be
    // a historical glitch. All the java code opens dex files using an array of Elements.
    ObjPtr<mirror::Object> dex_file;
    if (element_class == element->GetClass()) {
      dex_file = dex_file_field->GetObject(element);
    } else if (dexfile_class == element->GetClass()) {
      dex_file = element;
    } else {
      LOG(ERROR) << "Unsupported element in dex_elements: "
                 << mirror::Class::PrettyClass(element->GetClass());
      return false;
    }

    if (!CollectDexFilesFromJavaDexFile(dex_file, cookie_field, out_dex_files)) {
      return false;
    }
  }
  return true;
}

// Adds the `class_loader` info to the `context`.
// The dex file present in `dex_elements` array (if not null) will be added at the end of
// the classpath.
// This method is recursive (w.r.t. the class loader parent) and will stop once it reaches the
// BootClassLoader. Note that the class loader chain is expected to be short.
bool ClassLoaderContext::AddInfoToContextFromClassLoader(
      ScopedObjectAccessAlreadyRunnable& soa,
      Handle<mirror::ClassLoader> class_loader,
      Handle<mirror::ObjectArray<mirror::Object>> dex_elements)
    REQUIRES_SHARED(Locks::mutator_lock_) {
  if (ClassLinker::IsBootClassLoader(soa, class_loader.Get())) {
    // Nothing to do for the boot class loader as we don't add its dex files to the context.
    return true;
  }

  ClassLoaderContext::ClassLoaderType type;
  if (IsPathOrDexClassLoader(soa, class_loader)) {
    type = kPathClassLoader;
  } else if (IsDelegateLastClassLoader(soa, class_loader)) {
    type = kDelegateLastClassLoader;
  } else {
    LOG(WARNING) << "Unsupported class loader";
    return false;
  }

  // Inspect the class loader for its dex files.
  std::vector<const DexFile*> dex_files_loaded;
  CollectDexFilesFromSupportedClassLoader(soa, class_loader, &dex_files_loaded);

  // If we have a dex_elements array extract its dex elements now.
  // This is used in two situations:
  //   1) when a new ClassLoader is created DexPathList will open each dex file sequentially
  //      passing the list of already open dex files each time. This ensures that we see the
  //      correct context even if the ClassLoader under construction is not fully build.
  //   2) when apk splits are loaded on the fly, the framework will load their dex files by
  //      appending them to the current class loader. When the new code paths are loaded in
  //      BaseDexClassLoader, the paths already present in the class loader will be passed
  //      in the dex_elements array.
  if (dex_elements != nullptr) {
    GetDexFilesFromDexElementsArray(soa, dex_elements, &dex_files_loaded);
  }

  class_loader_chain_.push_back(ClassLoaderContext::ClassLoaderInfo(type));
  ClassLoaderInfo& info = class_loader_chain_.back();
  for (const DexFile* dex_file : dex_files_loaded) {
    info.classpath.push_back(dex_file->GetLocation());
    info.checksums.push_back(dex_file->GetLocationChecksum());
    info.opened_dex_files.emplace_back(dex_file);
  }

  // We created the ClassLoaderInfo for the current loader. Move on to its parent.

  StackHandleScope<1> hs(Thread::Current());
  Handle<mirror::ClassLoader> parent = hs.NewHandle(class_loader->GetParent());

  // Note that dex_elements array is null here. The elements are considered to be part of the
  // current class loader and are not passed to the parents.
  ScopedNullHandle<mirror::ObjectArray<mirror::Object>> null_dex_elements;
  return AddInfoToContextFromClassLoader(soa, parent, null_dex_elements);
}

std::unique_ptr<ClassLoaderContext> ClassLoaderContext::CreateContextForClassLoader(
    jobject class_loader,
    jobjectArray dex_elements) {
  ScopedObjectAccess soa(Thread::Current());
  StackHandleScope<2> hs(soa.Self());
  Handle<mirror::ClassLoader> h_class_loader =
      hs.NewHandle(soa.Decode<mirror::ClassLoader>(class_loader));
  Handle<mirror::ObjectArray<mirror::Object>> h_dex_elements =
      hs.NewHandle(soa.Decode<mirror::ObjectArray<mirror::Object>>(dex_elements));

  CHECK(h_class_loader != nullptr);

  std::unique_ptr<ClassLoaderContext> result(new ClassLoaderContext(/*owns_the_dex_files*/ false));
  if (result->AddInfoToContextFromClassLoader(soa, h_class_loader, h_dex_elements)) {
    return result;
  } else {
    return nullptr;
  }
}

}  // namespace art