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 //===- llvm/Support/FileSystem.h - File System OS Concept -------*- 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
 //
 //===----------------------------------------------------------------------===//
 //
 // This file declares the llvm::sys::fs namespace. It is designed after
 // TR2/boost filesystem (v3), but modified to remove exception handling and the
 // path class.
 //
 // All functions return an error_code and their actual work via the last out
 // argument. The out argument is defined if and only if errc::success is
 // returned. A function may return any error code in the generic or system
 // category. However, they shall be equivalent to any error conditions listed
 // in each functions respective documentation if the condition applies. [ note:
 // this does not guarantee that error_code will be in the set of explicitly
 // listed codes, but it does guarantee that if any of the explicitly listed
 // errors occur, the correct error_code will be used ]. All functions may
 // return errc::not_enough_memory if there is not enough memory to complete the
 // operation.
 //
 //===----------------------------------------------------------------------===//
 
 #ifndef LLVM_SUPPORT_FILESYSTEM_H
 #define LLVM_SUPPORT_FILESYSTEM_H
 
 #include "llvm/ADT/SmallString.h"
 #include "llvm/ADT/StringRef.h"
 #include "llvm/ADT/Twine.h"
 #include "llvm/Config/llvm-config.h"
 #include "llvm/Support/Chrono.h"
 #include "llvm/Support/Error.h"
 #include "llvm/Support/ErrorHandling.h"
 #include "llvm/Support/ErrorOr.h"
 #include "llvm/Support/FileSystem/UniqueID.h"
 #include "llvm/Support/MD5.h"
 #include <cassert>
 #include <cstdint>
 #include <ctime>
 #include <memory>
 #include <string>
 #include <system_error>
 #include <vector>
 
 namespace llvm {
 namespace sys {
 namespace fs {
 
 #if defined(_WIN32)
 // A Win32 HANDLE is a typedef of void*
 using file_t = void *;
 #else
 using file_t = int;
 #endif
 
 extern const file_t kInvalidFile;
 
 /// An enumeration for the file system's view of the type.
 enum class file_type {
   status_error,
   file_not_found,
   regular_file,
   directory_file,
   symlink_file,
   block_file,
   character_file,
   fifo_file,
   socket_file,
   type_unknown
 };
 
 /// space_info - Self explanatory.
 struct space_info {
   uint64_t capacity;
   uint64_t free;
   uint64_t available;
 };
 
 enum perms {
   no_perms = 0,
   owner_read = 0400,
   owner_write = 0200,
   owner_exe = 0100,
   owner_all = owner_read | owner_write | owner_exe,
   group_read = 040,
   group_write = 020,
   group_exe = 010,
   group_all = group_read | group_write | group_exe,
   others_read = 04,
   others_write = 02,
   others_exe = 01,
   others_all = others_read | others_write | others_exe,
   all_read = owner_read | group_read | others_read,
   all_write = owner_write | group_write | others_write,
   all_exe = owner_exe | group_exe | others_exe,
   all_all = owner_all | group_all | others_all,
   set_uid_on_exe = 04000,
   set_gid_on_exe = 02000,
   sticky_bit = 01000,
   all_perms = all_all | set_uid_on_exe | set_gid_on_exe | sticky_bit,
   perms_not_known = 0xFFFF
 };
 
 // Helper functions so that you can use & and | to manipulate perms bits:
 inline perms operator|(perms l, perms r) {
   return static_cast<perms>(static_cast<unsigned short>(l) |
                             static_cast<unsigned short>(r));
 }
 inline perms operator&(perms l, perms r) {
   return static_cast<perms>(static_cast<unsigned short>(l) &
                             static_cast<unsigned short>(r));
 }
 inline perms &operator|=(perms &l, perms r) {
   l = l | r;
   return l;
 }
 inline perms &operator&=(perms &l, perms r) {
   l = l & r;
   return l;
 }
 inline perms operator~(perms x) {
   // Avoid UB by explicitly truncating the (unsigned) ~ result.
   return static_cast<perms>(
       static_cast<unsigned short>(~static_cast<unsigned short>(x)));
 }
 
 /// Represents the result of a call to directory_iterator::status(). This is a
 /// subset of the information returned by a regular sys::fs::status() call, and
 /// represents the information provided by Windows FileFirstFile/FindNextFile.
 class basic_file_status {
 protected:
   #if defined(LLVM_ON_UNIX)
   time_t fs_st_atime = 0;
   time_t fs_st_mtime = 0;
   uint32_t fs_st_atime_nsec = 0;
   uint32_t fs_st_mtime_nsec = 0;
   uid_t fs_st_uid = 0;
   gid_t fs_st_gid = 0;
   off_t fs_st_size = 0;
   #elif defined (_WIN32)
   uint32_t LastAccessedTimeHigh = 0;
   uint32_t LastAccessedTimeLow = 0;
   uint32_t LastWriteTimeHigh = 0;
   uint32_t LastWriteTimeLow = 0;
   uint32_t FileSizeHigh = 0;
   uint32_t FileSizeLow = 0;
   #endif
   file_type Type = file_type::status_error;
   perms Perms = perms_not_known;
 
 public:
   basic_file_status() = default;
 
   explicit basic_file_status(file_type Type) : Type(Type) {}
 
   #if defined(LLVM_ON_UNIX)
   basic_file_status(file_type Type, perms Perms, time_t ATime,
                     uint32_t ATimeNSec, time_t MTime, uint32_t MTimeNSec,
                     uid_t UID, gid_t GID, off_t Size)
       : fs_st_atime(ATime), fs_st_mtime(MTime),
         fs_st_atime_nsec(ATimeNSec), fs_st_mtime_nsec(MTimeNSec),
         fs_st_uid(UID), fs_st_gid(GID),
         fs_st_size(Size), Type(Type), Perms(Perms) {}
 #elif defined(_WIN32)
   basic_file_status(file_type Type, perms Perms, uint32_t LastAccessTimeHigh,
                     uint32_t LastAccessTimeLow, uint32_t LastWriteTimeHigh,
                     uint32_t LastWriteTimeLow, uint32_t FileSizeHigh,
                     uint32_t FileSizeLow)
       : LastAccessedTimeHigh(LastAccessTimeHigh),
         LastAccessedTimeLow(LastAccessTimeLow),
         LastWriteTimeHigh(LastWriteTimeHigh),
         LastWriteTimeLow(LastWriteTimeLow), FileSizeHigh(FileSizeHigh),
         FileSizeLow(FileSizeLow), Type(Type), Perms(Perms) {}
   #endif
 
   // getters
   file_type type() const { return Type; }
   perms permissions() const { return Perms; }
 
   /// The file access time as reported from the underlying file system.
   ///
   /// Also see comments on \c getLastModificationTime() related to the precision
   /// of the returned value.
   TimePoint<> getLastAccessedTime() const;
 
   /// The file modification time as reported from the underlying file system.
   ///
   /// The returned value allows for nanosecond precision but the actual
   /// resolution is an implementation detail of the underlying file system.
   /// There is no guarantee for what kind of resolution you can expect, the
   /// resolution can differ across platforms and even across mountpoints on the
   /// same machine.
   TimePoint<> getLastModificationTime() const;
 
   #if defined(LLVM_ON_UNIX)
   uint32_t getUser() const { return fs_st_uid; }
   uint32_t getGroup() const { return fs_st_gid; }
   uint64_t getSize() const { return fs_st_size; }
   #elif defined (_WIN32)
   uint32_t getUser() const {
     return 9999; // Not applicable to Windows, so...
   }
 
   uint32_t getGroup() const {
     return 9999; // Not applicable to Windows, so...
   }
 
   uint64_t getSize() const {
     return (uint64_t(FileSizeHigh) << 32) + FileSizeLow;
   }
   #endif
 
   // setters
   void type(file_type v) { Type = v; }
   void permissions(perms p) { Perms = p; }
 };
 
 /// Represents the result of a call to sys::fs::status().
 class file_status : public basic_file_status {
   friend bool equivalent(file_status A, file_status B);
 
   #if defined(LLVM_ON_UNIX)
   dev_t fs_st_dev = 0;
   nlink_t fs_st_nlinks = 0;
   ino_t fs_st_ino = 0;
   #elif defined (_WIN32)
   uint32_t NumLinks = 0;
   uint32_t VolumeSerialNumber = 0;
   uint64_t PathHash = 0;
   #endif
 
 public:
   file_status() = default;
 
   explicit file_status(file_type Type) : basic_file_status(Type) {}
 
   #if defined(LLVM_ON_UNIX)
   file_status(file_type Type, perms Perms, dev_t Dev, nlink_t Links, ino_t Ino,
               time_t ATime, uint32_t ATimeNSec,
               time_t MTime, uint32_t MTimeNSec,
               uid_t UID, gid_t GID, off_t Size)
       : basic_file_status(Type, Perms, ATime, ATimeNSec, MTime, MTimeNSec,
                           UID, GID, Size),
         fs_st_dev(Dev), fs_st_nlinks(Links), fs_st_ino(Ino) {}
   #elif defined(_WIN32)
   file_status(file_type Type, perms Perms, uint32_t LinkCount,
               uint32_t LastAccessTimeHigh, uint32_t LastAccessTimeLow,
               uint32_t LastWriteTimeHigh, uint32_t LastWriteTimeLow,
               uint32_t VolumeSerialNumber, uint32_t FileSizeHigh,
               uint32_t FileSizeLow, uint64_t PathHash)
       : basic_file_status(Type, Perms, LastAccessTimeHigh, LastAccessTimeLow,
                           LastWriteTimeHigh, LastWriteTimeLow, FileSizeHigh,
                           FileSizeLow),
         NumLinks(LinkCount), VolumeSerialNumber(VolumeSerialNumber),
         PathHash(PathHash) {}
   #endif
 
   UniqueID getUniqueID() const;
   uint32_t getLinkCount() const;
 };
 
 /// @}
 /// @name Physical Operators
 /// @{
 
 /// Make \a path an absolute path.
 ///
 /// Makes \a path absolute using the \a current_directory if it is not already.
 /// An empty \a path will result in the \a current_directory.
 ///
 /// /absolute/path   => /absolute/path
 /// relative/../path => <current-directory>/relative/../path
 ///
 /// @param path A path that is modified to be an absolute path.
 void make_absolute(const Twine &current_directory, SmallVectorImpl<char> &path);
 
 /// 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 errc::success if \a path has been made absolute, otherwise a
 ///          platform-specific error_code.
 std::error_code make_absolute(SmallVectorImpl<char> &path);
 
 /// Create all the non-existent directories in path.
 ///
 /// @param path Directories to create.
 /// @returns errc::success if is_directory(path), otherwise a platform
 ///          specific error_code. If IgnoreExisting is false, also returns
 ///          error if the directory already existed.
 std::error_code create_directories(const Twine &path,
                                    bool IgnoreExisting = true,
                                    perms Perms = owner_all | group_all);
 
 /// Create the directory in path.
 ///
 /// @param path Directory to create.
 /// @returns errc::success if is_directory(path), otherwise a platform
 ///          specific error_code. If IgnoreExisting is false, also returns
 ///          error if the directory already existed.
 std::error_code create_directory(const Twine &path, bool IgnoreExisting = true,
                                  perms Perms = owner_all | group_all);
 
 /// Create a link from \a from to \a to.
 ///
 /// The link may be a soft or a hard link, depending on the platform. The caller
 /// may not assume which one. Currently on windows it creates a hard link since
 /// soft links require extra privileges. On unix, it creates a soft link since
 /// hard links don't work on SMB file systems.
 ///
 /// @param to The path to hard link to.
 /// @param from The path to hard link from. This is created.
 /// @returns errc::success if the link was created, otherwise a platform
 /// specific error_code.
 std::error_code create_link(const Twine &to, const Twine &from);
 
 /// Create a hard link from \a from to \a to, or return an error.
 ///
 /// @param to The path to hard link to.
 /// @param from The path to hard link from. This is created.
 /// @returns errc::success if the link was created, otherwise a platform
 /// specific error_code.
 std::error_code create_hard_link(const Twine &to, const Twine &from);
 
 /// Collapse all . and .. patterns, resolve all symlinks, and optionally
 ///        expand ~ expressions to the user's home directory.
 ///
 /// @param path The path to resolve.
 /// @param output The location to store the resolved path.
 /// @param expand_tilde If true, resolves ~ expressions to the user's home
 ///                     directory.
 std::error_code real_path(const Twine &path, SmallVectorImpl<char> &output,
                           bool expand_tilde = false);
 
 /// Expands ~ expressions to the user's home directory. On Unix ~user
 /// directories are resolved as well.
 ///
 /// @param path The path to resolve.
 void expand_tilde(const Twine &path, SmallVectorImpl<char> &output);
 
 /// Get the current path.
 ///
 /// @param result Holds the current path on return.
 /// @returns errc::success if the current path has been stored in result,
 ///          otherwise a platform-specific error_code.
 std::error_code current_path(SmallVectorImpl<char> &result);
 
 /// Set the current path.
 ///
 /// @param path The path to set.
 /// @returns errc::success if the current path was successfully set,
 ///          otherwise a platform-specific error_code.
 std::error_code set_current_path(const Twine &path);
 
 /// Remove path. Equivalent to POSIX remove().
 ///
 /// @param path Input path.
 /// @returns errc::success if path has been removed or didn't exist, otherwise a
 ///          platform-specific error code. If IgnoreNonExisting is false, also
 ///          returns error if the file didn't exist.
 std::error_code remove(const Twine &path, bool IgnoreNonExisting = true);
 
 /// Recursively delete a directory.
 ///
 /// @param path Input path.
 /// @returns errc::success if path has been removed or didn't exist, otherwise a
 ///          platform-specific error code.
 std::error_code remove_directories(const Twine &path, bool IgnoreErrors = true);
 
 /// Rename \a from to \a to.
 ///
 /// Files are renamed as if by POSIX rename(), except that on Windows there may
 /// be a short interval of time during which the destination file does not
 /// exist.
 ///
 /// @param from The path to rename from.
 /// @param to The path to rename to. This is created.
 std::error_code rename(const Twine &from, const Twine &to);
 
 /// Copy the contents of \a From to \a To.
 ///
 /// @param From The path to copy from.
 /// @param To The path to copy to. This is created.
 std::error_code copy_file(const Twine &From, const Twine &To);
 
 /// Copy the contents of \a From to \a To.
 ///
 /// @param From The path to copy from.
 /// @param ToFD The open file descriptor of the destination file.
 std::error_code copy_file(const Twine &From, int ToFD);
 
 /// Resize path to size. File is resized as if by POSIX truncate().
 ///
 /// @param FD Input file descriptor.
 /// @param Size Size to resize to.
 /// @returns errc::success if \a path has been resized to \a size, otherwise a
 ///          platform-specific error_code.
 std::error_code resize_file(int FD, uint64_t Size);
 
 /// Resize \p FD to \p Size before mapping \a mapped_file_region::readwrite. On
 /// non-Windows, this calls \a resize_file(). On Windows, this is a no-op,
 /// since the subsequent mapping (via \c CreateFileMapping) automatically
 /// extends the file.
 inline std::error_code resize_file_before_mapping_readwrite(int FD,
                                                             uint64_t Size) {
 #ifdef _WIN32
   (void)FD;
   (void)Size;
   return std::error_code();
 #else
   return resize_file(FD, Size);
 #endif
 }
 
 /// Compute an MD5 hash of a file's contents.
 ///
 /// @param FD Input file descriptor.
 /// @returns An MD5Result with the hash computed, if successful, otherwise a
 ///          std::error_code.
 ErrorOr<MD5::MD5Result> md5_contents(int FD);
 
 /// Version of compute_md5 that doesn't require an open file descriptor.
 ErrorOr<MD5::MD5Result> md5_contents(const Twine &Path);
 
 /// @}
 /// @name Physical Observers
 /// @{
 
 /// Does file exist?
 ///
 /// @param status A basic_file_status previously returned from stat.
 /// @returns True if the file represented by status exists, false if it does
 ///          not.
 bool exists(const basic_file_status &status);
 
 enum class AccessMode { Exist, Write, Execute };
 
 /// Can the file be accessed?
 ///
 /// @param Path Input path.
 /// @returns errc::success if the path can be accessed, otherwise a
 ///          platform-specific error_code.
 std::error_code access(const Twine &Path, AccessMode Mode);
 
 /// Does file exist?
 ///
 /// @param Path Input path.
 /// @returns True if it exists, false otherwise.
 inline bool exists(const Twine &Path) {
   return !access(Path, AccessMode::Exist);
 }
 
 /// Can we execute this file?
 ///
 /// @param Path Input path.
 /// @returns True if we can execute it, false otherwise.
 bool can_execute(const Twine &Path);
 
 /// Can we write this file?
 ///
 /// @param Path Input path.
 /// @returns True if we can write to it, false otherwise.
 inline bool can_write(const Twine &Path) {
   return !access(Path, AccessMode::Write);
 }
 
 /// Do file_status's represent the same thing?
 ///
 /// @param A Input file_status.
 /// @param B Input file_status.
 ///
 /// assert(status_known(A) || status_known(B));
 ///
 /// @returns True if A and B both represent the same file system entity, false
 ///          otherwise.
 bool equivalent(file_status A, file_status B);
 
 /// Do paths represent the same thing?
 ///
 /// assert(status_known(A) || status_known(B));
 ///
 /// @param A Input path A.
 /// @param B Input path B.
 /// @param result Set to true if stat(A) and stat(B) have the same device and
 ///               inode (or equivalent).
 /// @returns errc::success if result has been successfully set, otherwise a
 ///          platform-specific error_code.
 std::error_code equivalent(const Twine &A, const Twine &B, bool &result);
 
 /// Simpler version of equivalent for clients that don't need to
 ///        differentiate between an error and false.
 inline bool equivalent(const Twine &A, const Twine &B) {
   bool result;
   return !equivalent(A, B, result) && result;
 }
 
 /// Is the file mounted on a local filesystem?
 ///
 /// @param path Input path.
 /// @param result Set to true if \a path is on fixed media such as a hard disk,
 ///               false if it is not.
 /// @returns errc::success if result has been successfully set, otherwise a
 ///          platform specific error_code.
 std::error_code is_local(const Twine &path, bool &result);
 
 /// Version of is_local accepting an open file descriptor.
 std::error_code is_local(int FD, bool &result);
 
 /// Simpler version of is_local for clients that don't need to
 ///        differentiate between an error and false.
 inline bool is_local(const Twine &Path) {
   bool Result;
   return !is_local(Path, Result) && Result;
 }
 
 /// Simpler version of is_local accepting an open file descriptor for
 ///        clients that don't need to differentiate between an error and false.
 inline bool is_local(int FD) {
   bool Result;
   return !is_local(FD, Result) && Result;
 }
 
 /// Does status represent a directory?
 ///
 /// @param Path The path to get the type of.
 /// @param Follow For symbolic links, indicates whether to return the file type
 ///               of the link itself, or of the target.
 /// @returns A value from the file_type enumeration indicating the type of file.
 file_type get_file_type(const Twine &Path, bool Follow = true);
 
 /// Does status represent a directory?
 ///
 /// @param status A basic_file_status previously returned from status.
 /// @returns status.type() == file_type::directory_file.
 bool is_directory(const basic_file_status &status);
 
 /// Is path a directory?
 ///
 /// @param path Input path.
 /// @param result Set to true if \a path is a directory (after following
 ///               symlinks, false if it is not. Undefined otherwise.
 /// @returns errc::success if result has been successfully set, otherwise a
 ///          platform-specific error_code.
 std::error_code is_directory(const Twine &path, bool &result);
 
 /// Simpler version of is_directory for clients that don't need to
 ///        differentiate between an error and false.
 inline bool is_directory(const Twine &Path) {
   bool Result;
   return !is_directory(Path, Result) && Result;
 }
 
 /// Does status represent a regular file?
 ///
 /// @param status A basic_file_status previously returned from status.
 /// @returns status_known(status) && status.type() == file_type::regular_file.
 bool is_regular_file(const basic_file_status &status);
 
 /// Is path a regular file?
 ///
 /// @param path Input path.
 /// @param result Set to true if \a path is a regular file (after following
 ///               symlinks), false if it is not. Undefined otherwise.
 /// @returns errc::success if result has been successfully set, otherwise a
 ///          platform-specific error_code.
 std::error_code is_regular_file(const Twine &path, bool &result);
 
 /// Simpler version of is_regular_file for clients that don't need to
 ///        differentiate between an error and false.
 inline bool is_regular_file(const Twine &Path) {
   bool Result;
   if (is_regular_file(Path, Result))
     return false;
   return Result;
 }
 
 /// Does status represent a symlink file?
 ///
 /// @param status A basic_file_status previously returned from status.
 /// @returns status_known(status) && status.type() == file_type::symlink_file.
 bool is_symlink_file(const basic_file_status &status);
 
 /// Is path a symlink file?
 ///
 /// @param path Input path.
 /// @param result Set to true if \a path is a symlink file, false if it is not.
 ///               Undefined otherwise.
 /// @returns errc::success if result has been successfully set, otherwise a
 ///          platform-specific error_code.
 std::error_code is_symlink_file(const Twine &path, bool &result);
 
 /// Simpler version of is_symlink_file for clients that don't need to
 ///        differentiate between an error and false.
 inline bool is_symlink_file(const Twine &Path) {
   bool Result;
   if (is_symlink_file(Path, Result))
     return false;
   return Result;
 }
 
 /// Does this status represent something that exists but is not a
 ///        directory or regular file?
 ///
 /// @param status A basic_file_status previously returned from status.
 /// @returns exists(s) && !is_regular_file(s) && !is_directory(s)
 bool is_other(const basic_file_status &status);
 
 /// Is path something that exists but is not a directory,
 ///        regular file, or symlink?
 ///
 /// @param path Input path.
 /// @param result Set to true if \a path exists, but is not a directory, regular
 ///               file, or a symlink, false if it does not. Undefined otherwise.
 /// @returns errc::success if result has been successfully set, otherwise a
 ///          platform-specific error_code.
 std::error_code is_other(const Twine &path, bool &result);
 
 /// Get file status as if by POSIX stat().
 ///
 /// @param path Input path.
 /// @param result Set to the file status.
 /// @param follow When true, follows symlinks.  Otherwise, the symlink itself is
 ///               statted.
 /// @returns errc::success if result has been successfully set, otherwise a
 ///          platform-specific error_code.
 std::error_code status(const Twine &path, file_status &result,
                        bool follow = true);
 
 /// A version for when a file descriptor is already available.
 std::error_code status(int FD, file_status &Result);
 
 #ifdef _WIN32
 /// A version for when a file descriptor is already available.
 std::error_code status(file_t FD, file_status &Result);
 #endif
 
 /// Get file creation mode mask of the process.
 ///
 /// @returns Mask reported by umask(2)
 /// @note There is no umask on Windows. This function returns 0 always
 ///       on Windows. This function does not return an error_code because
 ///       umask(2) never fails. It is not thread safe.
 unsigned getUmask();
 
 /// Set file permissions.
 ///
 /// @param Path File to set permissions on.
 /// @param Permissions New file permissions.
 /// @returns errc::success if the permissions were successfully set, otherwise
 ///          a platform-specific error_code.
 /// @note On Windows, all permissions except *_write are ignored. Using any of
 ///       owner_write, group_write, or all_write will make the file writable.
 ///       Otherwise, the file will be marked as read-only.
 std::error_code setPermissions(const Twine &Path, perms Permissions);
 
 /// Vesion of setPermissions accepting a file descriptor.
 /// TODO Delete the path based overload once we implement the FD based overload
 /// on Windows.
 std::error_code setPermissions(int FD, perms Permissions);
 
 /// Get file permissions.
 ///
 /// @param Path File to get permissions from.
 /// @returns the permissions if they were successfully retrieved, otherwise a
 ///          platform-specific error_code.
 /// @note On Windows, if the file does not have the FILE_ATTRIBUTE_READONLY
 ///       attribute, all_all will be returned. Otherwise, all_read | all_exe
 ///       will be returned.
 ErrorOr<perms> getPermissions(const Twine &Path);
 
 /// Get file size.
 ///
 /// @param Path Input path.
 /// @param Result Set to the size of the file in \a Path.
 /// @returns errc::success if result has been successfully set, otherwise a
 ///          platform-specific error_code.
 inline std::error_code file_size(const Twine &Path, uint64_t &Result) {
   file_status Status;
   std::error_code EC = status(Path, Status);
   if (EC)
     return EC;
   Result = Status.getSize();
   return std::error_code();
 }
 
 /// Set the file modification and access time.
 ///
 /// @returns errc::success if the file times were successfully set, otherwise a
 ///          platform-specific error_code or errc::function_not_supported on
 ///          platforms where the functionality isn't available.
 std::error_code setLastAccessAndModificationTime(int FD, TimePoint<> AccessTime,
                                                  TimePoint<> ModificationTime);
 
 /// Simpler version that sets both file modification and access time to the same
 /// time.
 inline std::error_code setLastAccessAndModificationTime(int FD,
                                                         TimePoint<> Time) {
   return setLastAccessAndModificationTime(FD, Time, Time);
 }
 
 /// Is status available?
 ///
 /// @param s Input file status.
 /// @returns True if status() != status_error.
 bool status_known(const basic_file_status &s);
 
 /// Is status available?
 ///
 /// @param path Input path.
 /// @param result Set to true if status() != status_error.
 /// @returns errc::success if result has been successfully set, otherwise a
 ///          platform-specific error_code.
 std::error_code status_known(const Twine &path, bool &result);
 
 enum CreationDisposition : unsigned {
   /// CD_CreateAlways - When opening a file:
   ///   * If it already exists, truncate it.
   ///   * If it does not already exist, create a new file.
   CD_CreateAlways = 0,
 
   /// CD_CreateNew - When opening a file:
   ///   * If it already exists, fail.
   ///   * If it does not already exist, create a new file.
   CD_CreateNew = 1,
 
   /// CD_OpenExisting - When opening a file:
   ///   * If it already exists, open the file with the offset set to 0.
   ///   * If it does not already exist, fail.
   CD_OpenExisting = 2,
 
   /// CD_OpenAlways - When opening a file:
   ///   * If it already exists, open the file with the offset set to 0.
   ///   * If it does not already exist, create a new file.
   CD_OpenAlways = 3,
 };
 
 enum FileAccess : unsigned {
   FA_Read = 1,
   FA_Write = 2,
 };
 
 enum OpenFlags : unsigned {
   OF_None = 0,
 
   /// The file should be opened in text mode on platforms like z/OS that make
   /// this distinction.
   OF_Text = 1,
 
   /// The file should use a carriage linefeed '\r\n'. This flag should only be
   /// used with OF_Text. Only makes a difference on Windows.
   OF_CRLF = 2,
 
   /// The file should be opened in text mode and use a carriage linefeed '\r\n'.
   /// This flag has the same functionality as OF_Text on z/OS but adds a
   /// carriage linefeed on Windows.
   OF_TextWithCRLF = OF_Text | OF_CRLF,
 
   /// The file should be opened in append mode.
   OF_Append = 4,
 
   /// The returned handle can be used for deleting the file. Only makes a
   /// difference on windows.
   OF_Delete = 8,
 
   /// When a child process is launched, this file should remain open in the
   /// child process.
   OF_ChildInherit = 16,
 
   /// Force files Atime to be updated on access. Only makes a difference on
   /// Windows.
   OF_UpdateAtime = 32,
 };
 
 /// Create a potentially unique file name but does not create it.
 ///
 /// Generates a unique path suitable for a temporary file but does not
 /// open or create the file. The name is based on \a Model with '%'
 /// replaced by a random char in [0-9a-f]. If \a MakeAbsolute is true
 /// then the system's temp directory is prepended first. If \a MakeAbsolute
 /// is false the current directory will be used instead.
 ///
 /// This function does not check if the file exists. If you want to be sure
 /// that the file does not yet exist, you should use enough '%' characters
 /// in your model to ensure this. Each '%' gives 4-bits of entropy so you can
 /// use 32 of them to get 128 bits of entropy.
 ///
 /// Example: clang-%%-%%-%%-%%-%%.s => clang-a0-b1-c2-d3-e4.s
 ///
 /// @param Model Name to base unique path off of.
 /// @param ResultPath Set to the file's path.
 /// @param MakeAbsolute Whether to use the system temp directory.
 void createUniquePath(const Twine &Model, SmallVectorImpl<char> &ResultPath,
                       bool MakeAbsolute);
 
 /// Create a uniquely named file.
 ///
 /// Generates a unique path suitable for a temporary file and then opens it as a
 /// file. The name is based on \a Model with '%' replaced by a random char in
 /// [0-9a-f]. If \a Model is not an absolute path, the temporary file will be
 /// created in the current directory.
 ///
 /// Example: clang-%%-%%-%%-%%-%%.s => clang-a0-b1-c2-d3-e4.s
 ///
 /// This is an atomic operation. Either the file is created and opened, or the
 /// file system is left untouched.
 ///
 /// The intended use is for files that are to be kept, possibly after
 /// renaming them. For example, when running 'clang -c foo.o', the file can
 /// be first created as foo-abc123.o and then renamed.
 ///
 /// @param Model Name to base unique path off of.
 /// @param ResultFD Set to the opened file's file descriptor.
 /// @param ResultPath Set to the opened file's absolute path.
 /// @param Flags Set to the opened file's flags.
 /// @param Mode Set to the opened file's permissions.
 /// @returns errc::success if Result{FD,Path} have been successfully set,
 ///          otherwise a platform-specific error_code.
 std::error_code createUniqueFile(const Twine &Model, int &ResultFD,
                                  SmallVectorImpl<char> &ResultPath,
                                  OpenFlags Flags = OF_None,
                                  unsigned Mode = all_read | all_write);
 
 /// Simpler version for clients that don't want an open file. An empty
 /// file will still be created.
 std::error_code createUniqueFile(const Twine &Model,
                                  SmallVectorImpl<char> &ResultPath,
                                  unsigned Mode = all_read | all_write);
 
 /// Represents a temporary file.
 ///
 /// The temporary file must be eventually discarded or given a final name and
 /// kept.
 ///
 /// The destructor doesn't implicitly discard because there is no way to
 /// properly handle errors in a destructor.
 class TempFile {
   bool Done = false;
   TempFile(StringRef Name, int FD);
 
 public:
   /// This creates a temporary file with createUniqueFile and schedules it for
   /// deletion with sys::RemoveFileOnSignal.
   static Expected<TempFile> create(const Twine &Model,
                                    unsigned Mode = all_read | all_write,
                                    OpenFlags ExtraFlags = OF_None);
   TempFile(TempFile &&Other);
   TempFile &operator=(TempFile &&Other);
 
   // Name of the temporary file.
   std::string TmpName;
 
   // The open file descriptor.
   int FD = -1;
 
 #ifdef _WIN32
   // Whether we need to manually remove the file on close.
   bool RemoveOnClose = false;
 #endif
 
   // Keep this with the given name.
   Error keep(const Twine &Name);
 
   // Keep this with the temporary name.
   Error keep();
 
   // Delete the file.
   Error discard();
 
   // This checks that keep or delete was called.
   ~TempFile();
 };
 
 /// Create a file in the system temporary directory.
 ///
 /// The filename is of the form prefix-random_chars.suffix. Since the directory
 /// is not know to the caller, Prefix and Suffix cannot have path separators.
 /// The files are created with mode 0600.
 ///
 /// This should be used for things like a temporary .s that is removed after
 /// running the assembler.
 std::error_code createTemporaryFile(const Twine &Prefix, StringRef Suffix,
                                     int &ResultFD,
                                     SmallVectorImpl<char> &ResultPath,
                                     OpenFlags Flags = OF_None);
 
 /// Simpler version for clients that don't want an open file. An empty
 /// file will still be created.
 std::error_code createTemporaryFile(const Twine &Prefix, StringRef Suffix,
                                     SmallVectorImpl<char> &ResultPath,
                                     OpenFlags Flags = OF_None);
 
 std::error_code createUniqueDirectory(const Twine &Prefix,
                                       SmallVectorImpl<char> &ResultPath);
 
 /// Get a unique name, not currently exisiting in the filesystem. Subject
 /// to race conditions, prefer to use createUniqueFile instead.
 ///
 /// Similar to createUniqueFile, but instead of creating a file only
 /// checks if it exists. This function is subject to race conditions, if you
 /// want to use the returned name to actually create a file, use
 /// createUniqueFile instead.
 std::error_code getPotentiallyUniqueFileName(const Twine &Model,
                                              SmallVectorImpl<char> &ResultPath);
 
 /// Get a unique temporary file name, not currently exisiting in the
 /// filesystem. Subject to race conditions, prefer to use createTemporaryFile
 /// instead.
 ///
 /// Similar to createTemporaryFile, but instead of creating a file only
 /// checks if it exists. This function is subject to race conditions, if you
 /// want to use the returned name to actually create a file, use
 /// createTemporaryFile instead.
 std::error_code
 getPotentiallyUniqueTempFileName(const Twine &Prefix, StringRef Suffix,
                                  SmallVectorImpl<char> &ResultPath);
 
 inline OpenFlags operator|(OpenFlags A, OpenFlags B) {
   return OpenFlags(unsigned(A) | unsigned(B));
 }
 
 inline OpenFlags &operator|=(OpenFlags &A, OpenFlags B) {
   A = A | B;
   return A;
 }
 
 inline FileAccess operator|(FileAccess A, FileAccess B) {
   return FileAccess(unsigned(A) | unsigned(B));
 }
 
 inline FileAccess &operator|=(FileAccess &A, FileAccess B) {
   A = A | B;
   return A;
 }
 
 /// @brief Opens a file with the specified creation disposition, access mode,
 /// and flags and returns a file descriptor.
 ///
 /// The caller is responsible for closing the file descriptor once they are
 /// finished with it.
 ///
 /// @param Name The path of the file to open, relative or absolute.
 /// @param ResultFD If the file could be opened successfully, its descriptor
 ///                 is stored in this location. Otherwise, this is set to -1.
 /// @param Disp Value specifying the existing-file behavior.
 /// @param Access Value specifying whether to open the file in read, write, or
 ///               read-write mode.
 /// @param Flags Additional flags.
 /// @param Mode The access permissions of the file, represented in octal.
 /// @returns errc::success if \a Name has been opened, otherwise a
 ///          platform-specific error_code.
 std::error_code openFile(const Twine &Name, int &ResultFD,
                          CreationDisposition Disp, FileAccess Access,
                          OpenFlags Flags, unsigned Mode = 0666);
 
 /// @brief Opens a file with the specified creation disposition, access mode,
 /// and flags and returns a platform-specific file object.
 ///
 /// The caller is responsible for closing the file object once they are
 /// finished with it.
 ///
 /// @param Name The path of the file to open, relative or absolute.
 /// @param Disp Value specifying the existing-file behavior.
 /// @param Access Value specifying whether to open the file in read, write, or
 ///               read-write mode.
 /// @param Flags Additional flags.
 /// @param Mode The access permissions of the file, represented in octal.
 /// @returns errc::success if \a Name has been opened, otherwise a
 ///          platform-specific error_code.
 Expected<file_t> openNativeFile(const Twine &Name, CreationDisposition Disp,
                                 FileAccess Access, OpenFlags Flags,
                                 unsigned Mode = 0666);
 
 /// Converts from a Posix file descriptor number to a native file handle.
 /// On Windows, this retreives the underlying handle. On non-Windows, this is a
 /// no-op.
 file_t convertFDToNativeFile(int FD);
 
 #ifndef _WIN32
 inline file_t convertFDToNativeFile(int FD) { return FD; }
 #endif
 
 /// Return an open handle to standard in. On Unix, this is typically FD 0.
 /// Returns kInvalidFile when the stream is closed.
 file_t getStdinHandle();
 
 /// Return an open handle to standard out. On Unix, this is typically FD 1.
 /// Returns kInvalidFile when the stream is closed.
 file_t getStdoutHandle();
 
 /// Return an open handle to standard error. On Unix, this is typically FD 2.
 /// Returns kInvalidFile when the stream is closed.
 file_t getStderrHandle();
 
 /// Reads \p Buf.size() bytes from \p FileHandle into \p Buf. Returns the number
 /// of bytes actually read. On Unix, this is equivalent to `return ::read(FD,
 /// Buf.data(), Buf.size())`, with error reporting. Returns 0 when reaching EOF.
 ///
 /// @param FileHandle File to read from.
 /// @param Buf Buffer to read into.
 /// @returns The number of bytes read, or error.
 Expected<size_t> readNativeFile(file_t FileHandle, MutableArrayRef<char> Buf);
 
 /// Default chunk size for \a readNativeFileToEOF().
 enum : size_t { DefaultReadChunkSize = 4 * 4096 };
 
 /// Reads from \p FileHandle until EOF, appending to \p Buffer in chunks of
 /// size \p ChunkSize.
 ///
 /// This calls \a readNativeFile() in a loop. On Error, previous chunks that
 /// were read successfully are left in \p Buffer and returned.
 ///
 /// Note: For reading the final chunk at EOF, \p Buffer's capacity needs extra
 /// storage of \p ChunkSize.
 ///
 /// \param FileHandle File to read from.
 /// \param Buffer Where to put the file content.
 /// \param ChunkSize Size of chunks.
 /// \returns The error if EOF was not found.
 Error readNativeFileToEOF(file_t FileHandle, SmallVectorImpl<char> &Buffer,
                           ssize_t ChunkSize = DefaultReadChunkSize);
 
 /// Reads \p Buf.size() bytes from \p FileHandle at offset \p Offset into \p
 /// Buf. If 'pread' is available, this will use that, otherwise it will use
 /// 'lseek'. Returns the number of bytes actually read. Returns 0 when reaching
 /// EOF.
 ///
 /// @param FileHandle File to read from.
 /// @param Buf Buffer to read into.
 /// @param Offset Offset into the file at which the read should occur.
 /// @returns The number of bytes read, or error.
 Expected<size_t> readNativeFileSlice(file_t FileHandle,
                                      MutableArrayRef<char> Buf,
                                      uint64_t Offset);
 
 /// @brief Opens the file with the given name in a write-only or read-write
 /// mode, returning its open file descriptor. If the file does not exist, it
 /// is created.
 ///
 /// The caller is responsible for closing the file descriptor once they are
 /// finished with it.
 ///
 /// @param Name The path of the file to open, relative or absolute.
 /// @param ResultFD If the file could be opened successfully, its descriptor
 ///                 is stored in this location. Otherwise, this is set to -1.
 /// @param Flags Additional flags used to determine whether the file should be
 ///              opened in, for example, read-write or in write-only mode.
 /// @param Mode The access permissions of the file, represented in octal.
 /// @returns errc::success if \a Name has been opened, otherwise a
 ///          platform-specific error_code.
 inline std::error_code
 openFileForWrite(const Twine &Name, int &ResultFD,
                  CreationDisposition Disp = CD_CreateAlways,
                  OpenFlags Flags = OF_None, unsigned Mode = 0666) {
   return openFile(Name, ResultFD, Disp, FA_Write, Flags, Mode);
 }
 
 /// @brief Opens the file with the given name in a write-only or read-write
 /// mode, returning its open file descriptor. If the file does not exist, it
 /// is created.
 ///
 /// The caller is responsible for closing the freeing the file once they are
 /// finished with it.
 ///
 /// @param Name The path of the file to open, relative or absolute.
 /// @param Flags Additional flags used to determine whether the file should be
 ///              opened in, for example, read-write or in write-only mode.
 /// @param Mode The access permissions of the file, represented in octal.
 /// @returns a platform-specific file descriptor if \a Name has been opened,
 ///          otherwise an error object.
 inline Expected<file_t> openNativeFileForWrite(const Twine &Name,
                                                CreationDisposition Disp,
                                                OpenFlags Flags,
                                                unsigned Mode = 0666) {
   return openNativeFile(Name, Disp, FA_Write, Flags, Mode);
 }
 
 /// @brief Opens the file with the given name in a write-only or read-write
 /// mode, returning its open file descriptor. If the file does not exist, it
 /// is created.
 ///
 /// The caller is responsible for closing the file descriptor once they are
 /// finished with it.
 ///
 /// @param Name The path of the file to open, relative or absolute.
 /// @param ResultFD If the file could be opened successfully, its descriptor
 ///                 is stored in this location. Otherwise, this is set to -1.
 /// @param Flags Additional flags used to determine whether the file should be
 ///              opened in, for example, read-write or in write-only mode.
 /// @param Mode The access permissions of the file, represented in octal.
 /// @returns errc::success if \a Name has been opened, otherwise a
 ///          platform-specific error_code.
 inline std::error_code openFileForReadWrite(const Twine &Name, int &ResultFD,
                                             CreationDisposition Disp,
                                             OpenFlags Flags,
                                             unsigned Mode = 0666) {
   return openFile(Name, ResultFD, Disp, FA_Write | FA_Read, Flags, Mode);
 }
 
 /// @brief Opens the file with the given name in a write-only or read-write
 /// mode, returning its open file descriptor. If the file does not exist, it
 /// is created.
 ///
 /// The caller is responsible for closing the freeing the file once they are
 /// finished with it.
 ///
 /// @param Name The path of the file to open, relative or absolute.
 /// @param Flags Additional flags used to determine whether the file should be
 ///              opened in, for example, read-write or in write-only mode.
 /// @param Mode The access permissions of the file, represented in octal.
 /// @returns a platform-specific file descriptor if \a Name has been opened,
 ///          otherwise an error object.
 inline Expected<file_t> openNativeFileForReadWrite(const Twine &Name,
                                                    CreationDisposition Disp,
                                                    OpenFlags Flags,
                                                    unsigned Mode = 0666) {
   return openNativeFile(Name, Disp, FA_Write | FA_Read, Flags, Mode);
 }
 
 /// @brief Opens the file with the given name in a read-only mode, returning
 /// its open file descriptor.
 ///
 /// The caller is responsible for closing the file descriptor once they are
 /// finished with it.
 ///
 /// @param Name The path of the file to open, relative or absolute.
 /// @param ResultFD If the file could be opened successfully, its descriptor
 ///                 is stored in this location. Otherwise, this is set to -1.
 /// @param RealPath If nonnull, extra work is done to determine the real path
 ///                 of the opened file, and that path is stored in this
 ///                 location.
 /// @returns errc::success if \a Name has been opened, otherwise a
 ///          platform-specific error_code.
 std::error_code openFileForRead(const Twine &Name, int &ResultFD,
                                 OpenFlags Flags = OF_None,
                                 SmallVectorImpl<char> *RealPath = nullptr);
 
 /// @brief Opens the file with the given name in a read-only mode, returning
 /// its open file descriptor.
 ///
 /// The caller is responsible for closing the freeing the file once they are
 /// finished with it.
 ///
 /// @param Name The path of the file to open, relative or absolute.
 /// @param RealPath If nonnull, extra work is done to determine the real path
 ///                 of the opened file, and that path is stored in this
 ///                 location.
 /// @returns a platform-specific file descriptor if \a Name has been opened,
 ///          otherwise an error object.
 Expected<file_t>
 openNativeFileForRead(const Twine &Name, OpenFlags Flags = OF_None,
                       SmallVectorImpl<char> *RealPath = nullptr);
 
 /// Try to locks the file during the specified time.
 ///
 /// This function implements advisory locking on entire file. If it returns
 /// <em>errc::success</em>, the file is locked by the calling process. Until the
 /// process unlocks the file by calling \a unlockFile, all attempts to lock the
 /// same file will fail/block. The process that locked the file may assume that
 /// none of other processes read or write this file, provided that all processes
 /// lock the file prior to accessing its content.
 ///
 /// @param FD      The descriptor representing the file to lock.
 /// @param Timeout Time in milliseconds that the process should wait before
 ///                reporting lock failure. Zero value means try to get lock only
 ///                once.
 /// @returns errc::success if lock is successfully obtained,
 /// errc::no_lock_available if the file cannot be locked, or platform-specific
 /// error_code otherwise.
 ///
 /// @note Care should be taken when using this function in a multithreaded
 /// context, as it may not prevent other threads in the same process from
 /// obtaining a lock on the same file, even if they are using a different file
 /// descriptor.
 std::error_code
 tryLockFile(int FD,
             std::chrono::milliseconds Timeout = std::chrono::milliseconds(0));
 
 /// Lock the file.
 ///
 /// This function acts as @ref tryLockFile but it waits infinitely.
 std::error_code lockFile(int FD);
 
 /// Unlock the file.
 ///
 /// @param FD The descriptor representing the file to unlock.
 /// @returns errc::success if lock is successfully released or platform-specific
 /// error_code otherwise.
 std::error_code unlockFile(int FD);
 
 /// @brief Close the file object.  This should be used instead of ::close for
 /// portability. On error, the caller should assume the file is closed, as is
 /// the case for Process::SafelyCloseFileDescriptor
 ///
 /// @param F On input, this is the file to close.  On output, the file is
 /// set to kInvalidFile.
 ///
 /// @returns An error code if closing the file failed. Typically, an error here
 /// means that the filesystem may have failed to perform some buffered writes.
 std::error_code closeFile(file_t &F);
 
 #ifdef LLVM_ON_UNIX
 /// @brief Change ownership of a file.
 ///
 /// @param Owner The owner of the file to change to.
 /// @param Group The group of the file to change to.
 /// @returns errc::success if successfully updated file ownership, otherwise an
 ///          error code is returned.
 std::error_code changeFileOwnership(int FD, uint32_t Owner, uint32_t Group);
 #endif
 
 /// RAII class that facilitates file locking.
 class FileLocker {
   int FD; ///< Locked file handle.
   FileLocker(int FD) : FD(FD) {}
   friend class llvm::raw_fd_ostream;
 
 public:
   FileLocker(const FileLocker &L) = delete;
   FileLocker(FileLocker &&L) : FD(L.FD) { L.FD = -1; }
   ~FileLocker() {
     if (FD != -1)
       unlockFile(FD);
   }
   FileLocker &operator=(FileLocker &&L) {
     FD = L.FD;
     L.FD = -1;
     return *this;
   }
   FileLocker &operator=(const FileLocker &L) = delete;
   std::error_code unlock() {
     if (FD != -1) {
       std::error_code Result = unlockFile(FD);
       FD = -1;
       return Result;
     }
     return std::error_code();
   }
 };
 
 std::error_code getUniqueID(const Twine Path, UniqueID &Result);
 
 /// Get disk space usage information.
 ///
 /// Note: Users must be careful about "Time Of Check, Time Of Use" kind of bug.
 /// Note: Windows reports results according to the quota allocated to the user.
 ///
 /// @param Path Input path.
 /// @returns a space_info structure filled with the capacity, free, and
 /// available space on the device \a Path is on. A platform specific error_code
 /// is returned on error.
 ErrorOr<space_info> disk_space(const Twine &Path);
 
 /// This class represents a memory mapped file. It is based on
 /// boost::iostreams::mapped_file.
 class mapped_file_region {
 public:
   enum mapmode {
     readonly, ///< May only access map via const_data as read only.
     readwrite, ///< May access map via data and modify it. Written to path.
     priv ///< May modify via data, but changes are lost on destruction.
   };
 
 private:
   /// Platform-specific mapping state.
   size_t Size = 0;
   void *Mapping = nullptr;
 #ifdef _WIN32
   sys::fs::file_t FileHandle = nullptr;
 #endif
   mapmode Mode = readonly;
 
   void copyFrom(const mapped_file_region &Copied) {
     Size = Copied.Size;
     Mapping = Copied.Mapping;
 #ifdef _WIN32
     FileHandle = Copied.FileHandle;
 #endif
     Mode = Copied.Mode;
   }
 
   void moveFromImpl(mapped_file_region &Moved) {
     copyFrom(Moved);
     Moved.copyFrom(mapped_file_region());
   }
 
   void unmapImpl();
   void dontNeedImpl();
 
   std::error_code init(sys::fs::file_t FD, uint64_t Offset, mapmode Mode);
 
 public:
   mapped_file_region() = default;
   mapped_file_region(mapped_file_region &&Moved) { moveFromImpl(Moved); }
   mapped_file_region &operator=(mapped_file_region &&Moved) {
     unmap();
     moveFromImpl(Moved);
     return *this;
   }
 
   mapped_file_region(const mapped_file_region &) = delete;
   mapped_file_region &operator=(const mapped_file_region &) = delete;
 
   /// \param fd An open file descriptor to map. Does not take ownership of fd.
   mapped_file_region(sys::fs::file_t fd, mapmode mode, size_t length, uint64_t offset,
                      std::error_code &ec);
 
   ~mapped_file_region() { unmapImpl(); }
 
   /// Check if this is a valid mapping.
   explicit operator bool() const { return Mapping; }
 
   /// Unmap.
   void unmap() {
     unmapImpl();
     copyFrom(mapped_file_region());
   }
   void dontNeed() { dontNeedImpl(); }
 
   size_t size() const;
   char *data() const;
 
   /// Get a const view of the data. Modifying this memory has undefined
   /// behavior.
   const char *const_data() const;
 
   /// \returns The minimum alignment offset must be.
   static int alignment();
 };
 
 /// Return the path to the main executable, given the value of argv[0] from
 /// program startup and the address of main itself. In extremis, this function
 /// may fail and return an empty path.
 std::string getMainExecutable(const char *argv0, void *MainExecAddr);
 
 /// @}
 /// @name Iterators
 /// @{
 
 /// directory_entry - A single entry in a directory.
 class directory_entry {
   // FIXME: different platforms make different information available "for free"
   // when traversing a directory. The design of this class wraps most of the
   // information in basic_file_status, so on platforms where we can't populate
   // that whole structure, callers end up paying for a stat().
   // std::filesystem::directory_entry may be a better model.
   std::string Path;
   file_type Type = file_type::type_unknown; // Most platforms can provide this.
   bool FollowSymlinks = true;               // Affects the behavior of status().
   basic_file_status Status;                 // If available.
 
 public:
   explicit directory_entry(const Twine &Path, bool FollowSymlinks = true,
                            file_type Type = file_type::type_unknown,
                            basic_file_status Status = basic_file_status())
       : Path(Path.str()), Type(Type), FollowSymlinks(FollowSymlinks),
         Status(Status) {}
 
   directory_entry() = default;
 
   void replace_filename(const Twine &Filename, file_type Type,
                         basic_file_status Status = basic_file_status());
 
   const std::string &path() const { return Path; }
   // Get basic information about entry file (a subset of fs::status()).
   // On most platforms this is a stat() call.
   // On windows the information was already retrieved from the directory.
   ErrorOr<basic_file_status> status() const;
   // Get the type of this file.
   // On most platforms (Linux/Mac/Windows/BSD), this was already retrieved.
   // On some platforms (e.g. Solaris) this is a stat() call.
   file_type type() const {
     if (Type != file_type::type_unknown)
       return Type;
     auto S = status();
     return S ? S->type() : file_type::type_unknown;
   }
 
   bool operator==(const directory_entry& RHS) const { return Path == RHS.Path; }
   bool operator!=(const directory_entry& RHS) const { return !(*this == RHS); }
   bool operator< (const directory_entry& RHS) const;
   bool operator<=(const directory_entry& RHS) const;
   bool operator> (const directory_entry& RHS) const;
   bool operator>=(const directory_entry& RHS) const;
 };
 
 namespace detail {
 
   struct DirIterState;
 
   std::error_code directory_iterator_construct(DirIterState &, StringRef, bool);
   std::error_code directory_iterator_increment(DirIterState &);
   std::error_code directory_iterator_destruct(DirIterState &);
 
   /// Keeps state for the directory_iterator.
   struct DirIterState {
     ~DirIterState() {
       directory_iterator_destruct(*this);
     }
 
     intptr_t IterationHandle = 0;
     directory_entry CurrentEntry;
   };
 
 } // end namespace detail
 
 /// directory_iterator - Iterates through the entries in path. There is no
 /// operator++ because we need an error_code. If it's really needed we can make
 /// it call report_fatal_error on error.
 class directory_iterator {
   std::shared_ptr<detail::DirIterState> State;
   bool FollowSymlinks = true;
 
 public:
   explicit directory_iterator(const Twine &path, std::error_code &ec,
                               bool follow_symlinks = true)
       : FollowSymlinks(follow_symlinks) {
     State = std::make_shared<detail::DirIterState>();
     SmallString<128> path_storage;
     ec = detail::directory_iterator_construct(
         *State, path.toStringRef(path_storage), FollowSymlinks);
   }
 
   explicit directory_iterator(const directory_entry &de, std::error_code &ec,
                               bool follow_symlinks = true)
       : FollowSymlinks(follow_symlinks) {
     State = std::make_shared<detail::DirIterState>();
     ec = detail::directory_iterator_construct(
         *State, de.path(), FollowSymlinks);
   }
 
   /// Construct end iterator.
   directory_iterator() = default;
 
   // No operator++ because we need error_code.
   directory_iterator &increment(std::error_code &ec) {
     ec = directory_iterator_increment(*State);
     return *this;
   }
 
   const directory_entry &operator*() const { return State->CurrentEntry; }
   const directory_entry *operator->() const { return &State->CurrentEntry; }
 
   bool operator==(const directory_iterator &RHS) const {
     if (State == RHS.State)
       return true;
     if (!RHS.State)
       return State->CurrentEntry == directory_entry();
     if (!State)
       return RHS.State->CurrentEntry == directory_entry();
     return State->CurrentEntry == RHS.State->CurrentEntry;
   }
 
   bool operator!=(const directory_iterator &RHS) const {
     return !(*this == RHS);
   }
 };
 
 namespace detail {
 
   /// Keeps state for the recursive_directory_iterator.
   struct RecDirIterState {
     std::vector<directory_iterator> Stack;
     uint16_t Level = 0;
     bool HasNoPushRequest = false;
   };
 
 } // end namespace detail
 
 /// recursive_directory_iterator - Same as directory_iterator except for it
 /// recurses down into child directories.
 class recursive_directory_iterator {
   std::shared_ptr<detail::RecDirIterState> State;
   bool Follow;
 
 public:
   recursive_directory_iterator() = default;
   explicit recursive_directory_iterator(const Twine &path, std::error_code &ec,
                                         bool follow_symlinks = true)
       : State(std::make_shared<detail::RecDirIterState>()),
         Follow(follow_symlinks) {
     State->Stack.push_back(directory_iterator(path, ec, Follow));
     if (State->Stack.back() == directory_iterator())
       State.reset();
   }
 
   // No operator++ because we need error_code.
   recursive_directory_iterator &increment(std::error_code &ec) {
     const directory_iterator end_itr = {};
 
     if (State->HasNoPushRequest)
       State->HasNoPushRequest = false;
     else {
       file_type type = State->Stack.back()->type();
       if (type == file_type::symlink_file && Follow) {
         // Resolve the symlink: is it a directory to recurse into?
         ErrorOr<basic_file_status> status = State->Stack.back()->status();
         if (status)
           type = status->type();
         // Otherwise broken symlink, and we'll continue.
       }
       if (type == file_type::directory_file) {
         State->Stack.push_back(
             directory_iterator(*State->Stack.back(), ec, Follow));
         if (State->Stack.back() != end_itr) {
           ++State->Level;
           return *this;
         }
         State->Stack.pop_back();
       }
     }
 
     while (!State->Stack.empty()
            && State->Stack.back().increment(ec) == end_itr) {
       State->Stack.pop_back();
       --State->Level;
     }
 
     // Check if we are done. If so, create an end iterator.
     if (State->Stack.empty())
       State.reset();
 
     return *this;
   }
 
   const directory_entry &operator*() const { return *State->Stack.back(); }
   const directory_entry *operator->() const { return &*State->Stack.back(); }
 
   // observers
   /// Gets the current level. Starting path is at level 0.
   int level() const { return State->Level; }
 
   /// Returns true if no_push has been called for this directory_entry.
   bool no_push_request() const { return State->HasNoPushRequest; }
 
   // modifiers
   /// Goes up one level if Level > 0.
   void pop() {
     assert(State && "Cannot pop an end iterator!");
     assert(State->Level > 0 && "Cannot pop an iterator with level < 1");
 
     const directory_iterator end_itr = {};
     std::error_code ec;
     do {
       if (ec)
         report_fatal_error("Error incrementing directory iterator.");
       State->Stack.pop_back();
       --State->Level;
     } while (!State->Stack.empty()
              && State->Stack.back().increment(ec) == end_itr);
 
     // Check if we are done. If so, create an end iterator.
     if (State->Stack.empty())
       State.reset();
   }
 
   /// Does not go down into the current directory_entry.
   void no_push() { State->HasNoPushRequest = true; }
 
   bool operator==(const recursive_directory_iterator &RHS) const {
     return State == RHS.State;
   }
 
   bool operator!=(const recursive_directory_iterator &RHS) const {
     return !(*this == RHS);
   }
 };
 
 /// @}
 
 } // end namespace fs
 } // end namespace sys
 } // end namespace llvm
 
 #endif // LLVM_SUPPORT_FILESYSTEM_H

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