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diff --git a/include/clang/Basic/VirtualFileSystem.h b/include/clang/Basic/VirtualFileSystem.h
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+//===- VirtualFileSystem.h - Virtual File System Layer ----------*- C++ -*-===//
+//
+//                     The LLVM Compiler Infrastructure
+//
+// This file is distributed under the University of Illinois Open Source
+// License. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+/// \file
+/// \brief Defines the virtual file system interface vfs::FileSystem.
+//===----------------------------------------------------------------------===//
+
+#ifndef LLVM_CLANG_BASIC_VIRTUAL_FILE_SYSTEM_H
+#define LLVM_CLANG_BASIC_VIRTUAL_FILE_SYSTEM_H
+
+#include "clang/Basic/LLVM.h"
+#include "llvm/ADT/IntrusiveRefCntPtr.h"
+#include "llvm/ADT/Optional.h"
+#include "llvm/Support/ErrorOr.h"
+#include "llvm/Support/FileSystem.h"
+#include "llvm/Support/raw_ostream.h"
+#include "llvm/Support/SourceMgr.h"
+
+namespace llvm {
+class MemoryBuffer;
+}
+
+namespace clang {
+namespace vfs {
+
+/// \brief The result of a \p status operation.
+class Status {
+  std::string Name;
+  llvm::sys::fs::UniqueID UID;
+  llvm::sys::TimeValue MTime;
+  uint32_t User;
+  uint32_t Group;
+  uint64_t Size;
+  llvm::sys::fs::file_type Type;
+  llvm::sys::fs::perms Perms;
+
+public:
+  bool IsVFSMapped; // FIXME: remove when files support multiple names
+
+public:
+  Status() : Type(llvm::sys::fs::file_type::status_error) {}
+  Status(const llvm::sys::fs::file_status &Status);
+  Status(StringRef Name, StringRef RealName, llvm::sys::fs::UniqueID UID,
+         llvm::sys::TimeValue MTime, uint32_t User, uint32_t Group,
+         uint64_t Size, llvm::sys::fs::file_type Type,
+         llvm::sys::fs::perms Perms);
+
+  /// \brief Returns the name that should be used for this file or directory.
+  StringRef getName() const { return Name; }
+  void setName(StringRef N) { Name = N; }
+
+  /// @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::TimeValue 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; }
+  void setType(llvm::sys::fs::file_type v) { Type = v; }
+  void setPermissions(llvm::sys::fs::perms p) { Perms = p; }
+  /// @}
+  /// @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;
+  /// @}
+};
+
+/// \brief Represents an open file.
+class File {
+public:
+  /// \brief 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();
+  /// \brief Get the status of the file.
+  virtual llvm::ErrorOr<Status> status() = 0;
+  /// \brief Get the contents of the file as a \p MemoryBuffer.
+  virtual std::error_code getBuffer(const Twine &Name,
+                                    std::unique_ptr<llvm::MemoryBuffer> &Result,
+                                    int64_t FileSize = -1,
+                                    bool RequiresNullTerminator = true,
+                                    bool IsVolatile = false) = 0;
+  /// \brief Closes the file.
+  virtual std::error_code close() = 0;
+  /// \brief Sets the name to use for this file.
+  virtual void setName(StringRef Name) = 0;
+};
+
+namespace detail {
+/// \brief An interface for virtual file systems to provide an iterator over the
+/// (non-recursive) contents of a directory.
+struct DirIterImpl {
+  virtual ~DirIterImpl();
+  /// \brief Sets \c CurrentEntry to the next entry in the directory on success,
+  /// or returns a system-defined \c error_code.
+  virtual std::error_code increment() = 0;
+  Status CurrentEntry;
+};
+} // end namespace detail
+
+/// \brief 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(I) {
+    assert(Impl.get() != nullptr && "requires non-null implementation");
+    if (!Impl->CurrentEntry.isStatusKnown())
+      Impl.reset(); // Normalize the end iterator to Impl == nullptr.
+  }
+
+  /// \brief Construct an 'end' iterator.
+  directory_iterator() { }
+
+  /// \brief 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 (EC || !Impl->CurrentEntry.isStatusKnown())
+      Impl.reset(); // Normalize the end iterator to Impl == nullptr.
+    return *this;
+  }
+
+  const Status &operator*() const { return Impl->CurrentEntry; }
+  const Status *operator->() const { return &Impl->CurrentEntry; }
+
+  bool operator==(const directory_iterator &RHS) const {
+    if (Impl && RHS.Impl)
+      return Impl->CurrentEntry.equivalent(RHS.Impl->CurrentEntry);
+    return !Impl && !RHS.Impl;
+  }
+  bool operator!=(const directory_iterator &RHS) const {
+    return !(*this == RHS);
+  }
+};
+
+class FileSystem;
+
+/// \brief An input iterator over the recursive contents of a virtual path,
+/// similar to llvm::sys::fs::recursive_directory_iterator.
+class recursive_directory_iterator {
+  typedef std::stack<directory_iterator, std::vector<directory_iterator>>
+      IterState;
+
+  FileSystem *FS;
+  std::shared_ptr<IterState> State; // Input iterator semantics on copy.
+
+public:
+  recursive_directory_iterator(FileSystem &FS, const Twine &Path,
+                               std::error_code &EC);
+  /// \brief Construct an 'end' iterator.
+  recursive_directory_iterator() { }
+
+  /// \brief Equivalent to operator++, with an error code.
+  recursive_directory_iterator &increment(std::error_code &EC);
+
+  const Status &operator*() const { return *State->top(); }
+  const Status *operator->() const { return &*State->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);
+  }
+};
+
+/// \brief The virtual file system interface.
+class FileSystem : public llvm::ThreadSafeRefCountedBase<FileSystem> {
+public:
+  virtual ~FileSystem();
+
+  /// \brief Get the status of the entry at \p Path, if one exists.
+  virtual llvm::ErrorOr<Status> status(const Twine &Path) = 0;
+  /// \brief Get a \p File object for the file at \p Path, if one exists.
+  virtual std::error_code openFileForRead(const Twine &Path,
+                                          std::unique_ptr<File> &Result) = 0;
+
+  /// This is a convenience method that opens a file, gets its content and then
+  /// closes the file.
+  std::error_code getBufferForFile(const Twine &Name,
+                                   std::unique_ptr<llvm::MemoryBuffer> &Result,
+                                   int64_t FileSize = -1,
+                                   bool RequiresNullTerminator = true,
+                                   bool IsVolatile = false);
+
+  /// \brief 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;
+};
+
+/// \brief Gets an \p vfs::FileSystem for the 'real' file system, as seen by
+/// the operating system.
+IntrusiveRefCntPtr<FileSystem> getRealFileSystem();
+
+/// \brief 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 {
+  typedef SmallVector<IntrusiveRefCntPtr<FileSystem>, 1> FileSystemList;
+  /// \brief The stack of file systems, implemented as a list in order of
+  /// their addition.
+  FileSystemList FSList;
+
+public:
+  OverlayFileSystem(IntrusiveRefCntPtr<FileSystem> Base);
+  /// \brief Pushes a file system on top of the stack.
+  void pushOverlay(IntrusiveRefCntPtr<FileSystem> FS);
+
+  llvm::ErrorOr<Status> status(const Twine &Path) override;
+  std::error_code openFileForRead(const Twine &Path,
+                                  std::unique_ptr<File> &Result) override;
+  directory_iterator dir_begin(const Twine &Dir, std::error_code &EC) override;
+
+  typedef FileSystemList::reverse_iterator iterator;
+  
+  /// \brief Get an iterator pointing to the most recently added file system.
+  iterator overlays_begin() { return FSList.rbegin(); }
+
+  /// \brief Get an iterator pointing one-past the least recently added file
+  /// system.
+  iterator overlays_end() { return FSList.rend(); }
+};
+
+/// \brief Get a globally unique ID for a virtual file or directory.
+llvm::sys::fs::UniqueID getNextVirtualUniqueID();
+
+/// \brief Gets a \p FileSystem for a virtual file system described in YAML
+/// format.
+///
+/// Takes ownership of \p Buffer.
+IntrusiveRefCntPtr<FileSystem>
+getVFSFromYAML(llvm::MemoryBuffer *Buffer,
+               llvm::SourceMgr::DiagHandlerTy DiagHandler,
+               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 YAMLVFSWriter {
+  std::vector<YAMLVFSEntry> Mappings;
+  Optional<bool> IsCaseSensitive;
+
+public:
+  YAMLVFSWriter() {}
+  void addFileMapping(StringRef VirtualPath, StringRef RealPath);
+  void setCaseSensitivity(bool CaseSensitive) {
+    IsCaseSensitive = CaseSensitive;
+  }
+  void write(llvm::raw_ostream &OS);
+};
+
+} // end namespace vfs
+} // end namespace clang
+#endif // LLVM_CLANG_BASIC_VIRTUAL_FILE_SYSTEM_H