base/files/file.h

// Copyright 2012 The Chromium Authors
// Use of this source code is governed by a BSD-style license that can be
// found in the LICENSE file.

#ifndef BASE_FILES_FILE_H_
#define BASE_FILES_FILE_H_

#include <stdint.h>

#include <string>

#include "base/base_export.h"
#include "base/containers/span.h"
#include "base/files/file_path.h"
#include "base/files/file_tracing.h"
#include "base/files/platform_file.h"
#include "base/time/time.h"
#include "base/trace_event/base_tracing_forward.h"
#include "build/build_config.h"

struct stat;

namespace base {

using stat_wrapper_t = struct stat;

// Thin wrapper around an OS-level file.
// Note that this class does not provide any support for asynchronous IO, other
// than the ability to create asynchronous handles on Windows.
//
// Note about const: this class does not attempt to determine if the underlying
// file system object is affected by a particular method in order to consider
// that method const or not. Only methods that deal with member variables in an
// obvious non-modifying way are marked as const. Any method that forward calls
// to the OS is not considered const, even if there is no apparent change to
// member variables.
//
// On POSIX, if the given file is a symbolic link, most of the methods apply to
// the file that the symbolic link resolves to.
class BASE_EXPORT File {
 public:
  // FLAG_(OPEN|CREATE).* are mutually exclusive. You should specify exactly one
  // of the five (possibly combining with other flags) when opening or creating
  // a file.
  // FLAG_(WRITE|APPEND) are mutually exclusive. This is so that APPEND behavior
  // will be consistent with O_APPEND on POSIX.
  enum Flags : uint32_t {
    FLAG_OPEN = 1 << 0,            // Opens a file, only if it exists.
    FLAG_CREATE = 1 << 1,          // Creates a new file, only if it does not
                                   // already exist.
    FLAG_OPEN_ALWAYS = 1 << 2,     // May create a new file.
    FLAG_CREATE_ALWAYS = 1 << 3,   // May overwrite an old file.
    FLAG_OPEN_TRUNCATED = 1 << 4,  // Opens a file and truncates it, only if it
                                   // exists.
    FLAG_READ = 1 << 5,
    FLAG_WRITE = 1 << 6,
    FLAG_APPEND = 1 << 7,
    FLAG_WIN_EXCLUSIVE_READ = 1 << 8,   // Windows only. Opposite of SHARE.
    FLAG_WIN_EXCLUSIVE_WRITE = 1 << 9,  // Windows only. Opposite of SHARE.
    FLAG_ASYNC = 1 << 10,
    FLAG_WIN_TEMPORARY = 1 << 11,  // Windows only.
    FLAG_WIN_HIDDEN = 1 << 12,     // Windows only.
    FLAG_DELETE_ON_CLOSE = 1 << 13,
    FLAG_WRITE_ATTRIBUTES = 1 << 14,  // File opened in a mode allowing writing
                                      // attributes, such as with SetTimes().
    FLAG_WIN_SHARE_DELETE = 1 << 15,  // Windows only.
    FLAG_TERMINAL_DEVICE = 1 << 16,   // Serial port flags.
    FLAG_WIN_BACKUP_SEMANTICS = 1 << 17,  // Windows only.
    FLAG_WIN_EXECUTE = 1 << 18,           // Windows only.
    FLAG_WIN_SEQUENTIAL_SCAN = 1 << 19,   // Windows only.
    FLAG_CAN_DELETE_ON_CLOSE = 1 << 20,  // Requests permission to delete a file
                                         // via DeleteOnClose() (Windows only).
                                         // See DeleteOnClose() for details.
    FLAG_WIN_NO_EXECUTE =
        1 << 21,  // Windows only. Marks the file with a deny ACE that prevents
                  // opening the file with EXECUTE access. Cannot be used with
                  // FILE_WIN_EXECUTE flag. See also PreventExecuteMapping.
  };

  // This enum has been recorded in multiple histograms using PlatformFileError
  // enum. If the order of the fields needs to change, please ensure that those
  // histograms are obsolete or have been moved to a different enum.
  //
  // FILE_ERROR_ACCESS_DENIED is returned when a call fails because of a
  // filesystem restriction. FILE_ERROR_SECURITY is returned when a browser
  // policy doesn't allow the operation to be executed.
  enum Error {
    FILE_OK = 0,
    FILE_ERROR_FAILED = -1,
    FILE_ERROR_IN_USE = -2,
    FILE_ERROR_EXISTS = -3,
    FILE_ERROR_NOT_FOUND = -4,
    FILE_ERROR_ACCESS_DENIED = -5,
    FILE_ERROR_TOO_MANY_OPENED = -6,
    FILE_ERROR_NO_MEMORY = -7,
    FILE_ERROR_NO_SPACE = -8,
    FILE_ERROR_NOT_A_DIRECTORY = -9,
    FILE_ERROR_INVALID_OPERATION = -10,
    FILE_ERROR_SECURITY = -11,
    FILE_ERROR_ABORT = -12,
    FILE_ERROR_NOT_A_FILE = -13,
    FILE_ERROR_NOT_EMPTY = -14,
    FILE_ERROR_INVALID_URL = -15,
    FILE_ERROR_IO = -16,
    // Put new entries here and increment FILE_ERROR_MAX.
    FILE_ERROR_MAX = -17
  };

  // This explicit mapping matches both FILE_ on Windows and SEEK_ on Linux.
  enum Whence {
    FROM_BEGIN   = 0,
    FROM_CURRENT = 1,
    FROM_END     = 2
  };

  // Used to hold information about a given file.
  // If you add more fields to this structure (platform-specific fields are OK),
  // make sure to update all functions that use it in file_util_{win|posix}.cc,
  // too, and the ParamTraits<base::File::Info> implementation in
  // ipc/ipc_message_utils.cc.
  struct BASE_EXPORT Info {
    Info();
    ~Info();
#if BUILDFLAG(IS_POSIX) || BUILDFLAG(IS_FUCHSIA)
    // Fills this struct with values from |stat_info|.
    void FromStat(const stat_wrapper_t& stat_info);
#endif

    // The size of the file in bytes.  Undefined when is_directory is true.
    int64_t size = 0;

    // True if the file corresponds to a directory.
    bool is_directory = false;

    // True if the file corresponds to a symbolic link.  For Windows currently
    // not supported and thus always false.
    bool is_symbolic_link = false;

    // The last modified time of a file.
    Time last_modified;

    // The last accessed time of a file.
    Time last_accessed;

    // The creation time of a file.
    Time creation_time;
  };

  File();

  // Creates or opens the given file. This will fail with 'access denied' if the
  // |path| contains path traversal ('..') components.
  File(const FilePath& path, uint32_t flags);

  // Takes ownership of |platform_file| and sets async to false.
  explicit File(ScopedPlatformFile platform_file);
  explicit File(PlatformFile platform_file);

  // Takes ownership of |platform_file| and sets async to the given value.
  // This constructor exists because on Windows you can't check if platform_file
  // is async or not.
  File(ScopedPlatformFile platform_file, bool async);
  File(PlatformFile platform_file, bool async);

  // Creates an object with a specific error_details code.
  explicit File(Error error_details);

  File(File&& other);

  File(const File&) = delete;
  File& operator=(const File&) = delete;

  ~File();

  File& operator=(File&& other);

  // Creates or opens the given file.
  void Initialize(const FilePath& path, uint32_t flags);

  // Returns |true| if the handle / fd wrapped by this object is valid.  This
  // method doesn't interact with the file system and is thus safe to be called
  // from threads that disallow blocking.
  bool IsValid() const;

  // Returns true if a new file was created (or an old one truncated to zero
  // length to simulate a new file, which can happen with
  // FLAG_CREATE_ALWAYS), and false otherwise.
  bool created() const { return created_; }

  // Returns the OS result of opening this file. Note that the way to verify
  // the success of the operation is to use IsValid(), not this method:
  //   File file(path, flags);
  //   if (!file.IsValid())
  //     return;
  Error error_details() const { return error_details_; }

  PlatformFile GetPlatformFile() const;
  PlatformFile TakePlatformFile();

  // Destroying this object closes the file automatically.
  void Close();

  // Changes current position in the file to an |offset| relative to an origin
  // defined by |whence|. Returns the resultant current position in the file
  // (relative to the start) or -1 in case of error.
  int64_t Seek(Whence whence, int64_t offset);

  // Simplified versions of Read() and friends (see below) that check the int
  // return value and just return a boolean. They return true if and only if
  // the function read in / wrote out exactly |data.size()| bytes of data.
  bool ReadAndCheck(int64_t offset, span<uint8_t> data);
  bool ReadAtCurrentPosAndCheck(span<uint8_t> data);
  bool WriteAndCheck(int64_t offset, span<const uint8_t> data);
  bool WriteAtCurrentPosAndCheck(span<const uint8_t> data);

  // Reads the given number of bytes (or until EOF is reached) starting with the
  // given offset. Returns the number of bytes read, or -1 on error. Note that
  // this function makes a best effort to read all data on all platforms, so it
  // is not intended for stream oriented files but instead for cases when the
  // normal expectation is that actually |size| bytes are read unless there is
  // an error.
  int Read(int64_t offset, char* data, int size);

  // Same as above but without seek.
  int ReadAtCurrentPos(char* data, int size);

  // Reads the given number of bytes (or until EOF is reached) starting with the
  // given offset, but does not make any effort to read all data on all
  // platforms. Returns the number of bytes read, or -1 on error.
  int ReadNoBestEffort(int64_t offset, char* data, int size);

  // Same as above but without seek.
  int ReadAtCurrentPosNoBestEffort(char* data, int size);

  // Writes the given buffer into the file at the given offset, overwritting any
  // data that was previously there. Returns the number of bytes written, or -1
  // on error. Note that this function makes a best effort to write all data on
  // all platforms. |data| can be nullptr when |size| is 0.
  // Ignores the offset and writes to the end of the file if the file was opened
  // with FLAG_APPEND.
  int Write(int64_t offset, const char* data, int size);

  // Save as above but without seek.
  int WriteAtCurrentPos(const char* data, int size);

  // Save as above but does not make any effort to write all data on all
  // platforms. Returns the number of bytes written, or -1 on error.
  int WriteAtCurrentPosNoBestEffort(const char* data, int size);

  // Returns the current size of this file, or a negative number on failure.
  int64_t GetLength();

  // Truncates the file to the given length. If |length| is greater than the
  // current size of the file, the file is extended with zeros. If the file
  // doesn't exist, |false| is returned.
  bool SetLength(int64_t length);

  // Instructs the filesystem to flush the file to disk. (POSIX: fsync, Windows:
  // FlushFileBuffers).
  // Calling Flush() does not guarantee file integrity and thus is not a valid
  // substitute for file integrity checks and recovery codepaths for malformed
  // files. It can also be *really* slow, so avoid blocking on Flush(),
  // especially please don't block shutdown on Flush().
  // Latency percentiles of Flush() across all platforms as of July 2016:
  // 50 %     > 5 ms
  // 10 %     > 58 ms
  //  1 %     > 357 ms
  //  0.1 %   > 1.8 seconds
  //  0.01 %  > 7.6 seconds
  bool Flush();

  // Updates the file times.
  bool SetTimes(Time last_access_time, Time last_modified_time);

  // Returns some basic information for the given file.
  bool GetInfo(Info* info);

#if !BUILDFLAG( \
    IS_FUCHSIA)  // Fuchsia's POSIX API does not support file locking.
  enum class LockMode {
    kShared,
    kExclusive,
  };

  // Attempts to take an exclusive write lock on the file. Returns immediately
  // (i.e. does not wait for another process to unlock the file). If the lock
  // was obtained, the result will be FILE_OK. A lock only guarantees
  // that other processes may not also take a lock on the same file with the
  // same API - it may still be opened, renamed, unlinked, etc.
  //
  // Common semantics:
  //  * Locks are held by processes, but not inherited by child processes.
  //  * Locks are released by the OS on file close or process termination.
  //  * Locks are reliable only on local filesystems.
  //  * Duplicated file handles may also write to locked files.
  // Windows-specific semantics:
  //  * Locks are mandatory for read/write APIs, advisory for mapping APIs.
  //  * Within a process, locking the same file (by the same or new handle)
  //    will fail.
  // POSIX-specific semantics:
  //  * Locks are advisory only.
  //  * Within a process, locking the same file (by the same or new handle)
  //    will succeed. The new lock replaces the old lock.
  //  * Closing any descriptor on a given file releases the lock.
  Error Lock(LockMode mode);

  // Unlock a file previously locked.
  Error Unlock();

#endif  // !BUILDFLAG(IS_FUCHSIA)

  // Returns a new object referencing this file for use within the current
  // process. Handling of FLAG_DELETE_ON_CLOSE varies by OS. On POSIX, the File
  // object that was created or initialized with this flag will have unlinked
  // the underlying file when it was created or opened. On Windows, the
  // underlying file is deleted when the last handle to it is closed.
  File Duplicate() const;

  bool async() const { return async_; }

  // Serialise this object into a trace.
  void WriteIntoTrace(perfetto::TracedValue context) const;

#if BUILDFLAG(IS_WIN)
  // Sets or clears the DeleteFile disposition on the file. Returns true if
  // the disposition was set or cleared, as indicated by |delete_on_close|.
  //
  // Microsoft Windows deletes a file only when the DeleteFile disposition is
  // set on a file when the last handle to the last underlying kernel File
  // object is closed. This disposition is be set by:
  // - Calling the Win32 DeleteFile function with the path to a file.
  // - Opening/creating a file with FLAG_DELETE_ON_CLOSE and then closing all
  //   handles to that File object.
  // - Opening/creating a file with FLAG_CAN_DELETE_ON_CLOSE and subsequently
  //   calling DeleteOnClose(true).
  //
  // In all cases, all pre-existing handles to the file must have been opened
  // with FLAG_WIN_SHARE_DELETE. Once the disposition has been set by any of the
  // above means, no new File objects can be created for the file.
  //
  // So:
  // - Use FLAG_WIN_SHARE_DELETE when creating/opening a file to allow another
  //   entity on the system to cause it to be deleted when it is closed. (Note:
  //   another entity can delete the file the moment after it is closed, so not
  //   using this permission doesn't provide any protections.)
  // - Use FLAG_DELETE_ON_CLOSE for any file that is to be deleted after use.
  //   The OS will ensure it is deleted even in the face of process termination.
  //   Note that it's possible for deletion to be cancelled via another File
  //   object referencing the same file using DeleteOnClose(false) to clear the
  //   DeleteFile disposition after the original File is closed.
  // - Use FLAG_CAN_DELETE_ON_CLOSE in conjunction with DeleteOnClose() to alter
  //   the DeleteFile disposition on an open handle. This fine-grained control
  //   allows for marking a file for deletion during processing so that it is
  //   deleted in the event of untimely process termination, and then clearing
  //   this state once the file is suitable for persistence.
  bool DeleteOnClose(bool delete_on_close);
#endif

#if BUILDFLAG(IS_WIN)
  static Error OSErrorToFileError(DWORD last_error);
#elif BUILDFLAG(IS_POSIX) || BUILDFLAG(IS_FUCHSIA)
  static Error OSErrorToFileError(int saved_errno);
#endif

  // Gets the last global error (errno or GetLastError()) and converts it to the
  // closest base::File::Error equivalent via OSErrorToFileError(). The returned
  // value is only trustworthy immediately after another base::File method
  // fails. base::File never resets the global error to zero.
  static Error GetLastFileError();

  // Converts an error value to a human-readable form. Used for logging.
  static std::string ErrorToString(Error error);

#if BUILDFLAG(IS_POSIX) || BUILDFLAG(IS_FUCHSIA)
  // Wrapper for stat().
  static int Stat(const char* path, stat_wrapper_t* sb);
  // Wrapper for fstat().
  static int Fstat(int fd, stat_wrapper_t* sb);
  // Wrapper for lstat().
  static int Lstat(const char* path, stat_wrapper_t* sb);
#endif

  // This function can be used to augment `flags` with the correct flags
  // required to create a File that can be safely passed to an untrusted
  // process. It must be called if the File is intended to be transferred to an
  // untrusted process, but can still be safely called even if the File is not
  // intended to be transferred.
  static constexpr uint32_t AddFlagsForPassingToUntrustedProcess(
      uint32_t flags) {
    if (flags & File::FLAG_WRITE || flags & File::FLAG_APPEND ||
        flags & File::FLAG_WRITE_ATTRIBUTES) {
      flags |= File::FLAG_WIN_NO_EXECUTE;
    }
    return flags;
  }

 private:
  friend class FileTracing::ScopedTrace;

  // Creates or opens the given file. Only called if |path| has no
  // traversal ('..') components.
  void DoInitialize(const FilePath& path, uint32_t flags);

  void SetPlatformFile(PlatformFile file);

  ScopedPlatformFile file_;

  // A path to use for tracing purposes. Set if file tracing is enabled during
  // |Initialize()|.
  FilePath tracing_path_;

  // Object tied to the lifetime of |this| that enables/disables tracing.
  FileTracing::ScopedEnabler trace_enabler_;

  Error error_details_ = FILE_ERROR_FAILED;
  bool created_ = false;
  bool async_ = false;
};

}  // namespace base

#endif  // BASE_FILES_FILE_H_