1 files changed, 2070 insertions, 0 deletions

diff --git a/contrib/llvm/lib/Support/VirtualFileSystem.cpp b/contrib/llvm/lib/Support/VirtualFileSystem.cpp
new file mode 100644
index 000000000000..f2a8a1bb27af
--- /dev/null
+++ b/contrib/llvm/lib/Support/VirtualFileSystem.cpp
@@ -0,0 +1,2070 @@
+//===- VirtualFileSystem.cpp - Virtual File System Layer ------------------===//
+//
+//                     The LLVM Compiler Infrastructure
+//
+// This file is distributed under the University of Illinois Open Source
+// License. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+//
+// This file implements the VirtualFileSystem interface.
+//
+//===----------------------------------------------------------------------===//
+
+#include "llvm/Support/VirtualFileSystem.h"
+#include "llvm/ADT/ArrayRef.h"
+#include "llvm/ADT/DenseMap.h"
+#include "llvm/ADT/IntrusiveRefCntPtr.h"
+#include "llvm/ADT/None.h"
+#include "llvm/ADT/Optional.h"
+#include "llvm/ADT/STLExtras.h"
+#include "llvm/ADT/SmallString.h"
+#include "llvm/ADT/SmallVector.h"
+#include "llvm/ADT/StringRef.h"
+#include "llvm/ADT/StringSet.h"
+#include "llvm/ADT/Twine.h"
+#include "llvm/ADT/iterator_range.h"
+#include "llvm/Config/llvm-config.h"
+#include "llvm/Support/Casting.h"
+#include "llvm/Support/Chrono.h"
+#include "llvm/Support/Compiler.h"
+#include "llvm/Support/Debug.h"
+#include "llvm/Support/Errc.h"
+#include "llvm/Support/ErrorHandling.h"
+#include "llvm/Support/ErrorOr.h"
+#include "llvm/Support/FileSystem.h"
+#include "llvm/Support/MemoryBuffer.h"
+#include "llvm/Support/Path.h"
+#include "llvm/Support/Process.h"
+#include "llvm/Support/SMLoc.h"
+#include "llvm/Support/SourceMgr.h"
+#include "llvm/Support/YAMLParser.h"
+#include "llvm/Support/raw_ostream.h"
+#include <algorithm>
+#include <atomic>
+#include <cassert>
+#include <cstdint>
+#include <iterator>
+#include <limits>
+#include <map>
+#include <memory>
+#include <mutex>
+#include <string>
+#include <system_error>
+#include <utility>
+#include <vector>
+
+using namespace llvm;
+using namespace llvm::vfs;
+
+using llvm::sys::fs::file_status;
+using llvm::sys::fs::file_type;
+using llvm::sys::fs::perms;
+using llvm::sys::fs::UniqueID;
+
+Status::Status(const file_status &Status)
+    : UID(Status.getUniqueID()), MTime(Status.getLastModificationTime()),
+      User(Status.getUser()), Group(Status.getGroup()), Size(Status.getSize()),
+      Type(Status.type()), Perms(Status.permissions()) {}
+
+Status::Status(StringRef Name, UniqueID UID, sys::TimePoint<> MTime,
+               uint32_t User, uint32_t Group, uint64_t Size, file_type Type,
+               perms Perms)
+    : Name(Name), UID(UID), MTime(MTime), User(User), Group(Group), Size(Size),
+      Type(Type), Perms(Perms) {}
+
+Status Status::copyWithNewName(const Status &In, StringRef NewName) {
+  return Status(NewName, In.getUniqueID(), In.getLastModificationTime(),
+                In.getUser(), In.getGroup(), In.getSize(), In.getType(),
+                In.getPermissions());
+}
+
+Status Status::copyWithNewName(const file_status &In, StringRef NewName) {
+  return Status(NewName, In.getUniqueID(), In.getLastModificationTime(),
+                In.getUser(), In.getGroup(), In.getSize(), In.type(),
+                In.permissions());
+}
+
+bool Status::equivalent(const Status &Other) const {
+  assert(isStatusKnown() && Other.isStatusKnown());
+  return getUniqueID() == Other.getUniqueID();
+}
+
+bool Status::isDirectory() const { return Type == file_type::directory_file; }
+
+bool Status::isRegularFile() const { return Type == file_type::regular_file; }
+
+bool Status::isOther() const {
+  return exists() && !isRegularFile() && !isDirectory() && !isSymlink();
+}
+
+bool Status::isSymlink() const { return Type == file_type::symlink_file; }
+
+bool Status::isStatusKnown() const { return Type != file_type::status_error; }
+
+bool Status::exists() const {
+  return isStatusKnown() && Type != file_type::file_not_found;
+}
+
+File::~File() = default;
+
+FileSystem::~FileSystem() = default;
+
+ErrorOr<std::unique_ptr<MemoryBuffer>>
+FileSystem::getBufferForFile(const llvm::Twine &Name, int64_t FileSize,
+                             bool RequiresNullTerminator, bool IsVolatile) {
+  auto F = openFileForRead(Name);
+  if (!F)
+    return F.getError();
+
+  return (*F)->getBuffer(Name, FileSize, RequiresNullTerminator, IsVolatile);
+}
+
+std::error_code FileSystem::makeAbsolute(SmallVectorImpl<char> &Path) const {
+  if (llvm::sys::path::is_absolute(Path))
+    return {};
+
+  auto WorkingDir = getCurrentWorkingDirectory();
+  if (!WorkingDir)
+    return WorkingDir.getError();
+
+  llvm::sys::fs::make_absolute(WorkingDir.get(), Path);
+  return {};
+}
+
+std::error_code FileSystem::getRealPath(const Twine &Path,
+                                        SmallVectorImpl<char> &Output) const {
+  return errc::operation_not_permitted;
+}
+
+std::error_code FileSystem::isLocal(const Twine &Path, bool &Result) {
+  return errc::operation_not_permitted;
+}
+
+bool FileSystem::exists(const Twine &Path) {
+  auto Status = status(Path);
+  return Status && Status->exists();
+}
+
+#ifndef NDEBUG
+static bool isTraversalComponent(StringRef Component) {
+  return Component.equals("..") || Component.equals(".");
+}
+
+static bool pathHasTraversal(StringRef Path) {
+  using namespace llvm::sys;
+
+  for (StringRef Comp : llvm::make_range(path::begin(Path), path::end(Path)))
+    if (isTraversalComponent(Comp))
+      return true;
+  return false;
+}
+#endif
+
+//===-----------------------------------------------------------------------===/
+// RealFileSystem implementation
+//===-----------------------------------------------------------------------===/
+
+namespace {
+
+/// Wrapper around a raw file descriptor.
+class RealFile : public File {
+  friend class RealFileSystem;
+
+  int FD;
+  Status S;
+  std::string RealName;
+
+  RealFile(int FD, StringRef NewName, StringRef NewRealPathName)
+      : FD(FD), S(NewName, {}, {}, {}, {}, {},
+                  llvm::sys::fs::file_type::status_error, {}),
+        RealName(NewRealPathName.str()) {
+    assert(FD >= 0 && "Invalid or inactive file descriptor");
+  }
+
+public:
+  ~RealFile() override;
+
+  ErrorOr<Status> status() override;
+  ErrorOr<std::string> getName() override;
+  ErrorOr<std::unique_ptr<MemoryBuffer>> getBuffer(const Twine &Name,
+                                                   int64_t FileSize,
+                                                   bool RequiresNullTerminator,
+                                                   bool IsVolatile) override;
+  std::error_code close() override;
+};
+
+} // namespace
+
+RealFile::~RealFile() { close(); }
+
+ErrorOr<Status> RealFile::status() {
+  assert(FD != -1 && "cannot stat closed file");
+  if (!S.isStatusKnown()) {
+    file_status RealStatus;
+    if (std::error_code EC = sys::fs::status(FD, RealStatus))
+      return EC;
+    S = Status::copyWithNewName(RealStatus, S.getName());
+  }
+  return S;
+}
+
+ErrorOr<std::string> RealFile::getName() {
+  return RealName.empty() ? S.getName().str() : RealName;
+}
+
+ErrorOr<std::unique_ptr<MemoryBuffer>>
+RealFile::getBuffer(const Twine &Name, int64_t FileSize,
+                    bool RequiresNullTerminator, bool IsVolatile) {
+  assert(FD != -1 && "cannot get buffer for closed file");
+  return MemoryBuffer::getOpenFile(FD, Name, FileSize, RequiresNullTerminator,
+                                   IsVolatile);
+}
+
+std::error_code RealFile::close() {
+  std::error_code EC = sys::Process::SafelyCloseFileDescriptor(FD);
+  FD = -1;
+  return EC;
+}
+
+namespace {
+
+/// The file system according to your operating system.
+class RealFileSystem : public FileSystem {
+public:
+  ErrorOr<Status> status(const Twine &Path) override;
+  ErrorOr<std::unique_ptr<File>> openFileForRead(const Twine &Path) override;
+  directory_iterator dir_begin(const Twine &Dir, std::error_code &EC) override;
+
+  llvm::ErrorOr<std::string> getCurrentWorkingDirectory() const override;
+  std::error_code setCurrentWorkingDirectory(const Twine &Path) override;
+  std::error_code isLocal(const Twine &Path, bool &Result) override;
+  std::error_code getRealPath(const Twine &Path,
+                              SmallVectorImpl<char> &Output) const override;
+
+private:
+  mutable std::mutex CWDMutex;
+  mutable std::string CWDCache;
+};
+
+} // namespace
+
+ErrorOr<Status> RealFileSystem::status(const Twine &Path) {
+  sys::fs::file_status RealStatus;
+  if (std::error_code EC = sys::fs::status(Path, RealStatus))
+    return EC;
+  return Status::copyWithNewName(RealStatus, Path.str());
+}
+
+ErrorOr<std::unique_ptr<File>>
+RealFileSystem::openFileForRead(const Twine &Name) {
+  int FD;
+  SmallString<256> RealName;
+  if (std::error_code EC =
+          sys::fs::openFileForRead(Name, FD, sys::fs::OF_None, &RealName))
+    return EC;
+  return std::unique_ptr<File>(new RealFile(FD, Name.str(), RealName.str()));
+}
+
+llvm::ErrorOr<std::string> RealFileSystem::getCurrentWorkingDirectory() const {
+  std::lock_guard<std::mutex> Lock(CWDMutex);
+  if (!CWDCache.empty())
+    return CWDCache;
+  SmallString<256> Dir;
+  if (std::error_code EC = llvm::sys::fs::current_path(Dir))
+    return EC;
+  CWDCache = Dir.str();
+  return CWDCache;
+}
+
+std::error_code RealFileSystem::setCurrentWorkingDirectory(const Twine &Path) {
+  // FIXME: chdir is thread hostile; on the other hand, creating the same
+  // behavior as chdir is complex: chdir resolves the path once, thus
+  // guaranteeing that all subsequent relative path operations work
+  // on the same path the original chdir resulted in. This makes a
+  // difference for example on network filesystems, where symlinks might be
+  // switched during runtime of the tool. Fixing this depends on having a
+  // file system abstraction that allows openat() style interactions.
+  if (auto EC = llvm::sys::fs::set_current_path(Path))
+    return EC;
+
+  // Invalidate cache.
+  std::lock_guard<std::mutex> Lock(CWDMutex);
+  CWDCache.clear();
+  return std::error_code();
+}
+
+std::error_code RealFileSystem::isLocal(const Twine &Path, bool &Result) {
+  return llvm::sys::fs::is_local(Path, Result);
+}
+
+std::error_code
+RealFileSystem::getRealPath(const Twine &Path,
+                            SmallVectorImpl<char> &Output) const {
+  return llvm::sys::fs::real_path(Path, Output);
+}
+
+IntrusiveRefCntPtr<FileSystem> vfs::getRealFileSystem() {
+  static IntrusiveRefCntPtr<FileSystem> FS = new RealFileSystem();
+  return FS;
+}
+
+namespace {
+
+class RealFSDirIter : public llvm::vfs::detail::DirIterImpl {
+  llvm::sys::fs::directory_iterator Iter;
+
+public:
+  RealFSDirIter(const Twine &Path, std::error_code &EC) : Iter(Path, EC) {
+    if (Iter != llvm::sys::fs::directory_iterator())
+      CurrentEntry = directory_entry(Iter->path(), Iter->type());
+  }
+
+  std::error_code increment() override {
+    std::error_code EC;
+    Iter.increment(EC);
+    CurrentEntry = (Iter == llvm::sys::fs::directory_iterator())
+                       ? directory_entry()
+                       : directory_entry(Iter->path(), Iter->type());
+    return EC;
+  }
+};
+
+} // namespace
+
+directory_iterator RealFileSystem::dir_begin(const Twine &Dir,
+                                             std::error_code &EC) {
+  return directory_iterator(std::make_shared<RealFSDirIter>(Dir, EC));
+}
+
+//===-----------------------------------------------------------------------===/
+// OverlayFileSystem implementation
+//===-----------------------------------------------------------------------===/
+
+OverlayFileSystem::OverlayFileSystem(IntrusiveRefCntPtr<FileSystem> BaseFS) {
+  FSList.push_back(std::move(BaseFS));
+}
+
+void OverlayFileSystem::pushOverlay(IntrusiveRefCntPtr<FileSystem> FS) {
+  FSList.push_back(FS);
+  // Synchronize added file systems by duplicating the working directory from
+  // the first one in the list.
+  FS->setCurrentWorkingDirectory(getCurrentWorkingDirectory().get());
+}
+
+ErrorOr<Status> OverlayFileSystem::status(const Twine &Path) {
+  // FIXME: handle symlinks that cross file systems
+  for (iterator I = overlays_begin(), E = overlays_end(); I != E; ++I) {
+    ErrorOr<Status> Status = (*I)->status(Path);
+    if (Status || Status.getError() != llvm::errc::no_such_file_or_directory)
+      return Status;
+  }
+  return make_error_code(llvm::errc::no_such_file_or_directory);
+}
+
+ErrorOr<std::unique_ptr<File>>
+OverlayFileSystem::openFileForRead(const llvm::Twine &Path) {
+  // FIXME: handle symlinks that cross file systems
+  for (iterator I = overlays_begin(), E = overlays_end(); I != E; ++I) {
+    auto Result = (*I)->openFileForRead(Path);
+    if (Result || Result.getError() != llvm::errc::no_such_file_or_directory)
+      return Result;
+  }
+  return make_error_code(llvm::errc::no_such_file_or_directory);
+}
+
+llvm::ErrorOr<std::string>
+OverlayFileSystem::getCurrentWorkingDirectory() const {
+  // All file systems are synchronized, just take the first working directory.
+  return FSList.front()->getCurrentWorkingDirectory();
+}
+
+std::error_code
+OverlayFileSystem::setCurrentWorkingDirectory(const Twine &Path) {
+  for (auto &FS : FSList)
+    if (std::error_code EC = FS->setCurrentWorkingDirectory(Path))
+      return EC;
+  return {};
+}
+
+std::error_code OverlayFileSystem::isLocal(const Twine &Path, bool &Result) {
+  for (auto &FS : FSList)
+    if (FS->exists(Path))
+      return FS->isLocal(Path, Result);
+  return errc::no_such_file_or_directory;
+}
+
+std::error_code
+OverlayFileSystem::getRealPath(const Twine &Path,
+                               SmallVectorImpl<char> &Output) const {
+  for (auto &FS : FSList)
+    if (FS->exists(Path))
+      return FS->getRealPath(Path, Output);
+  return errc::no_such_file_or_directory;
+}
+
+llvm::vfs::detail::DirIterImpl::~DirIterImpl() = default;
+
+namespace {
+
+class OverlayFSDirIterImpl : public llvm::vfs::detail::DirIterImpl {
+  OverlayFileSystem &Overlays;
+  std::string Path;
+  OverlayFileSystem::iterator CurrentFS;
+  directory_iterator CurrentDirIter;
+  llvm::StringSet<> SeenNames;
+
+  std::error_code incrementFS() {
+    assert(CurrentFS != Overlays.overlays_end() && "incrementing past end");
+    ++CurrentFS;
+    for (auto E = Overlays.overlays_end(); CurrentFS != E; ++CurrentFS) {
+      std::error_code EC;
+      CurrentDirIter = (*CurrentFS)->dir_begin(Path, EC);
+      if (EC && EC != errc::no_such_file_or_directory)
+        return EC;
+      if (CurrentDirIter != directory_iterator())
+        break; // found
+    }
+    return {};
+  }
+
+  std::error_code incrementDirIter(bool IsFirstTime) {
+    assert((IsFirstTime || CurrentDirIter != directory_iterator()) &&
+           "incrementing past end");
+    std::error_code EC;
+    if (!IsFirstTime)
+      CurrentDirIter.increment(EC);
+    if (!EC && CurrentDirIter == directory_iterator())
+      EC = incrementFS();
+    return EC;
+  }
+
+  std::error_code incrementImpl(bool IsFirstTime) {
+    while (true) {
+      std::error_code EC = incrementDirIter(IsFirstTime);
+      if (EC || CurrentDirIter == directory_iterator()) {
+        CurrentEntry = directory_entry();
+        return EC;
+      }
+      CurrentEntry = *CurrentDirIter;
+      StringRef Name = llvm::sys::path::filename(CurrentEntry.path());
+      if (SeenNames.insert(Name).second)
+        return EC; // name not seen before
+    }
+    llvm_unreachable("returned above");
+  }
+
+public:
+  OverlayFSDirIterImpl(const Twine &Path, OverlayFileSystem &FS,
+                       std::error_code &EC)
+      : Overlays(FS), Path(Path.str()), CurrentFS(Overlays.overlays_begin()) {
+    CurrentDirIter = (*CurrentFS)->dir_begin(Path, EC);
+    EC = incrementImpl(true);
+  }
+
+  std::error_code increment() override { return incrementImpl(false); }
+};
+
+} // namespace
+
+directory_iterator OverlayFileSystem::dir_begin(const Twine &Dir,
+                                                std::error_code &EC) {
+  return directory_iterator(
+      std::make_shared<OverlayFSDirIterImpl>(Dir, *this, EC));
+}
+
+void ProxyFileSystem::anchor() {}
+
+namespace llvm {
+namespace vfs {
+
+namespace detail {
+
+enum InMemoryNodeKind { IME_File, IME_Directory, IME_HardLink };
+
+/// The in memory file system is a tree of Nodes. Every node can either be a
+/// file , hardlink or a directory.
+class InMemoryNode {
+  InMemoryNodeKind Kind;
+  std::string FileName;
+
+public:
+  InMemoryNode(llvm::StringRef FileName, InMemoryNodeKind Kind)
+      : Kind(Kind), FileName(llvm::sys::path::filename(FileName)) {}
+  virtual ~InMemoryNode() = default;
+
+  /// Get the filename of this node (the name without the directory part).
+  StringRef getFileName() const { return FileName; }
+  InMemoryNodeKind getKind() const { return Kind; }
+  virtual std::string toString(unsigned Indent) const = 0;
+};
+
+class InMemoryFile : public InMemoryNode {
+  Status Stat;
+  std::unique_ptr<llvm::MemoryBuffer> Buffer;
+
+public:
+  InMemoryFile(Status Stat, std::unique_ptr<llvm::MemoryBuffer> Buffer)
+      : InMemoryNode(Stat.getName(), IME_File), Stat(std::move(Stat)),
+        Buffer(std::move(Buffer)) {}
+
+  /// Return the \p Status for this node. \p RequestedName should be the name
+  /// through which the caller referred to this node. It will override
+  /// \p Status::Name in the return value, to mimic the behavior of \p RealFile.
+  Status getStatus(StringRef RequestedName) const {
+    return Status::copyWithNewName(Stat, RequestedName);
+  }
+  llvm::MemoryBuffer *getBuffer() const { return Buffer.get(); }
+
+  std::string toString(unsigned Indent) const override {
+    return (std::string(Indent, ' ') + Stat.getName() + "\n").str();
+  }
+
+  static bool classof(const InMemoryNode *N) {
+    return N->getKind() == IME_File;
+  }
+};
+
+namespace {
+
+class InMemoryHardLink : public InMemoryNode {
+  const InMemoryFile &ResolvedFile;
+
+public:
+  InMemoryHardLink(StringRef Path, const InMemoryFile &ResolvedFile)
+      : InMemoryNode(Path, IME_HardLink), ResolvedFile(ResolvedFile) {}
+  const InMemoryFile &getResolvedFile() const { return ResolvedFile; }
+
+  std::string toString(unsigned Indent) const override {
+    return std::string(Indent, ' ') + "HardLink to -> " +
+           ResolvedFile.toString(0);
+  }
+
+  static bool classof(const InMemoryNode *N) {
+    return N->getKind() == IME_HardLink;
+  }
+};
+
+/// Adapt a InMemoryFile for VFS' File interface.  The goal is to make
+/// \p InMemoryFileAdaptor mimic as much as possible the behavior of
+/// \p RealFile.
+class InMemoryFileAdaptor : public File {
+  const InMemoryFile &Node;
+  /// The name to use when returning a Status for this file.
+  std::string RequestedName;
+
+public:
+  explicit InMemoryFileAdaptor(const InMemoryFile &Node,
+                               std::string RequestedName)
+      : Node(Node), RequestedName(std::move(RequestedName)) {}
+
+  llvm::ErrorOr<Status> status() override {
+    return Node.getStatus(RequestedName);
+  }
+
+  llvm::ErrorOr<std::unique_ptr<llvm::MemoryBuffer>>
+  getBuffer(const Twine &Name, int64_t FileSize, bool RequiresNullTerminator,
+            bool IsVolatile) override {
+    llvm::MemoryBuffer *Buf = Node.getBuffer();
+    return llvm::MemoryBuffer::getMemBuffer(
+        Buf->getBuffer(), Buf->getBufferIdentifier(), RequiresNullTerminator);
+  }
+
+  std::error_code close() override { return {}; }
+};
+} // namespace
+
+class InMemoryDirectory : public InMemoryNode {
+  Status Stat;
+  llvm::StringMap<std::unique_ptr<InMemoryNode>> Entries;
+
+public:
+  InMemoryDirectory(Status Stat)
+      : InMemoryNode(Stat.getName(), IME_Directory), Stat(std::move(Stat)) {}
+
+  /// Return the \p Status for this node. \p RequestedName should be the name
+  /// through which the caller referred to this node. It will override
+  /// \p Status::Name in the return value, to mimic the behavior of \p RealFile.
+  Status getStatus(StringRef RequestedName) const {
+    return Status::copyWithNewName(Stat, RequestedName);
+  }
+  InMemoryNode *getChild(StringRef Name) {
+    auto I = Entries.find(Name);
+    if (I != Entries.end())
+      return I->second.get();
+    return nullptr;
+  }
+
+  InMemoryNode *addChild(StringRef Name, std::unique_ptr<InMemoryNode> Child) {
+    return Entries.insert(make_pair(Name, std::move(Child)))
+        .first->second.get();
+  }
+
+  using const_iterator = decltype(Entries)::const_iterator;
+
+  const_iterator begin() const { return Entries.begin(); }
+  const_iterator end() const { return Entries.end(); }
+
+  std::string toString(unsigned Indent) const override {
+    std::string Result =
+        (std::string(Indent, ' ') + Stat.getName() + "\n").str();
+    for (const auto &Entry : Entries)
+      Result += Entry.second->toString(Indent + 2);
+    return Result;
+  }
+
+  static bool classof(const InMemoryNode *N) {
+    return N->getKind() == IME_Directory;
+  }
+};
+
+namespace {
+Status getNodeStatus(const InMemoryNode *Node, StringRef RequestedName) {
+  if (auto Dir = dyn_cast<detail::InMemoryDirectory>(Node))
+    return Dir->getStatus(RequestedName);
+  if (auto File = dyn_cast<detail::InMemoryFile>(Node))
+    return File->getStatus(RequestedName);
+  if (auto Link = dyn_cast<detail::InMemoryHardLink>(Node))
+    return Link->getResolvedFile().getStatus(RequestedName);
+  llvm_unreachable("Unknown node type");
+}
+} // namespace
+} // namespace detail
+
+InMemoryFileSystem::InMemoryFileSystem(bool UseNormalizedPaths)
+    : Root(new detail::InMemoryDirectory(
+          Status("", getNextVirtualUniqueID(), llvm::sys::TimePoint<>(), 0, 0,
+                 0, llvm::sys::fs::file_type::directory_file,
+                 llvm::sys::fs::perms::all_all))),
+      UseNormalizedPaths(UseNormalizedPaths) {}
+
+InMemoryFileSystem::~InMemoryFileSystem() = default;
+
+std::string InMemoryFileSystem::toString() const {
+  return Root->toString(/*Indent=*/0);
+}
+
+bool InMemoryFileSystem::addFile(const Twine &P, time_t ModificationTime,
+                                 std::unique_ptr<llvm::MemoryBuffer> Buffer,
+                                 Optional<uint32_t> User,
+                                 Optional<uint32_t> Group,
+                                 Optional<llvm::sys::fs::file_type> Type,
+                                 Optional<llvm::sys::fs::perms> Perms,
+                                 const detail::InMemoryFile *HardLinkTarget) {
+  SmallString<128> Path;
+  P.toVector(Path);
+
+  // Fix up relative paths. This just prepends the current working directory.
+  std::error_code EC = makeAbsolute(Path);
+  assert(!EC);
+  (void)EC;
+
+  if (useNormalizedPaths())
+    llvm::sys::path::remove_dots(Path, /*remove_dot_dot=*/true);
+
+  if (Path.empty())
+    return false;
+
+  detail::InMemoryDirectory *Dir = Root.get();
+  auto I = llvm::sys::path::begin(Path), E = sys::path::end(Path);
+  const auto ResolvedUser = User.getValueOr(0);
+  const auto ResolvedGroup = Group.getValueOr(0);
+  const auto ResolvedType = Type.getValueOr(sys::fs::file_type::regular_file);
+  const auto ResolvedPerms = Perms.getValueOr(sys::fs::all_all);
+  assert(!(HardLinkTarget && Buffer) && "HardLink cannot have a buffer");
+  // Any intermediate directories we create should be accessible by
+  // the owner, even if Perms says otherwise for the final path.
+  const auto NewDirectoryPerms = ResolvedPerms | sys::fs::owner_all;
+  while (true) {
+    StringRef Name = *I;
+    detail::InMemoryNode *Node = Dir->getChild(Name);
+    ++I;
+    if (!Node) {
+      if (I == E) {
+        // End of the path.
+        std::unique_ptr<detail::InMemoryNode> Child;
+        if (HardLinkTarget)
+          Child.reset(new detail::InMemoryHardLink(P.str(), *HardLinkTarget));
+        else {
+          // Create a new file or directory.
+          Status Stat(P.str(), getNextVirtualUniqueID(),
+                      llvm::sys::toTimePoint(ModificationTime), ResolvedUser,
+                      ResolvedGroup, Buffer->getBufferSize(), ResolvedType,
+                      ResolvedPerms);
+          if (ResolvedType == sys::fs::file_type::directory_file) {
+            Child.reset(new detail::InMemoryDirectory(std::move(Stat)));
+          } else {
+            Child.reset(
+                new detail::InMemoryFile(std::move(Stat), std::move(Buffer)));
+          }
+        }
+        Dir->addChild(Name, std::move(Child));
+        return true;
+      }
+
+      // Create a new directory. Use the path up to here.
+      Status Stat(
+          StringRef(Path.str().begin(), Name.end() - Path.str().begin()),
+          getNextVirtualUniqueID(), llvm::sys::toTimePoint(ModificationTime),
+          ResolvedUser, ResolvedGroup, 0, sys::fs::file_type::directory_file,
+          NewDirectoryPerms);
+      Dir = cast<detail::InMemoryDirectory>(Dir->addChild(
+          Name, llvm::make_unique<detail::InMemoryDirectory>(std::move(Stat))));
+      continue;
+    }
+
+    if (auto *NewDir = dyn_cast<detail::InMemoryDirectory>(Node)) {
+      Dir = NewDir;
+    } else {
+      assert((isa<detail::InMemoryFile>(Node) ||
+              isa<detail::InMemoryHardLink>(Node)) &&
+             "Must be either file, hardlink or directory!");
+
+      // Trying to insert a directory in place of a file.
+      if (I != E)
+        return false;
+
+      // Return false only if the new file is different from the existing one.
+      if (auto Link = dyn_cast<detail::InMemoryHardLink>(Node)) {
+        return Link->getResolvedFile().getBuffer()->getBuffer() ==
+               Buffer->getBuffer();
+      }
+      return cast<detail::InMemoryFile>(Node)->getBuffer()->getBuffer() ==
+             Buffer->getBuffer();
+    }
+  }
+}
+
+bool InMemoryFileSystem::addFile(const Twine &P, time_t ModificationTime,
+                                 std::unique_ptr<llvm::MemoryBuffer> Buffer,
+                                 Optional<uint32_t> User,
+                                 Optional<uint32_t> Group,
+                                 Optional<llvm::sys::fs::file_type> Type,
+                                 Optional<llvm::sys::fs::perms> Perms) {
+  return addFile(P, ModificationTime, std::move(Buffer), User, Group, Type,
+                 Perms, /*HardLinkTarget=*/nullptr);
+}
+
+bool InMemoryFileSystem::addFileNoOwn(const Twine &P, time_t ModificationTime,
+                                      llvm::MemoryBuffer *Buffer,
+                                      Optional<uint32_t> User,
+                                      Optional<uint32_t> Group,
+                                      Optional<llvm::sys::fs::file_type> Type,
+                                      Optional<llvm::sys::fs::perms> Perms) {
+  return addFile(P, ModificationTime,
+                 llvm::MemoryBuffer::getMemBuffer(
+                     Buffer->getBuffer(), Buffer->getBufferIdentifier()),
+                 std::move(User), std::move(Group), std::move(Type),
+                 std::move(Perms));
+}
+
+static ErrorOr<const detail::InMemoryNode *>
+lookupInMemoryNode(const InMemoryFileSystem &FS, detail::InMemoryDirectory *Dir,
+                   const Twine &P) {
+  SmallString<128> Path;
+  P.toVector(Path);
+
+  // Fix up relative paths. This just prepends the current working directory.
+  std::error_code EC = FS.makeAbsolute(Path);
+  assert(!EC);
+  (void)EC;
+
+  if (FS.useNormalizedPaths())
+    llvm::sys::path::remove_dots(Path, /*remove_dot_dot=*/true);
+
+  if (Path.empty())
+    return Dir;
+
+  auto I = llvm::sys::path::begin(Path), E = llvm::sys::path::end(Path);
+  while (true) {
+    detail::InMemoryNode *Node = Dir->getChild(*I);
+    ++I;
+    if (!Node)
+      return errc::no_such_file_or_directory;
+
+    // Return the file if it's at the end of the path.
+    if (auto File = dyn_cast<detail::InMemoryFile>(Node)) {
+      if (I == E)
+        return File;
+      return errc::no_such_file_or_directory;
+    }
+
+    // If Node is HardLink then return the resolved file.
+    if (auto File = dyn_cast<detail::InMemoryHardLink>(Node)) {
+      if (I == E)
+        return &File->getResolvedFile();
+      return errc::no_such_file_or_directory;
+    }
+    // Traverse directories.
+    Dir = cast<detail::InMemoryDirectory>(Node);
+    if (I == E)
+      return Dir;
+  }
+}
+
+bool InMemoryFileSystem::addHardLink(const Twine &FromPath,
+                                     const Twine &ToPath) {
+  auto FromNode = lookupInMemoryNode(*this, Root.get(), FromPath);
+  auto ToNode = lookupInMemoryNode(*this, Root.get(), ToPath);
+  // FromPath must not have been added before. ToPath must have been added
+  // before. Resolved ToPath must be a File.
+  if (!ToNode || FromNode || !isa<detail::InMemoryFile>(*ToNode))
+    return false;
+  return this->addFile(FromPath, 0, nullptr, None, None, None, None,
+                       cast<detail::InMemoryFile>(*ToNode));
+}
+
+llvm::ErrorOr<Status> InMemoryFileSystem::status(const Twine &Path) {
+  auto Node = lookupInMemoryNode(*this, Root.get(), Path);
+  if (Node)
+    return detail::getNodeStatus(*Node, Path.str());
+  return Node.getError();
+}
+
+llvm::ErrorOr<std::unique_ptr<File>>
+InMemoryFileSystem::openFileForRead(const Twine &Path) {
+  auto Node = lookupInMemoryNode(*this, Root.get(), Path);
+  if (!Node)
+    return Node.getError();
+
+  // When we have a file provide a heap-allocated wrapper for the memory buffer
+  // to match the ownership semantics for File.
+  if (auto *F = dyn_cast<detail::InMemoryFile>(*Node))
+    return std::unique_ptr<File>(
+        new detail::InMemoryFileAdaptor(*F, Path.str()));
+
+  // FIXME: errc::not_a_file?
+  return make_error_code(llvm::errc::invalid_argument);
+}
+
+namespace {
+
+/// Adaptor from InMemoryDir::iterator to directory_iterator.
+class InMemoryDirIterator : public llvm::vfs::detail::DirIterImpl {
+  detail::InMemoryDirectory::const_iterator I;
+  detail::InMemoryDirectory::const_iterator E;
+  std::string RequestedDirName;
+
+  void setCurrentEntry() {
+    if (I != E) {
+      SmallString<256> Path(RequestedDirName);
+      llvm::sys::path::append(Path, I->second->getFileName());
+      sys::fs::file_type Type;
+      switch (I->second->getKind()) {
+      case detail::IME_File:
+      case detail::IME_HardLink:
+        Type = sys::fs::file_type::regular_file;
+        break;
+      case detail::IME_Directory:
+        Type = sys::fs::file_type::directory_file;
+        break;
+      }
+      CurrentEntry = directory_entry(Path.str(), Type);
+    } else {
+      // When we're at the end, make CurrentEntry invalid and DirIterImpl will
+      // do the rest.
+      CurrentEntry = directory_entry();
+    }
+  }
+
+public:
+  InMemoryDirIterator() = default;
+
+  explicit InMemoryDirIterator(const detail::InMemoryDirectory &Dir,
+                               std::string RequestedDirName)
+      : I(Dir.begin()), E(Dir.end()),
+        RequestedDirName(std::move(RequestedDirName)) {
+    setCurrentEntry();
+  }
+
+  std::error_code increment() override {
+    ++I;
+    setCurrentEntry();
+    return {};
+  }
+};
+
+} // namespace
+
+directory_iterator InMemoryFileSystem::dir_begin(const Twine &Dir,
+                                                 std::error_code &EC) {
+  auto Node = lookupInMemoryNode(*this, Root.get(), Dir);
+  if (!Node) {
+    EC = Node.getError();
+    return directory_iterator(std::make_shared<InMemoryDirIterator>());
+  }
+
+  if (auto *DirNode = dyn_cast<detail::InMemoryDirectory>(*Node))
+    return directory_iterator(
+        std::make_shared<InMemoryDirIterator>(*DirNode, Dir.str()));
+
+  EC = make_error_code(llvm::errc::not_a_directory);
+  return directory_iterator(std::make_shared<InMemoryDirIterator>());
+}
+
+std::error_code InMemoryFileSystem::setCurrentWorkingDirectory(const Twine &P) {
+  SmallString<128> Path;
+  P.toVector(Path);
+
+  // Fix up relative paths. This just prepends the current working directory.
+  std::error_code EC = makeAbsolute(Path);
+  assert(!EC);
+  (void)EC;
+
+  if (useNormalizedPaths())
+    llvm::sys::path::remove_dots(Path, /*remove_dot_dot=*/true);
+
+  if (!Path.empty())
+    WorkingDirectory = Path.str();
+  return {};
+}
+
+std::error_code
+InMemoryFileSystem::getRealPath(const Twine &Path,
+                                SmallVectorImpl<char> &Output) const {
+  auto CWD = getCurrentWorkingDirectory();
+  if (!CWD || CWD->empty())
+    return errc::operation_not_permitted;
+  Path.toVector(Output);
+  if (auto EC = makeAbsolute(Output))
+    return EC;
+  llvm::sys::path::remove_dots(Output, /*remove_dot_dot=*/true);
+  return {};
+}
+
+std::error_code InMemoryFileSystem::isLocal(const Twine &Path, bool &Result) {
+  Result = false;
+  return {};
+}
+
+} // namespace vfs
+} // namespace llvm
+
+//===-----------------------------------------------------------------------===/
+// RedirectingFileSystem implementation
+//===-----------------------------------------------------------------------===/
+
+// FIXME: reuse implementation common with OverlayFSDirIterImpl as these
+// iterators are conceptually similar.
+class llvm::vfs::VFSFromYamlDirIterImpl
+    : public llvm::vfs::detail::DirIterImpl {
+  std::string Dir;
+  RedirectingFileSystem::RedirectingDirectoryEntry::iterator Current, End;
+
+  // To handle 'fallthrough' mode we need to iterate at first through
+  // RedirectingDirectoryEntry and then through ExternalFS. These operations are
+  // done sequentially, we just need to keep a track of what kind of iteration
+  // we are currently performing.
+
+  /// Flag telling if we should iterate through ExternalFS or stop at the last
+  /// RedirectingDirectoryEntry::iterator.
+  bool IterateExternalFS;
+  /// Flag telling if we have switched to iterating through ExternalFS.
+  bool IsExternalFSCurrent = false;
+  FileSystem &ExternalFS;
+  directory_iterator ExternalDirIter;
+  llvm::StringSet<> SeenNames;
+
+  /// To combine multiple iterations, different methods are responsible for
+  /// different iteration steps.
+  /// @{
+
+  /// Responsible for dispatching between RedirectingDirectoryEntry iteration
+  /// and ExternalFS iteration.
+  std::error_code incrementImpl(bool IsFirstTime);
+  /// Responsible for RedirectingDirectoryEntry iteration.
+  std::error_code incrementContent(bool IsFirstTime);
+  /// Responsible for ExternalFS iteration.
+  std::error_code incrementExternal();
+  /// @}
+
+public:
+  VFSFromYamlDirIterImpl(
+      const Twine &Path,
+      RedirectingFileSystem::RedirectingDirectoryEntry::iterator Begin,
+      RedirectingFileSystem::RedirectingDirectoryEntry::iterator End,
+      bool IterateExternalFS, FileSystem &ExternalFS, std::error_code &EC);
+
+  std::error_code increment() override;
+};
+
+llvm::ErrorOr<std::string>
+RedirectingFileSystem::getCurrentWorkingDirectory() const {
+  return ExternalFS->getCurrentWorkingDirectory();
+}
+
+std::error_code
+RedirectingFileSystem::setCurrentWorkingDirectory(const Twine &Path) {
+  return ExternalFS->setCurrentWorkingDirectory(Path);
+}
+
+std::error_code RedirectingFileSystem::isLocal(const Twine &Path,
+                                               bool &Result) {
+  return ExternalFS->isLocal(Path, Result);
+}
+
+directory_iterator RedirectingFileSystem::dir_begin(const Twine &Dir,
+                                                    std::error_code &EC) {
+  ErrorOr<RedirectingFileSystem::Entry *> E = lookupPath(Dir);
+  if (!E) {
+    EC = E.getError();
+    if (IsFallthrough && EC == errc::no_such_file_or_directory)
+      return ExternalFS->dir_begin(Dir, EC);
+    return {};
+  }
+  ErrorOr<Status> S = status(Dir, *E);
+  if (!S) {
+    EC = S.getError();
+    return {};
+  }
+  if (!S->isDirectory()) {
+    EC = std::error_code(static_cast<int>(errc::not_a_directory),
+                         std::system_category());
+    return {};
+  }
+
+  auto *D = cast<RedirectingFileSystem::RedirectingDirectoryEntry>(*E);
+  return directory_iterator(std::make_shared<VFSFromYamlDirIterImpl>(
+      Dir, D->contents_begin(), D->contents_end(),
+      /*IterateExternalFS=*/IsFallthrough, *ExternalFS, EC));
+}
+
+void RedirectingFileSystem::setExternalContentsPrefixDir(StringRef PrefixDir) {
+  ExternalContentsPrefixDir = PrefixDir.str();
+}
+
+StringRef RedirectingFileSystem::getExternalContentsPrefixDir() const {
+  return ExternalContentsPrefixDir;
+}
+
+#if !defined(NDEBUG) || defined(LLVM_ENABLE_DUMP)
+LLVM_DUMP_METHOD void RedirectingFileSystem::dump() const {
+  for (const auto &Root : Roots)
+    dumpEntry(Root.get());
+}
+
+LLVM_DUMP_METHOD void
+RedirectingFileSystem::dumpEntry(RedirectingFileSystem::Entry *E,
+                                 int NumSpaces) const {
+  StringRef Name = E->getName();
+  for (int i = 0, e = NumSpaces; i < e; ++i)
+    dbgs() << " ";
+  dbgs() << "'" << Name.str().c_str() << "'"
+         << "\n";
+
+  if (E->getKind() == RedirectingFileSystem::EK_Directory) {
+    auto *DE = dyn_cast<RedirectingFileSystem::RedirectingDirectoryEntry>(E);
+    assert(DE && "Should be a directory");
+
+    for (std::unique_ptr<Entry> &SubEntry :
+         llvm::make_range(DE->contents_begin(), DE->contents_end()))
+      dumpEntry(SubEntry.get(), NumSpaces + 2);
+  }
+}
+#endif
+
+/// A helper class to hold the common YAML parsing state.
+class llvm::vfs::RedirectingFileSystemParser {
+  yaml::Stream &Stream;
+
+  void error(yaml::Node *N, const Twine &Msg) { Stream.printError(N, Msg); }
+
+  // false on error
+  bool parseScalarString(yaml::Node *N, StringRef &Result,
+                         SmallVectorImpl<char> &Storage) {
+    const auto *S = dyn_cast<yaml::ScalarNode>(N);
+
+    if (!S) {
+      error(N, "expected string");
+      return false;
+    }
+    Result = S->getValue(Storage);
+    return true;
+  }
+
+  // false on error
+  bool parseScalarBool(yaml::Node *N, bool &Result) {
+    SmallString<5> Storage;
+    StringRef Value;
+    if (!parseScalarString(N, Value, Storage))
+      return false;
+
+    if (Value.equals_lower("true") || Value.equals_lower("on") ||
+        Value.equals_lower("yes") || Value == "1") {
+      Result = true;
+      return true;
+    } else if (Value.equals_lower("false") || Value.equals_lower("off") ||
+               Value.equals_lower("no") || Value == "0") {
+      Result = false;
+      return true;
+    }
+
+    error(N, "expected boolean value");
+    return false;
+  }
+
+  struct KeyStatus {
+    bool Required;
+    bool Seen = false;
+
+    KeyStatus(bool Required = false) : Required(Required) {}
+  };
+
+  using KeyStatusPair = std::pair<StringRef, KeyStatus>;
+
+  // false on error
+  bool checkDuplicateOrUnknownKey(yaml::Node *KeyNode, StringRef Key,
+                                  DenseMap<StringRef, KeyStatus> &Keys) {
+    if (!Keys.count(Key)) {
+      error(KeyNode, "unknown key");
+      return false;
+    }
+    KeyStatus &S = Keys[Key];
+    if (S.Seen) {
+      error(KeyNode, Twine("duplicate key '") + Key + "'");
+      return false;
+    }
+    S.Seen = true;
+    return true;
+  }
+
+  // false on error
+  bool checkMissingKeys(yaml::Node *Obj, DenseMap<StringRef, KeyStatus> &Keys) {
+    for (const auto &I : Keys) {
+      if (I.second.Required && !I.second.Seen) {
+        error(Obj, Twine("missing key '") + I.first + "'");
+        return false;
+      }
+    }
+    return true;
+  }
+
+  RedirectingFileSystem::Entry *
+  lookupOrCreateEntry(RedirectingFileSystem *FS, StringRef Name,
+                      RedirectingFileSystem::Entry *ParentEntry = nullptr) {
+    if (!ParentEntry) { // Look for a existent root
+      for (const auto &Root : FS->Roots) {
+        if (Name.equals(Root->getName())) {
+          ParentEntry = Root.get();
+          return ParentEntry;
+        }
+      }
+    } else { // Advance to the next component
+      auto *DE = dyn_cast<RedirectingFileSystem::RedirectingDirectoryEntry>(
+          ParentEntry);
+      for (std::unique_ptr<RedirectingFileSystem::Entry> &Content :
+           llvm::make_range(DE->contents_begin(), DE->contents_end())) {
+        auto *DirContent =
+            dyn_cast<RedirectingFileSystem::RedirectingDirectoryEntry>(
+                Content.get());
+        if (DirContent && Name.equals(Content->getName()))
+          return DirContent;
+      }
+    }
+
+    // ... or create a new one
+    std::unique_ptr<RedirectingFileSystem::Entry> E =
+        llvm::make_unique<RedirectingFileSystem::RedirectingDirectoryEntry>(
+            Name, Status("", getNextVirtualUniqueID(),
+                         std::chrono::system_clock::now(), 0, 0, 0,
+                         file_type::directory_file, sys::fs::all_all));
+
+    if (!ParentEntry) { // Add a new root to the overlay
+      FS->Roots.push_back(std::move(E));
+      ParentEntry = FS->Roots.back().get();
+      return ParentEntry;
+    }
+
+    auto *DE =
+        dyn_cast<RedirectingFileSystem::RedirectingDirectoryEntry>(ParentEntry);
+    DE->addContent(std::move(E));
+    return DE->getLastContent();
+  }
+
+  void uniqueOverlayTree(RedirectingFileSystem *FS,
+                         RedirectingFileSystem::Entry *SrcE,
+                         RedirectingFileSystem::Entry *NewParentE = nullptr) {
+    StringRef Name = SrcE->getName();
+    switch (SrcE->getKind()) {
+    case RedirectingFileSystem::EK_Directory: {
+      auto *DE =
+          dyn_cast<RedirectingFileSystem::RedirectingDirectoryEntry>(SrcE);
+      assert(DE && "Must be a directory");
+      // Empty directories could be present in the YAML as a way to
+      // describe a file for a current directory after some of its subdir
+      // is parsed. This only leads to redundant walks, ignore it.
+      if (!Name.empty())
+        NewParentE = lookupOrCreateEntry(FS, Name, NewParentE);
+      for (std::unique_ptr<RedirectingFileSystem::Entry> &SubEntry :
+           llvm::make_range(DE->contents_begin(), DE->contents_end()))
+        uniqueOverlayTree(FS, SubEntry.get(), NewParentE);
+      break;
+    }
+    case RedirectingFileSystem::EK_File: {
+      auto *FE = dyn_cast<RedirectingFileSystem::RedirectingFileEntry>(SrcE);
+      assert(FE && "Must be a file");
+      assert(NewParentE && "Parent entry must exist");
+      auto *DE = dyn_cast<RedirectingFileSystem::RedirectingDirectoryEntry>(
+          NewParentE);
+      DE->addContent(
+          llvm::make_unique<RedirectingFileSystem::RedirectingFileEntry>(
+              Name, FE->getExternalContentsPath(), FE->getUseName()));
+      break;
+    }
+    }
+  }
+
+  std::unique_ptr<RedirectingFileSystem::Entry>
+  parseEntry(yaml::Node *N, RedirectingFileSystem *FS, bool IsRootEntry) {
+    auto *M = dyn_cast<yaml::MappingNode>(N);
+    if (!M) {
+      error(N, "expected mapping node for file or directory entry");
+      return nullptr;
+    }
+
+    KeyStatusPair Fields[] = {
+        KeyStatusPair("name", true),
+        KeyStatusPair("type", true),
+        KeyStatusPair("contents", false),
+        KeyStatusPair("external-contents", false),
+        KeyStatusPair("use-external-name", false),
+    };
+
+    DenseMap<StringRef, KeyStatus> Keys(std::begin(Fields), std::end(Fields));
+
+    bool HasContents = false; // external or otherwise
+    std::vector<std::unique_ptr<RedirectingFileSystem::Entry>>
+        EntryArrayContents;
+    std::string ExternalContentsPath;
+    std::string Name;
+    yaml::Node *NameValueNode;
+    auto UseExternalName =
+        RedirectingFileSystem::RedirectingFileEntry::NK_NotSet;
+    RedirectingFileSystem::EntryKind Kind;
+
+    for (auto &I : *M) {
+      StringRef Key;
+      // Reuse the buffer for key and value, since we don't look at key after
+      // parsing value.
+      SmallString<256> Buffer;
+      if (!parseScalarString(I.getKey(), Key, Buffer))
+        return nullptr;
+
+      if (!checkDuplicateOrUnknownKey(I.getKey(), Key, Keys))
+        return nullptr;
+
+      StringRef Value;
+      if (Key == "name") {
+        if (!parseScalarString(I.getValue(), Value, Buffer))
+          return nullptr;
+
+        NameValueNode = I.getValue();
+        if (FS->UseCanonicalizedPaths) {
+          SmallString<256> Path(Value);
+          // Guarantee that old YAML files containing paths with ".." and "."
+          // are properly canonicalized before read into the VFS.
+          Path = sys::path::remove_leading_dotslash(Path);
+          sys::path::remove_dots(Path, /*remove_dot_dot=*/true);
+          Name = Path.str();
+        } else {
+          Name = Value;
+        }
+      } else if (Key == "type") {
+        if (!parseScalarString(I.getValue(), Value, Buffer))
+          return nullptr;
+        if (Value == "file")
+          Kind = RedirectingFileSystem::EK_File;
+        else if (Value == "directory")
+          Kind = RedirectingFileSystem::EK_Directory;
+        else {
+          error(I.getValue(), "unknown value for 'type'");
+          return nullptr;
+        }
+      } else if (Key == "contents") {
+        if (HasContents) {
+          error(I.getKey(),
+                "entry already has 'contents' or 'external-contents'");
+          return nullptr;
+        }
+        HasContents = true;
+        auto *Contents = dyn_cast<yaml::SequenceNode>(I.getValue());
+        if (!Contents) {
+          // FIXME: this is only for directories, what about files?
+          error(I.getValue(), "expected array");
+          return nullptr;
+        }
+
+        for (auto &I : *Contents) {
+          if (std::unique_ptr<RedirectingFileSystem::Entry> E =
+                  parseEntry(&I, FS, /*IsRootEntry*/ false))
+            EntryArrayContents.push_back(std::move(E));
+          else
+            return nullptr;
+        }
+      } else if (Key == "external-contents") {
+        if (HasContents) {
+          error(I.getKey(),
+                "entry already has 'contents' or 'external-contents'");
+          return nullptr;
+        }
+        HasContents = true;
+        if (!parseScalarString(I.getValue(), Value, Buffer))
+          return nullptr;
+
+        SmallString<256> FullPath;
+        if (FS->IsRelativeOverlay) {
+          FullPath = FS->getExternalContentsPrefixDir();
+          assert(!FullPath.empty() &&
+                 "External contents prefix directory must exist");
+          llvm::sys::path::append(FullPath, Value);
+        } else {
+          FullPath = Value;
+        }
+
+        if (FS->UseCanonicalizedPaths) {
+          // Guarantee that old YAML files containing paths with ".." and "."
+          // are properly canonicalized before read into the VFS.
+          FullPath = sys::path::remove_leading_dotslash(FullPath);
+          sys::path::remove_dots(FullPath, /*remove_dot_dot=*/true);
+        }
+        ExternalContentsPath = FullPath.str();
+      } else if (Key == "use-external-name") {
+        bool Val;
+        if (!parseScalarBool(I.getValue(), Val))
+          return nullptr;
+        UseExternalName =
+            Val ? RedirectingFileSystem::RedirectingFileEntry::NK_External
+                : RedirectingFileSystem::RedirectingFileEntry::NK_Virtual;
+      } else {
+        llvm_unreachable("key missing from Keys");
+      }
+    }
+
+    if (Stream.failed())
+      return nullptr;
+
+    // check for missing keys
+    if (!HasContents) {
+      error(N, "missing key 'contents' or 'external-contents'");
+      return nullptr;
+    }
+    if (!checkMissingKeys(N, Keys))
+      return nullptr;
+
+    // check invalid configuration
+    if (Kind == RedirectingFileSystem::EK_Directory &&
+        UseExternalName !=
+            RedirectingFileSystem::RedirectingFileEntry::NK_NotSet) {
+      error(N, "'use-external-name' is not supported for directories");
+      return nullptr;
+    }
+
+    if (IsRootEntry && !sys::path::is_absolute(Name)) {
+      assert(NameValueNode && "Name presence should be checked earlier");
+      error(NameValueNode,
+            "entry with relative path at the root level is not discoverable");
+      return nullptr;
+    }
+
+    // Remove trailing slash(es), being careful not to remove the root path
+    StringRef Trimmed(Name);
+    size_t RootPathLen = sys::path::root_path(Trimmed).size();
+    while (Trimmed.size() > RootPathLen &&
+           sys::path::is_separator(Trimmed.back()))
+      Trimmed = Trimmed.slice(0, Trimmed.size() - 1);
+    // Get the last component
+    StringRef LastComponent = sys::path::filename(Trimmed);
+
+    std::unique_ptr<RedirectingFileSystem::Entry> Result;
+    switch (Kind) {
+    case RedirectingFileSystem::EK_File:
+      Result = llvm::make_unique<RedirectingFileSystem::RedirectingFileEntry>(
+          LastComponent, std::move(ExternalContentsPath), UseExternalName);
+      break;
+    case RedirectingFileSystem::EK_Directory:
+      Result =
+          llvm::make_unique<RedirectingFileSystem::RedirectingDirectoryEntry>(
+              LastComponent, std::move(EntryArrayContents),
+              Status("", getNextVirtualUniqueID(),
+                     std::chrono::system_clock::now(), 0, 0, 0,
+                     file_type::directory_file, sys::fs::all_all));
+      break;
+    }
+
+    StringRef Parent = sys::path::parent_path(Trimmed);
+    if (Parent.empty())
+      return Result;
+
+    // if 'name' contains multiple components, create implicit directory entries
+    for (sys::path::reverse_iterator I = sys::path::rbegin(Parent),
+                                     E = sys::path::rend(Parent);
+         I != E; ++I) {
+      std::vector<std::unique_ptr<RedirectingFileSystem::Entry>> Entries;
+      Entries.push_back(std::move(Result));
+      Result =
+          llvm::make_unique<RedirectingFileSystem::RedirectingDirectoryEntry>(
+              *I, std::move(Entries),
+              Status("", getNextVirtualUniqueID(),
+                     std::chrono::system_clock::now(), 0, 0, 0,
+                     file_type::directory_file, sys::fs::all_all));
+    }
+    return Result;
+  }
+
+public:
+  RedirectingFileSystemParser(yaml::Stream &S) : Stream(S) {}
+
+  // false on error
+  bool parse(yaml::Node *Root, RedirectingFileSystem *FS) {
+    auto *Top = dyn_cast<yaml::MappingNode>(Root);
+    if (!Top) {
+      error(Root, "expected mapping node");
+      return false;
+    }
+
+    KeyStatusPair Fields[] = {
+        KeyStatusPair("version", true),
+        KeyStatusPair("case-sensitive", false),
+        KeyStatusPair("use-external-names", false),
+        KeyStatusPair("overlay-relative", false),
+        KeyStatusPair("fallthrough", false),
+        KeyStatusPair("roots", true),
+    };
+
+    DenseMap<StringRef, KeyStatus> Keys(std::begin(Fields), std::end(Fields));
+    std::vector<std::unique_ptr<RedirectingFileSystem::Entry>> RootEntries;
+
+    // Parse configuration and 'roots'
+    for (auto &I : *Top) {
+      SmallString<10> KeyBuffer;
+      StringRef Key;
+      if (!parseScalarString(I.getKey(), Key, KeyBuffer))
+        return false;
+
+      if (!checkDuplicateOrUnknownKey(I.getKey(), Key, Keys))
+        return false;
+
+      if (Key == "roots") {
+        auto *Roots = dyn_cast<yaml::SequenceNode>(I.getValue());
+        if (!Roots) {
+          error(I.getValue(), "expected array");
+          return false;
+        }
+
+        for (auto &I : *Roots) {
+          if (std::unique_ptr<RedirectingFileSystem::Entry> E =
+                  parseEntry(&I, FS, /*IsRootEntry*/ true))
+            RootEntries.push_back(std::move(E));
+          else
+            return false;
+        }
+      } else if (Key == "version") {
+        StringRef VersionString;
+        SmallString<4> Storage;
+        if (!parseScalarString(I.getValue(), VersionString, Storage))
+          return false;
+        int Version;
+        if (VersionString.getAsInteger<int>(10, Version)) {
+          error(I.getValue(), "expected integer");
+          return false;
+        }
+        if (Version < 0) {
+          error(I.getValue(), "invalid version number");
+          return false;
+        }
+        if (Version != 0) {
+          error(I.getValue(), "version mismatch, expected 0");
+          return false;
+        }
+      } else if (Key == "case-sensitive") {
+        if (!parseScalarBool(I.getValue(), FS->CaseSensitive))
+          return false;
+      } else if (Key == "overlay-relative") {
+        if (!parseScalarBool(I.getValue(), FS->IsRelativeOverlay))
+          return false;
+      } else if (Key == "use-external-names") {
+        if (!parseScalarBool(I.getValue(), FS->UseExternalNames))
+          return false;
+      } else if (Key == "fallthrough") {
+        if (!parseScalarBool(I.getValue(), FS->IsFallthrough))
+          return false;
+      } else {
+        llvm_unreachable("key missing from Keys");
+      }
+    }
+
+    if (Stream.failed())
+      return false;
+
+    if (!checkMissingKeys(Top, Keys))
+      return false;
+
+    // Now that we sucessefully parsed the YAML file, canonicalize the internal
+    // representation to a proper directory tree so that we can search faster
+    // inside the VFS.
+    for (auto &E : RootEntries)
+      uniqueOverlayTree(FS, E.get());
+
+    return true;
+  }
+};
+
+RedirectingFileSystem *
+RedirectingFileSystem::create(std::unique_ptr<MemoryBuffer> Buffer,
+                              SourceMgr::DiagHandlerTy DiagHandler,
+                              StringRef YAMLFilePath, void *DiagContext,
+                              IntrusiveRefCntPtr<FileSystem> ExternalFS) {
+  SourceMgr SM;
+  yaml::Stream Stream(Buffer->getMemBufferRef(), SM);
+
+  SM.setDiagHandler(DiagHandler, DiagContext);
+  yaml::document_iterator DI = Stream.begin();
+  yaml::Node *Root = DI->getRoot();
+  if (DI == Stream.end() || !Root) {
+    SM.PrintMessage(SMLoc(), SourceMgr::DK_Error, "expected root node");
+    return nullptr;
+  }
+
+  RedirectingFileSystemParser P(Stream);
+
+  std::unique_ptr<RedirectingFileSystem> FS(
+      new RedirectingFileSystem(std::move(ExternalFS)));
+
+  if (!YAMLFilePath.empty()) {
+    // Use the YAML path from -ivfsoverlay to compute the dir to be prefixed
+    // to each 'external-contents' path.
+    //
+    // Example:
+    //    -ivfsoverlay dummy.cache/vfs/vfs.yaml
+    // yields:
+    //  FS->ExternalContentsPrefixDir => /<absolute_path_to>/dummy.cache/vfs
+    //
+    SmallString<256> OverlayAbsDir = sys::path::parent_path(YAMLFilePath);
+    std::error_code EC = llvm::sys::fs::make_absolute(OverlayAbsDir);
+    assert(!EC && "Overlay dir final path must be absolute");
+    (void)EC;
+    FS->setExternalContentsPrefixDir(OverlayAbsDir);
+  }
+
+  if (!P.parse(Root, FS.get()))
+    return nullptr;
+
+  return FS.release();
+}
+
+ErrorOr<RedirectingFileSystem::Entry *>
+RedirectingFileSystem::lookupPath(const Twine &Path_) const {
+  SmallString<256> Path;
+  Path_.toVector(Path);
+
+  // Handle relative paths
+  if (std::error_code EC = makeAbsolute(Path))
+    return EC;
+
+  // Canonicalize path by removing ".", "..", "./", etc components. This is
+  // a VFS request, do bot bother about symlinks in the path components
+  // but canonicalize in order to perform the correct entry search.
+  if (UseCanonicalizedPaths) {
+    Path = sys::path::remove_leading_dotslash(Path);
+    sys::path::remove_dots(Path, /*remove_dot_dot=*/true);
+  }
+
+  if (Path.empty())
+    return make_error_code(llvm::errc::invalid_argument);
+
+  sys::path::const_iterator Start = sys::path::begin(Path);
+  sys::path::const_iterator End = sys::path::end(Path);
+  for (const auto &Root : Roots) {
+    ErrorOr<RedirectingFileSystem::Entry *> Result =
+        lookupPath(Start, End, Root.get());
+    if (Result || Result.getError() != llvm::errc::no_such_file_or_directory)
+      return Result;
+  }
+  return make_error_code(llvm::errc::no_such_file_or_directory);
+}
+
+ErrorOr<RedirectingFileSystem::Entry *>
+RedirectingFileSystem::lookupPath(sys::path::const_iterator Start,
+                                  sys::path::const_iterator End,
+                                  RedirectingFileSystem::Entry *From) const {
+#ifndef _WIN32
+  assert(!isTraversalComponent(*Start) &&
+         !isTraversalComponent(From->getName()) &&
+         "Paths should not contain traversal components");
+#else
+  // FIXME: this is here to support windows, remove it once canonicalized
+  // paths become globally default.
+  if (Start->equals("."))
+    ++Start;
+#endif
+
+  StringRef FromName = From->getName();
+
+  // Forward the search to the next component in case this is an empty one.
+  if (!FromName.empty()) {
+    if (CaseSensitive ? !Start->equals(FromName)
+                      : !Start->equals_lower(FromName))
+      // failure to match
+      return make_error_code(llvm::errc::no_such_file_or_directory);
+
+    ++Start;
+
+    if (Start == End) {
+      // Match!
+      return From;
+    }
+  }
+
+  auto *DE = dyn_cast<RedirectingFileSystem::RedirectingDirectoryEntry>(From);
+  if (!DE)
+    return make_error_code(llvm::errc::not_a_directory);
+
+  for (const std::unique_ptr<RedirectingFileSystem::Entry> &DirEntry :
+       llvm::make_range(DE->contents_begin(), DE->contents_end())) {
+    ErrorOr<RedirectingFileSystem::Entry *> Result =
+        lookupPath(Start, End, DirEntry.get());
+    if (Result || Result.getError() != llvm::errc::no_such_file_or_directory)
+      return Result;
+  }
+  return make_error_code(llvm::errc::no_such_file_or_directory);
+}
+
+static Status getRedirectedFileStatus(const Twine &Path, bool UseExternalNames,
+                                      Status ExternalStatus) {
+  Status S = ExternalStatus;
+  if (!UseExternalNames)
+    S = Status::copyWithNewName(S, Path.str());
+  S.IsVFSMapped = true;
+  return S;
+}
+
+ErrorOr<Status> RedirectingFileSystem::status(const Twine &Path,
+                                              RedirectingFileSystem::Entry *E) {
+  assert(E != nullptr);
+  if (auto *F = dyn_cast<RedirectingFileSystem::RedirectingFileEntry>(E)) {
+    ErrorOr<Status> S = ExternalFS->status(F->getExternalContentsPath());
+    assert(!S || S->getName() == F->getExternalContentsPath());
+    if (S)
+      return getRedirectedFileStatus(Path, F->useExternalName(UseExternalNames),
+                                     *S);
+    return S;
+  } else { // directory
+    auto *DE = cast<RedirectingFileSystem::RedirectingDirectoryEntry>(E);
+    return Status::copyWithNewName(DE->getStatus(), Path.str());
+  }
+}
+
+ErrorOr<Status> RedirectingFileSystem::status(const Twine &Path) {
+  ErrorOr<RedirectingFileSystem::Entry *> Result = lookupPath(Path);
+  if (!Result) {
+    if (IsFallthrough &&
+        Result.getError() == llvm::errc::no_such_file_or_directory) {
+      return ExternalFS->status(Path);
+    }
+    return Result.getError();
+  }
+  return status(Path, *Result);
+}
+
+namespace {
+
+/// Provide a file wrapper with an overriden status.
+class FileWithFixedStatus : public File {
+  std::unique_ptr<File> InnerFile;
+  Status S;
+
+public:
+  FileWithFixedStatus(std::unique_ptr<File> InnerFile, Status S)
+      : InnerFile(std::move(InnerFile)), S(std::move(S)) {}
+
+  ErrorOr<Status> status() override { return S; }
+  ErrorOr<std::unique_ptr<llvm::MemoryBuffer>>
+
+  getBuffer(const Twine &Name, int64_t FileSize, bool RequiresNullTerminator,
+            bool IsVolatile) override {
+    return InnerFile->getBuffer(Name, FileSize, RequiresNullTerminator,
+                                IsVolatile);
+  }
+
+  std::error_code close() override { return InnerFile->close(); }
+};
+
+} // namespace
+
+ErrorOr<std::unique_ptr<File>>
+RedirectingFileSystem::openFileForRead(const Twine &Path) {
+  ErrorOr<RedirectingFileSystem::Entry *> E = lookupPath(Path);
+  if (!E) {
+    if (IsFallthrough &&
+        E.getError() == llvm::errc::no_such_file_or_directory) {
+      return ExternalFS->openFileForRead(Path);
+    }
+    return E.getError();
+  }
+
+  auto *F = dyn_cast<RedirectingFileSystem::RedirectingFileEntry>(*E);
+  if (!F) // FIXME: errc::not_a_file?
+    return make_error_code(llvm::errc::invalid_argument);
+
+  auto Result = ExternalFS->openFileForRead(F->getExternalContentsPath());
+  if (!Result)
+    return Result;
+
+  auto ExternalStatus = (*Result)->status();
+  if (!ExternalStatus)
+    return ExternalStatus.getError();
+
+  // FIXME: Update the status with the name and VFSMapped.
+  Status S = getRedirectedFileStatus(Path, F->useExternalName(UseExternalNames),
+                                     *ExternalStatus);
+  return std::unique_ptr<File>(
+      llvm::make_unique<FileWithFixedStatus>(std::move(*Result), S));
+}
+
+std::error_code
+RedirectingFileSystem::getRealPath(const Twine &Path,
+                                   SmallVectorImpl<char> &Output) const {
+  ErrorOr<RedirectingFileSystem::Entry *> Result = lookupPath(Path);
+  if (!Result) {
+    if (IsFallthrough &&
+        Result.getError() == llvm::errc::no_such_file_or_directory) {
+      return ExternalFS->getRealPath(Path, Output);
+    }
+    return Result.getError();
+  }
+
+  if (auto *F =
+          dyn_cast<RedirectingFileSystem::RedirectingFileEntry>(*Result)) {
+    return ExternalFS->getRealPath(F->getExternalContentsPath(), Output);
+  }
+  // Even if there is a directory entry, fall back to ExternalFS if allowed,
+  // because directories don't have a single external contents path.
+  return IsFallthrough ? ExternalFS->getRealPath(Path, Output)
+                       : llvm::errc::invalid_argument;
+}
+
+IntrusiveRefCntPtr<FileSystem>
+vfs::getVFSFromYAML(std::unique_ptr<MemoryBuffer> Buffer,
+                    SourceMgr::DiagHandlerTy DiagHandler,
+                    StringRef YAMLFilePath, void *DiagContext,
+                    IntrusiveRefCntPtr<FileSystem> ExternalFS) {
+  return RedirectingFileSystem::create(std::move(Buffer), DiagHandler,
+                                       YAMLFilePath, DiagContext,
+                                       std::move(ExternalFS));
+}
+
+static void getVFSEntries(RedirectingFileSystem::Entry *SrcE,
+                          SmallVectorImpl<StringRef> &Path,
+                          SmallVectorImpl<YAMLVFSEntry> &Entries) {
+  auto Kind = SrcE->getKind();
+  if (Kind == RedirectingFileSystem::EK_Directory) {
+    auto *DE = dyn_cast<RedirectingFileSystem::RedirectingDirectoryEntry>(SrcE);
+    assert(DE && "Must be a directory");
+    for (std::unique_ptr<RedirectingFileSystem::Entry> &SubEntry :
+         llvm::make_range(DE->contents_begin(), DE->contents_end())) {
+      Path.push_back(SubEntry->getName());
+      getVFSEntries(SubEntry.get(), Path, Entries);
+      Path.pop_back();
+    }
+    return;
+  }
+
+  assert(Kind == RedirectingFileSystem::EK_File && "Must be a EK_File");
+  auto *FE = dyn_cast<RedirectingFileSystem::RedirectingFileEntry>(SrcE);
+  assert(FE && "Must be a file");
+  SmallString<128> VPath;
+  for (auto &Comp : Path)
+    llvm::sys::path::append(VPath, Comp);
+  Entries.push_back(YAMLVFSEntry(VPath.c_str(), FE->getExternalContentsPath()));
+}
+
+void vfs::collectVFSFromYAML(std::unique_ptr<MemoryBuffer> Buffer,
+                             SourceMgr::DiagHandlerTy DiagHandler,
+                             StringRef YAMLFilePath,
+                             SmallVectorImpl<YAMLVFSEntry> &CollectedEntries,
+                             void *DiagContext,
+                             IntrusiveRefCntPtr<FileSystem> ExternalFS) {
+  RedirectingFileSystem *VFS = RedirectingFileSystem::create(
+      std::move(Buffer), DiagHandler, YAMLFilePath, DiagContext,
+      std::move(ExternalFS));
+  ErrorOr<RedirectingFileSystem::Entry *> RootE = VFS->lookupPath("/");
+  if (!RootE)
+    return;
+  SmallVector<StringRef, 8> Components;
+  Components.push_back("/");
+  getVFSEntries(*RootE, Components, CollectedEntries);
+}
+
+UniqueID vfs::getNextVirtualUniqueID() {
+  static std::atomic<unsigned> UID;
+  unsigned ID = ++UID;
+  // The following assumes that uint64_t max will never collide with a real
+  // dev_t value from the OS.
+  return UniqueID(std::numeric_limits<uint64_t>::max(), ID);
+}
+
+void YAMLVFSWriter::addFileMapping(StringRef VirtualPath, StringRef RealPath) {
+  assert(sys::path::is_absolute(VirtualPath) && "virtual path not absolute");
+  assert(sys::path::is_absolute(RealPath) && "real path not absolute");
+  assert(!pathHasTraversal(VirtualPath) && "path traversal is not supported");
+  Mappings.emplace_back(VirtualPath, RealPath);
+}
+
+namespace {
+
+class JSONWriter {
+  llvm::raw_ostream &OS;
+  SmallVector<StringRef, 16> DirStack;
+
+  unsigned getDirIndent() { return 4 * DirStack.size(); }
+  unsigned getFileIndent() { return 4 * (DirStack.size() + 1); }
+  bool containedIn(StringRef Parent, StringRef Path);
+  StringRef containedPart(StringRef Parent, StringRef Path);
+  void startDirectory(StringRef Path);
+  void endDirectory();
+  void writeEntry(StringRef VPath, StringRef RPath);
+
+public:
+  JSONWriter(llvm::raw_ostream &OS) : OS(OS) {}
+
+  void write(ArrayRef<YAMLVFSEntry> Entries, Optional<bool> UseExternalNames,
+             Optional<bool> IsCaseSensitive, Optional<bool> IsOverlayRelative,
+             StringRef OverlayDir);
+};
+
+} // namespace
+
+bool JSONWriter::containedIn(StringRef Parent, StringRef Path) {
+  using namespace llvm::sys;
+
+  // Compare each path component.
+  auto IParent = path::begin(Parent), EParent = path::end(Parent);
+  for (auto IChild = path::begin(Path), EChild = path::end(Path);
+       IParent != EParent && IChild != EChild; ++IParent, ++IChild) {
+    if (*IParent != *IChild)
+      return false;
+  }
+  // Have we exhausted the parent path?
+  return IParent == EParent;
+}
+
+StringRef JSONWriter::containedPart(StringRef Parent, StringRef Path) {
+  assert(!Parent.empty());
+  assert(containedIn(Parent, Path));
+  return Path.slice(Parent.size() + 1, StringRef::npos);
+}
+
+void JSONWriter::startDirectory(StringRef Path) {
+  StringRef Name =
+      DirStack.empty() ? Path : containedPart(DirStack.back(), Path);
+  DirStack.push_back(Path);
+  unsigned Indent = getDirIndent();
+  OS.indent(Indent) << "{\n";
+  OS.indent(Indent + 2) << "'type': 'directory',\n";
+  OS.indent(Indent + 2) << "'name': \"" << llvm::yaml::escape(Name) << "\",\n";
+  OS.indent(Indent + 2) << "'contents': [\n";
+}
+
+void JSONWriter::endDirectory() {
+  unsigned Indent = getDirIndent();
+  OS.indent(Indent + 2) << "]\n";
+  OS.indent(Indent) << "}";
+
+  DirStack.pop_back();
+}
+
+void JSONWriter::writeEntry(StringRef VPath, StringRef RPath) {
+  unsigned Indent = getFileIndent();
+  OS.indent(Indent) << "{\n";
+  OS.indent(Indent + 2) << "'type': 'file',\n";
+  OS.indent(Indent + 2) << "'name': \"" << llvm::yaml::escape(VPath) << "\",\n";
+  OS.indent(Indent + 2) << "'external-contents': \""
+                        << llvm::yaml::escape(RPath) << "\"\n";
+  OS.indent(Indent) << "}";
+}
+
+void JSONWriter::write(ArrayRef<YAMLVFSEntry> Entries,
+                       Optional<bool> UseExternalNames,
+                       Optional<bool> IsCaseSensitive,
+                       Optional<bool> IsOverlayRelative,
+                       StringRef OverlayDir) {
+  using namespace llvm::sys;
+
+  OS << "{\n"
+        "  'version': 0,\n";
+  if (IsCaseSensitive.hasValue())
+    OS << "  'case-sensitive': '"
+       << (IsCaseSensitive.getValue() ? "true" : "false") << "',\n";
+  if (UseExternalNames.hasValue())
+    OS << "  'use-external-names': '"
+       << (UseExternalNames.getValue() ? "true" : "false") << "',\n";
+  bool UseOverlayRelative = false;
+  if (IsOverlayRelative.hasValue()) {
+    UseOverlayRelative = IsOverlayRelative.getValue();
+    OS << "  'overlay-relative': '" << (UseOverlayRelative ? "true" : "false")
+       << "',\n";
+  }
+  OS << "  'roots': [\n";
+
+  if (!Entries.empty()) {
+    const YAMLVFSEntry &Entry = Entries.front();
+    startDirectory(path::parent_path(Entry.VPath));
+
+    StringRef RPath = Entry.RPath;
+    if (UseOverlayRelative) {
+      unsigned OverlayDirLen = OverlayDir.size();
+      assert(RPath.substr(0, OverlayDirLen) == OverlayDir &&
+             "Overlay dir must be contained in RPath");
+      RPath = RPath.slice(OverlayDirLen, RPath.size());
+    }
+
+    writeEntry(path::filename(Entry.VPath), RPath);
+
+    for (const auto &Entry : Entries.slice(1)) {
+      StringRef Dir = path::parent_path(Entry.VPath);
+      if (Dir == DirStack.back())
+        OS << ",\n";
+      else {
+        while (!DirStack.empty() && !containedIn(DirStack.back(), Dir)) {
+          OS << "\n";
+          endDirectory();
+        }
+        OS << ",\n";
+        startDirectory(Dir);
+      }
+      StringRef RPath = Entry.RPath;
+      if (UseOverlayRelative) {
+        unsigned OverlayDirLen = OverlayDir.size();
+        assert(RPath.substr(0, OverlayDirLen) == OverlayDir &&
+               "Overlay dir must be contained in RPath");
+        RPath = RPath.slice(OverlayDirLen, RPath.size());
+      }
+      writeEntry(path::filename(Entry.VPath), RPath);
+    }
+
+    while (!DirStack.empty()) {
+      OS << "\n";
+      endDirectory();
+    }
+    OS << "\n";
+  }
+
+  OS << "  ]\n"
+     << "}\n";
+}
+
+void YAMLVFSWriter::write(llvm::raw_ostream &OS) {
+  llvm::sort(Mappings, [](const YAMLVFSEntry &LHS, const YAMLVFSEntry &RHS) {
+    return LHS.VPath < RHS.VPath;
+  });
+
+  JSONWriter(OS).write(Mappings, UseExternalNames, IsCaseSensitive,
+                       IsOverlayRelative, OverlayDir);
+}
+
+VFSFromYamlDirIterImpl::VFSFromYamlDirIterImpl(
+    const Twine &_Path,
+    RedirectingFileSystem::RedirectingDirectoryEntry::iterator Begin,
+    RedirectingFileSystem::RedirectingDirectoryEntry::iterator End,
+    bool IterateExternalFS, FileSystem &ExternalFS, std::error_code &EC)
+    : Dir(_Path.str()), Current(Begin), End(End),
+      IterateExternalFS(IterateExternalFS), ExternalFS(ExternalFS) {
+  EC = incrementImpl(/*IsFirstTime=*/true);
+}
+
+std::error_code VFSFromYamlDirIterImpl::increment() {
+  return incrementImpl(/*IsFirstTime=*/false);
+}
+
+std::error_code VFSFromYamlDirIterImpl::incrementExternal() {
+  assert(!(IsExternalFSCurrent && ExternalDirIter == directory_iterator()) &&
+         "incrementing past end");
+  std::error_code EC;
+  if (IsExternalFSCurrent) {
+    ExternalDirIter.increment(EC);
+  } else if (IterateExternalFS) {
+    ExternalDirIter = ExternalFS.dir_begin(Dir, EC);
+    IsExternalFSCurrent = true;
+    if (EC && EC != errc::no_such_file_or_directory)
+      return EC;
+    EC = {};
+  }
+  if (EC || ExternalDirIter == directory_iterator()) {
+    CurrentEntry = directory_entry();
+  } else {
+    CurrentEntry = *ExternalDirIter;
+  }
+  return EC;
+}
+
+std::error_code VFSFromYamlDirIterImpl::incrementContent(bool IsFirstTime) {
+  assert((IsFirstTime || Current != End) && "cannot iterate past end");
+  if (!IsFirstTime)
+    ++Current;
+  while (Current != End) {
+    SmallString<128> PathStr(Dir);
+    llvm::sys::path::append(PathStr, (*Current)->getName());
+    sys::fs::file_type Type;
+    switch ((*Current)->getKind()) {
+    case RedirectingFileSystem::EK_Directory:
+      Type = sys::fs::file_type::directory_file;
+      break;
+    case RedirectingFileSystem::EK_File:
+      Type = sys::fs::file_type::regular_file;
+      break;
+    }
+    CurrentEntry = directory_entry(PathStr.str(), Type);
+    return {};
+  }
+  return incrementExternal();
+}
+
+std::error_code VFSFromYamlDirIterImpl::incrementImpl(bool IsFirstTime) {
+  while (true) {
+    std::error_code EC = IsExternalFSCurrent ? incrementExternal()
+                                             : incrementContent(IsFirstTime);
+    if (EC || CurrentEntry.path().empty())
+      return EC;
+    StringRef Name = llvm::sys::path::filename(CurrentEntry.path());
+    if (SeenNames.insert(Name).second)
+      return EC; // name not seen before
+  }
+  llvm_unreachable("returned above");
+}
+
+vfs::recursive_directory_iterator::recursive_directory_iterator(
+    FileSystem &FS_, const Twine &Path, std::error_code &EC)
+    : FS(&FS_) {
+  directory_iterator I = FS->dir_begin(Path, EC);
+  if (I != directory_iterator()) {
+    State = std::make_shared<detail::RecDirIterState>();
+    State->Stack.push(I);
+  }
+}
+
+vfs::recursive_directory_iterator &
+recursive_directory_iterator::increment(std::error_code &EC) {
+  assert(FS && State && !State->Stack.empty() && "incrementing past end");
+  assert(!State->Stack.top()->path().empty() && "non-canonical end iterator");
+  vfs::directory_iterator End;
+
+  if (State->HasNoPushRequest)
+    State->HasNoPushRequest = false;
+  else {
+    if (State->Stack.top()->type() == sys::fs::file_type::directory_file) {
+      vfs::directory_iterator I = FS->dir_begin(State->Stack.top()->path(), EC);
+      if (I != End) {
+        State->Stack.push(I);
+        return *this;
+      }
+    }
+  }
+
+  while (!State->Stack.empty() && State->Stack.top().increment(EC) == End)
+    State->Stack.pop();
+
+  if (State->Stack.empty())
+    State.reset(); // end iterator
+
+  return *this;
+}