text: //===- VirtualFileSystem.h - Virtual File System Layer ----------*- C++ -*-===//
  //
  // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
  // See https://llvm.org/LICENSE.txt for license information.
  // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
  //
  //===----------------------------------------------------------------------===//
  //
  /// \file
  /// Defines the virtual file system interface vfs::FileSystem.
  //
  //===----------------------------------------------------------------------===//
  
  #ifndef LLVM_SUPPORT_VIRTUALFILESYSTEM_H
  #define LLVM_SUPPORT_VIRTUALFILESYSTEM_H
  
  #include "llvm/ADT/IntrusiveRefCntPtr.h"
  #include "llvm/ADT/None.h"
  #include "llvm/ADT/Optional.h"
  #include "llvm/ADT/SmallVector.h"
  #include "llvm/ADT/StringRef.h"
  #include "llvm/ADT/Twine.h"
  #include "llvm/Support/Chrono.h"
  #include "llvm/Support/ErrorOr.h"
  #include "llvm/Support/FileSystem.h"
  #include "llvm/Support/Path.h"
  #include "llvm/Support/SourceMgr.h"
  #include <cassert>
  #include <cstdint>
  #include <ctime>
  #include <memory>
  #include <stack>
  #include <string>
  #include <system_error>
  #include <utility>
  #include <vector>
  
  namespace llvm {
  
  class MemoryBuffer;
  
  namespace vfs {
  
  /// The result of a \p status operation.
  class Status {
    std::string Name;
    llvm::sys::fs::UniqueID UID;
    llvm::sys::TimePoint<> MTime;
    uint32_t User;
    uint32_t Group;
    uint64_t Size;
    llvm::sys::fs::file_type Type = llvm::sys::fs::file_type::status_error;
    llvm::sys::fs::perms Perms;
  
  public:
    // FIXME: remove when files support multiple names
    bool IsVFSMapped = false;
  
    Status() = default;
    Status(const llvm::sys::fs::file_status &Status);
    Status(const Twine &Name, llvm::sys::fs::UniqueID UID,
           llvm::sys::TimePoint<> MTime, uint32_t User, uint32_t Group,
           uint64_t Size, llvm::sys::fs::file_type Type,
           llvm::sys::fs::perms Perms);
  
    /// Get a copy of a Status with a different name.
    static Status copyWithNewName(const Status &In, const Twine &NewName);
    static Status copyWithNewName(const llvm::sys::fs::file_status &In,
                                  const Twine &NewName);
  
    /// Returns the name that should be used for this file or directory.
    StringRef getName() const { return Name; }
  
    /// @name Status interface from llvm::sys::fs
    /// @{
    llvm::sys::fs::file_type getType() const { return Type; }
    llvm::sys::fs::perms getPermissions() const { return Perms; }
    llvm::sys::TimePoint<> getLastModificationTime() const { return MTime; }
    llvm::sys::fs::UniqueID getUniqueID() const { return UID; }
    uint32_t getUser() const { return User; }
    uint32_t getGroup() const { return Group; }
    uint64_t getSize() const { return Size; }
    /// @}
    /// @name Status queries
    /// These are static queries in llvm::sys::fs.
    /// @{
    bool equivalent(const Status &Other) const;
    bool isDirectory() const;
    bool isRegularFile() const;
    bool isOther() const;
    bool isSymlink() const;
    bool isStatusKnown() const;
    bool exists() const;
    /// @}
  };
  
  /// Represents an open file.
  class File {
  public:
    /// Destroy the file after closing it (if open).
    /// Sub-classes should generally call close() inside their destructors.  We
    /// cannot do that from the base class, since close is virtual.
    virtual ~File();
  
    /// Get the status of the file.
    virtual llvm::ErrorOr<Status> status() = 0;
  
    /// Get the name of the file
    virtual llvm::ErrorOr<std::string> getName() {
      if (auto Status = status())
        return Status->getName().str();
      else
        return Status.getError();
    }
  
    /// Get the contents of the file as a \p MemoryBuffer.
    virtual llvm::ErrorOr<std::unique_ptr<llvm::MemoryBuffer>>
    getBuffer(const Twine &Name, int64_t FileSize = -1,
              bool RequiresNullTerminator = true, bool IsVolatile = false) = 0;
  
    /// Closes the file.
    virtual std::error_code close() = 0;
  };
  
  /// A member of a directory, yielded by a directory_iterator.
  /// Only information available on most platforms is included.
  class directory_entry {
    std::string Path;
    llvm::sys::fs::file_type Type = llvm::sys::fs::file_type::type_unknown;
  
  public:
    directory_entry() = default;
    directory_entry(std::string Path, llvm::sys::fs::file_type Type)
        : Path(std::move(Path)), Type(Type) {}
  
    llvm::StringRef path() const { return Path; }
    llvm::sys::fs::file_type type() const { return Type; }
  };
  
  namespace detail {
  
  /// An interface for virtual file systems to provide an iterator over the
  /// (non-recursive) contents of a directory.
  struct DirIterImpl {
    virtual ~DirIterImpl();
  
    /// Sets \c CurrentEntry to the next entry in the directory on success,
    /// to directory_entry() at end,  or returns a system-defined \c error_code.
    virtual std::error_code increment() = 0;
  
    directory_entry CurrentEntry;
  };
  
  } // namespace detail
  
  /// An input iterator over the entries in a virtual path, similar to
  /// llvm::sys::fs::directory_iterator.
  class directory_iterator {
    std::shared_ptr<detail::DirIterImpl> Impl; // Input iterator semantics on copy
  
  public:
    directory_iterator(std::shared_ptr<detail::DirIterImpl> I)
        : Impl(std::move(I)) {
      assert(Impl.get() != nullptr && "requires non-null implementation");
      if (Impl->CurrentEntry.path().empty())
        Impl.reset(); // Normalize the end iterator to Impl == nullptr.
    }
  
    /// Construct an 'end' iterator.
    directory_iterator() = default;
  
    /// Equivalent to operator++, with an error code.
    directory_iterator &increment(std::error_code &EC) {
      assert(Impl && "attempting to increment past end");
      EC = Impl->increment();
      if (Impl->CurrentEntry.path().empty())
        Impl.reset(); // Normalize the end iterator to Impl == nullptr.
      return *this;
    }
  
    const directory_entry &operator*() const { return Impl->CurrentEntry; }
    const directory_entry *operator->() const { return &Impl->CurrentEntry; }
  
    bool operator==(const directory_iterator &RHS) const {
      if (Impl && RHS.Impl)
        return Impl->CurrentEntry.path() == RHS.Impl->CurrentEntry.path();
      return !Impl && !RHS.Impl;
    }
    bool operator!=(const directory_iterator &RHS) const {
      return !(*this == RHS);
    }
  };
  
  class FileSystem;
  
  namespace detail {
  
  /// Keeps state for the recursive_directory_iterator.
  struct RecDirIterState {
    std::stack<directory_iterator, std::vector<directory_iterator>> Stack;
    bool HasNoPushRequest = false;
  };
  
  } // end namespace detail
  
  /// An input iterator over the recursive contents of a virtual path,
  /// similar to llvm::sys::fs::recursive_directory_iterator.
  class recursive_directory_iterator {
    FileSystem *FS;
    std::shared_ptr<detail::RecDirIterState>
        State; // Input iterator semantics on copy.
  
  public:
    recursive_directory_iterator(FileSystem &FS, const Twine &Path,
                                 std::error_code &EC);
  
    /// Construct an 'end' iterator.
    recursive_directory_iterator() = default;
  
    /// Equivalent to operator++, with an error code.
    recursive_directory_iterator &increment(std::error_code &EC);
  
    const directory_entry &operator*() const { return *State->Stack.top(); }
    const directory_entry *operator->() const { return &*State->Stack.top(); }
  
    bool operator==(const recursive_directory_iterator &Other) const {
      return State == Other.State; // identity
    }
    bool operator!=(const recursive_directory_iterator &RHS) const {
      return !(*this == RHS);
    }
  
    /// Gets the current level. Starting path is at level 0.
    int level() const {
      assert(!State->Stack.empty() &&
             "Cannot get level without any iteration state");
      return State->Stack.size() - 1;
    }
  
    void no_push() { State->HasNoPushRequest = true; }
  };
  
  /// The virtual file system interface.
  class FileSystem : public llvm::ThreadSafeRefCountedBase<FileSystem> {
  public:
    virtual ~FileSystem();
  
    /// Get the status of the entry at \p Path, if one exists.
    virtual llvm::ErrorOr<Status> status(const Twine &Path) = 0;
  
    /// Get a \p File object for the file at \p Path, if one exists.
    virtual llvm::ErrorOr<std::unique_ptr<File>>
    openFileForRead(const Twine &Path) = 0;
  
    /// This is a convenience method that opens a file, gets its content and then
    /// closes the file.
    llvm::ErrorOr<std::unique_ptr<llvm::MemoryBuffer>>
    getBufferForFile(const Twine &Name, int64_t FileSize = -1,
                     bool RequiresNullTerminator = true, bool IsVolatile = false);
  
    /// Get a directory_iterator for \p Dir.
    /// \note The 'end' iterator is directory_iterator().
    virtual directory_iterator dir_begin(const Twine &Dir,
                                         std::error_code &EC) = 0;
  
    /// Set the working directory. This will affect all following operations on
    /// this file system and may propagate down for nested file systems.
    virtual std::error_code setCurrentWorkingDirectory(const Twine &Path) = 0;
  
    /// Get the working directory of this file system.
    virtual llvm::ErrorOr<std::string> getCurrentWorkingDirectory() const = 0;
  
    /// Gets real path of \p Path e.g. collapse all . and .. patterns, resolve
    /// symlinks. For real file system, this uses `llvm::sys::fs::real_path`.
    /// This returns errc::operation_not_permitted if not implemented by subclass.
    virtual std::error_code getRealPath(const Twine &Path,
                                        SmallVectorImpl<char> &Output) const;
  
    /// Check whether a file exists. Provided for convenience.
    bool exists(const Twine &Path);
  
    /// Is the file mounted on a local filesystem?
    virtual std::error_code isLocal(const Twine &Path, bool &Result);
  
    /// Make \a Path an absolute path.
    ///
    /// Makes \a Path absolute using the current directory if it is not already.
    /// An empty \a Path will result in the current directory.
    ///
    /// /absolute/path   => /absolute/path
    /// relative/../path => <current-directory>/relative/../path
    ///
    /// \param Path A path that is modified to be an absolute path.
    /// \returns success if \a path has been made absolute, otherwise a
    ///          platform-specific error_code.
    std::error_code makeAbsolute(SmallVectorImpl<char> &Path) const;
  };
  
  /// Gets an \p vfs::FileSystem for the 'real' file system, as seen by
  /// the operating system.
  /// The working directory is linked to the process's working directory.
  /// (This is usually thread-hostile).
  IntrusiveRefCntPtr<FileSystem> getRealFileSystem();
  
  /// Create an \p vfs::FileSystem for the 'real' file system, as seen by
  /// the operating system.
  /// It has its own working directory, independent of (but initially equal to)
  /// that of the process.
  std::unique_ptr<FileSystem> createPhysicalFileSystem();
  
  /// A file system that allows overlaying one \p AbstractFileSystem on top
  /// of another.
  ///
  /// Consists of a stack of >=1 \p FileSystem objects, which are treated as being
  /// one merged file system. When there is a directory that exists in more than
  /// one file system, the \p OverlayFileSystem contains a directory containing
  /// the union of their contents.  The attributes (permissions, etc.) of the
  /// top-most (most recently added) directory are used.  When there is a file
  /// that exists in more than one file system, the file in the top-most file
  /// system overrides the other(s).
  class OverlayFileSystem : public FileSystem {
    using FileSystemList = SmallVector<IntrusiveRefCntPtr<FileSystem>, 1>;
  
    /// The stack of file systems, implemented as a list in order of
    /// their addition.
    FileSystemList FSList;
  
  public:
    OverlayFileSystem(IntrusiveRefCntPtr<FileSystem> Base);
  
    /// Pushes a file system on top of the stack.
    void pushOverlay(IntrusiveRefCntPtr<FileSystem> FS);
  
    llvm::ErrorOr<Status> status(const Twine &Path) override;
    llvm::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;
  
    using iterator = FileSystemList::reverse_iterator;
    using const_iterator = FileSystemList::const_reverse_iterator;
    using reverse_iterator = FileSystemList::iterator;
    using const_reverse_iterator = FileSystemList::const_iterator;
  
    /// Get an iterator pointing to the most recently added file system.
    iterator overlays_begin() { return FSList.rbegin(); }
    const_iterator overlays_begin() const { return FSList.rbegin(); }
  
    /// Get an iterator pointing one-past the least recently added file system.
    iterator overlays_end() { return FSList.rend(); }
    const_iterator overlays_end() const { return FSList.rend(); }
  
    /// Get an iterator pointing to the least recently added file system.
    reverse_iterator overlays_rbegin() { return FSList.begin(); }
    const_reverse_iterator overlays_rbegin() const { return FSList.begin(); }
  
    /// Get an iterator pointing one-past the most recently added file system.
    reverse_iterator overlays_rend() { return FSList.end(); }
    const_reverse_iterator overlays_rend() const { return FSList.end(); }
  };
  
  /// By default, this delegates all calls to the underlying file system. This
  /// is useful when derived file systems want to override some calls and still
  /// proxy other calls.
  class ProxyFileSystem : public FileSystem {
  public:
    explicit ProxyFileSystem(IntrusiveRefCntPtr<FileSystem> FS)
        : FS(std::move(FS)) {}
  
    llvm::ErrorOr<Status> status(const Twine &Path) override {
      return FS->status(Path);
    }
    llvm::ErrorOr<std::unique_ptr<File>>
    openFileForRead(const Twine &Path) override {
      return FS->openFileForRead(Path);
    }
    directory_iterator dir_begin(const Twine &Dir, std::error_code &EC) override {
      return FS->dir_begin(Dir, EC);
    }
    llvm::ErrorOr<std::string> getCurrentWorkingDirectory() const override {
      return FS->getCurrentWorkingDirectory();
    }
    std::error_code setCurrentWorkingDirectory(const Twine &Path) override {
      return FS->setCurrentWorkingDirectory(Path);
    }
    std::error_code getRealPath(const Twine &Path,
                                SmallVectorImpl<char> &Output) const override {
      return FS->getRealPath(Path, Output);
    }
    std::error_code isLocal(const Twine &Path, bool &Result) override {
      return FS->isLocal(Path, Result);
    }
  
  protected:
    FileSystem &getUnderlyingFS() { return *FS; }
  
  private:
    IntrusiveRefCntPtr<FileSystem> FS;
  
    virtual void anchor();
  };
  
  namespace detail {
  
  class InMemoryDirectory;
  class InMemoryFile;
  
  } // namespace detail
  
  /// An in-memory file system.
  class InMemoryFileSystem : public FileSystem {
    std::unique_ptr<detail::InMemoryDirectory> Root;
    std::string WorkingDirectory;
    bool UseNormalizedPaths = true;
  
    /// If HardLinkTarget is non-null, a hardlink is created to the To path which
    /// must be a file. If it is null then it adds the file as the public addFile.
    bool addFile(const Twine &Path, 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);
  
  public:
    explicit InMemoryFileSystem(bool UseNormalizedPaths = true);
    ~InMemoryFileSystem() override;
  
    /// Add a file containing a buffer or a directory to the VFS with a
    /// path. The VFS owns the buffer.  If present, User, Group, Type
    /// and Perms apply to the newly-created file or directory.
    /// \return true if the file or directory was successfully added,
    /// false if the file or directory already exists in the file system with
    /// different contents.
    bool addFile(const Twine &Path, time_t ModificationTime,
                 std::unique_ptr<llvm::MemoryBuffer> Buffer,
                 Optional<uint32_t> User = None, Optional<uint32_t> Group = None,
                 Optional<llvm::sys::fs::file_type> Type = None,
                 Optional<llvm::sys::fs::perms> Perms = None);
  
    /// Add a hard link to a file.
    /// Here hard links are not intended to be fully equivalent to the classical
    /// filesystem. Both the hard link and the file share the same buffer and
    /// status (and thus have the same UniqueID). Because of this there is no way
    /// to distinguish between the link and the file after the link has been
    /// added.
    ///
    /// The To path must be an existing file or a hardlink. The From file must not
    /// have been added before. The To Path must not be a directory. The From Node
    /// is added as a hard link which points to the resolved file of To Node.
    /// \return true if the above condition is satisfied and hardlink was
    /// successfully created, false otherwise.
    bool addHardLink(const Twine &From, const Twine &To);
  
    /// Add a buffer to the VFS with a path. The VFS does not own the buffer.
    /// If present, User, Group, Type and Perms apply to the newly-created file
    /// or directory.
    /// \return true if the file or directory was successfully added,
    /// false if the file or directory already exists in the file system with
    /// different contents.
    bool addFileNoOwn(const Twine &Path, time_t ModificationTime,
                      llvm::MemoryBuffer *Buffer, Optional<uint32_t> User = None,
                      Optional<uint32_t> Group = None,
                      Optional<llvm::sys::fs::file_type> Type = None,
                      Optional<llvm::sys::fs::perms> Perms = None);
  
    std::string toString() const;
  
    /// Return true if this file system normalizes . and .. in paths.
    bool useNormalizedPaths() const { return UseNormalizedPaths; }
  
    llvm::ErrorOr<Status> status(const Twine &Path) override;
    llvm::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 {
      return WorkingDirectory;
    }
    /// Canonicalizes \p Path by combining with the current working
    /// directory and normalizing the path (e.g. remove dots). If the current
    /// working directory is not set, this returns errc::operation_not_permitted.
    ///
    /// This doesn't resolve symlinks as they are not supported in in-memory file
    /// system.
    std::error_code getRealPath(const Twine &Path,
                                SmallVectorImpl<char> &Output) const override;
    std::error_code isLocal(const Twine &Path, bool &Result) override;
    std::error_code setCurrentWorkingDirectory(const Twine &Path) override;
  };
  
  /// Get a globally unique ID for a virtual file or directory.
  llvm::sys::fs::UniqueID getNextVirtualUniqueID();
  
  /// Gets a \p FileSystem for a virtual file system described in YAML
  /// format.
  IntrusiveRefCntPtr<FileSystem>
  getVFSFromYAML(std::unique_ptr<llvm::MemoryBuffer> Buffer,
                 llvm::SourceMgr::DiagHandlerTy DiagHandler,
                 StringRef YAMLFilePath, void *DiagContext = nullptr,
                 IntrusiveRefCntPtr<FileSystem> ExternalFS = getRealFileSystem());
  
  struct YAMLVFSEntry {
    template <typename T1, typename T2>
    YAMLVFSEntry(T1 &&VPath, T2 &&RPath)
        : VPath(std::forward<T1>(VPath)), RPath(std::forward<T2>(RPath)) {}
    std::string VPath;
    std::string RPath;
  };
  
  class VFSFromYamlDirIterImpl;
  class RedirectingFileSystemParser;
  
  /// A virtual file system parsed from a YAML file.
  ///
  /// Currently, this class allows creating virtual directories and mapping
  /// virtual file paths to existing external files, available in \c ExternalFS.
  ///
  /// The basic structure of the parsed file is:
  /// \verbatim
  /// {
  ///   'version': <version number>,
  ///   <optional configuration>
  ///   'roots': [
  ///              <directory entries>
  ///            ]
  /// }
  /// \endverbatim
  ///
  /// All configuration options are optional.
  ///   'case-sensitive': <boolean, default=true>
  ///   'use-external-names': <boolean, default=true>
  ///   'overlay-relative': <boolean, default=false>
  ///   'fallthrough': <boolean, default=true>
  ///
  /// Virtual directories are represented as
  /// \verbatim
  /// {
  ///   'type': 'directory',
  ///   'name': <string>,
  ///   'contents': [ <file or directory entries> ]
  /// }
  /// \endverbatim
  ///
  /// The default attributes for virtual directories are:
  /// \verbatim
  /// MTime = now() when created
  /// Perms = 0777
  /// User = Group = 0
  /// Size = 0
  /// UniqueID = unspecified unique value
  /// \endverbatim
  ///
  /// Re-mapped files are represented as
  /// \verbatim
  /// {
  ///   'type': 'file',
  ///   'name': <string>,
  ///   'use-external-name': <boolean> # Optional
  ///   'external-contents': <path to external file>
  /// }
  /// \endverbatim
  ///
  /// and inherit their attributes from the external contents.
  ///
  /// In both cases, the 'name' field may contain multiple path components (e.g.
  /// /path/to/file). However, any directory that contains more than one child
  /// must be uniquely represented by a directory entry.
  class RedirectingFileSystem : public vfs::FileSystem {
  public:
    enum EntryKind { EK_Directory, EK_File };
  
    /// A single file or directory in the VFS.
    class Entry {
      EntryKind Kind;
      std::string Name;
  
    public:
      Entry(EntryKind K, StringRef Name) : Kind(K), Name(Name) {}
      virtual ~Entry() = default;
  
      StringRef getName() const { return Name; }
      EntryKind getKind() const { return Kind; }
    };
  
    class RedirectingDirectoryEntry : public Entry {
      std::vector<std::unique_ptr<Entry>> Contents;
      Status S;
  
    public:
      RedirectingDirectoryEntry(StringRef Name,
                                std::vector<std::unique_ptr<Entry>> Contents,
                                Status S)
          : Entry(EK_Directory, Name), Contents(std::move(Contents)),
            S(std::move(S)) {}
      RedirectingDirectoryEntry(StringRef Name, Status S)
          : Entry(EK_Directory, Name), S(std::move(S)) {}
  
      Status getStatus() { return S; }
  
      void addContent(std::unique_ptr<Entry> Content) {
        Contents.push_back(std::move(Content));
      }
  
      Entry *getLastContent() const { return Contents.back().get(); }
  
      using iterator = decltype(Contents)::iterator;
  
      iterator contents_begin() { return Contents.begin(); }
      iterator contents_end() { return Contents.end(); }
  
      static bool classof(const Entry *E) { return E->getKind() == EK_Directory; }
    };
  
    class RedirectingFileEntry : public Entry {
    public:
      enum NameKind { NK_NotSet, NK_External, NK_Virtual };
  
    private:
      std::string ExternalContentsPath;
      NameKind UseName;
  
    public:
      RedirectingFileEntry(StringRef Name, StringRef ExternalContentsPath,
                           NameKind UseName)
          : Entry(EK_File, Name), ExternalContentsPath(ExternalContentsPath),
            UseName(UseName) {}
  
      StringRef getExternalContentsPath() const { return ExternalContentsPath; }
  
      /// whether to use the external path as the name for this file.
      bool useExternalName(bool GlobalUseExternalName) const {
        return UseName == NK_NotSet ? GlobalUseExternalName
                                    : (UseName == NK_External);
      }
  
      NameKind getUseName() const { return UseName; }
  
      static bool classof(const Entry *E) { return E->getKind() == EK_File; }
    };
  
  private:
    friend class VFSFromYamlDirIterImpl;
    friend class RedirectingFileSystemParser;
  
    bool shouldUseExternalFS() const {
      return ExternalFSValidWD && IsFallthrough;
    }
  
    /// The root(s) of the virtual file system.
    std::vector<std::unique_ptr<Entry>> Roots;
  
    /// The current working directory of the file system.
    std::string WorkingDirectory;
  
    /// Whether the current working directory is valid for the external FS.
    bool ExternalFSValidWD = false;
  
    /// The file system to use for external references.
    IntrusiveRefCntPtr<FileSystem> ExternalFS;
  
    /// If IsRelativeOverlay is set, this represents the directory
    /// path that should be prefixed to each 'external-contents' entry
    /// when reading from YAML files.
    std::string ExternalContentsPrefixDir;
  
    /// @name Configuration
    /// @{
  
    /// Whether to perform case-sensitive comparisons.
    ///
    /// Currently, case-insensitive matching only works correctly with ASCII.
    bool CaseSensitive = true;
  
    /// IsRelativeOverlay marks whether a ExternalContentsPrefixDir path must
    /// be prefixed in every 'external-contents' when reading from YAML files.
    bool IsRelativeOverlay = false;
  
    /// Whether to use to use the value of 'external-contents' for the
    /// names of files.  This global value is overridable on a per-file basis.
    bool UseExternalNames = true;
  
    /// Whether to attempt a file lookup in external file system after it wasn't
    /// found in VFS.
    bool IsFallthrough = true;
    /// @}
  
    /// Virtual file paths and external files could be canonicalized without "..",
    /// "." and "./" in their paths. FIXME: some unittests currently fail on
    /// win32 when using remove_dots and remove_leading_dotslash on paths.
    bool UseCanonicalizedPaths =
  #ifdef _WIN32
        false;
  #else
        true;
  #endif
  
    RedirectingFileSystem(IntrusiveRefCntPtr<FileSystem> ExternalFS);
  
    /// Looks up the path <tt>[Start, End)</tt> in \p From, possibly
    /// recursing into the contents of \p From if it is a directory.
    ErrorOr<Entry *> lookupPath(llvm::sys::path::const_iterator Start,
                                llvm::sys::path::const_iterator End,
                                Entry *From) const;
  
    /// Get the status of a given an \c Entry.
    ErrorOr<Status> status(const Twine &Path, Entry *E);
  
  public:
    /// Looks up \p Path in \c Roots.
    ErrorOr<Entry *> lookupPath(const Twine &Path) const;
  
    /// Parses \p Buffer, which is expected to be in YAML format and
    /// returns a virtual file system representing its contents.
    static RedirectingFileSystem *
    create(std::unique_ptr<MemoryBuffer> Buffer,
           SourceMgr::DiagHandlerTy DiagHandler, StringRef YAMLFilePath,
           void *DiagContext, IntrusiveRefCntPtr<FileSystem> ExternalFS);
  
    ErrorOr<Status> status(const Twine &Path) override;
    ErrorOr<std::unique_ptr<File>> openFileForRead(const Twine &Path) override;
  
    std::error_code getRealPath(const Twine &Path,
                                SmallVectorImpl<char> &Output) const 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;
  
    directory_iterator dir_begin(const Twine &Dir, std::error_code &EC) override;
  
    void setExternalContentsPrefixDir(StringRef PrefixDir);
  
    StringRef getExternalContentsPrefixDir() const;
  
    void dump(raw_ostream &OS) const;
    void dumpEntry(raw_ostream &OS, Entry *E, int NumSpaces = 0) const;
  #if !defined(NDEBUG) || defined(LLVM_ENABLE_DUMP)
    LLVM_DUMP_METHOD void dump() const;
  #endif
  };
  
  /// Collect all pairs of <virtual path, real path> entries from the
  /// \p YAMLFilePath. This is used by the module dependency collector to forward
  /// the entries into the reproducer output VFS YAML file.
  void collectVFSFromYAML(
      std::unique_ptr<llvm::MemoryBuffer> Buffer,
      llvm::SourceMgr::DiagHandlerTy DiagHandler, StringRef YAMLFilePath,
      SmallVectorImpl<YAMLVFSEntry> &CollectedEntries,
      void *DiagContext = nullptr,
      IntrusiveRefCntPtr<FileSystem> ExternalFS = getRealFileSystem());
  
  class YAMLVFSWriter {
    std::vector<YAMLVFSEntry> Mappings;
    Optional<bool> IsCaseSensitive;
    Optional<bool> IsOverlayRelative;
    Optional<bool> UseExternalNames;
    std::string OverlayDir;
  
  public:
    YAMLVFSWriter() = default;
  
    void addFileMapping(StringRef VirtualPath, StringRef RealPath);
  
    void setCaseSensitivity(bool CaseSensitive) {
      IsCaseSensitive = CaseSensitive;
    }
  
    void setUseExternalNames(bool UseExtNames) { UseExternalNames = UseExtNames; }
  
    void setOverlayDir(StringRef OverlayDirectory) {
      IsOverlayRelative = true;
      OverlayDir.assign(OverlayDirectory.str());
    }
  
    const std::vector<YAMLVFSEntry> &getMappings() const { return Mappings; }
  
    void write(llvm::raw_ostream &OS);
  };
  
  } // namespace vfs
  } // namespace llvm
  
  #endif // LLVM_SUPPORT_VIRTUALFILESYSTEM_H
  