path: root/lib/std/fs/File.zig

const File = @This();

const builtin = @import("builtin");
const native_os = builtin.os.tag;
const is_windows = native_os == .windows;

const std = @import("../std.zig");
const Io = std.Io;
const Os = std.builtin.Os;
const Allocator = std.mem.Allocator;
const posix = std.posix;
const math = std.math;
const assert = std.debug.assert;
const linux = std.os.linux;
const windows = std.os.windows;
const maxInt = std.math.maxInt;
const Alignment = std.mem.Alignment;

/// The OS-specific file descriptor or file handle.
handle: Handle,

pub const Handle = Io.File.Handle;
pub const Mode = Io.File.Mode;
pub const INode = Io.File.INode;
pub const Uid = posix.uid_t;
pub const Gid = posix.gid_t;
pub const Kind = Io.File.Kind;

/// This is the default mode given to POSIX operating systems for creating
/// files. `0o666` is "-rw-rw-rw-" which is counter-intuitive at first,
/// since most people would expect "-rw-r--r--", for example, when using
/// the `touch` command, which would correspond to `0o644`. However, POSIX
/// libc implementations use `0o666` inside `fopen` and then rely on the
/// process-scoped "umask" setting to adjust this number for file creation.
pub const default_mode: Mode = if (Mode == u0) 0 else 0o666;

/// Deprecated in favor of `Io.File.OpenError`.
pub const OpenError = Io.File.OpenError || error{WouldBlock};
/// Deprecated in favor of `Io.File.OpenMode`.
pub const OpenMode = Io.File.OpenMode;
/// Deprecated in favor of `Io.File.Lock`.
pub const Lock = Io.File.Lock;
/// Deprecated in favor of `Io.File.OpenFlags`.
pub const OpenFlags = Io.File.OpenFlags;

pub const CreateFlags = struct {
    /// Whether the file will be created with read access.
    read: bool = false,

    /// If the file already exists, and is a regular file, and the access
    /// mode allows writing, it will be truncated to length 0.
    truncate: bool = true,

    /// Ensures that this open call creates the file, otherwise causes
    /// `error.PathAlreadyExists` to be returned.
    exclusive: bool = false,

    /// Open the file with an advisory lock to coordinate with other processes
    /// accessing it at the same time. An exclusive lock will prevent other
    /// processes from acquiring a lock. A shared lock will prevent other
    /// processes from acquiring a exclusive lock, but does not prevent
    /// other process from getting their own shared locks.
    ///
    /// The lock is advisory, except on Linux in very specific circumstances[1].
    /// This means that a process that does not respect the locking API can still get access
    /// to the file, despite the lock.
    ///
    /// On these operating systems, the lock is acquired atomically with
    /// opening the file:
    /// * Darwin
    /// * DragonFlyBSD
    /// * FreeBSD
    /// * Haiku
    /// * NetBSD
    /// * OpenBSD
    /// On these operating systems, the lock is acquired via a separate syscall
    /// after opening the file:
    /// * Linux
    /// * Windows
    ///
    /// [1]: https://www.kernel.org/doc/Documentation/filesystems/mandatory-locking.txt
    lock: Lock = .none,

    /// Sets whether or not to wait until the file is locked to return. If set to true,
    /// `error.WouldBlock` will be returned. Otherwise, the file will wait until the file
    /// is available to proceed.
    lock_nonblocking: bool = false,

    /// For POSIX systems this is the file system mode the file will
    /// be created with. On other systems this is always 0.
    mode: Mode = default_mode,
};

pub fn stdout() File {
    return .{ .handle = if (is_windows) windows.peb().ProcessParameters.hStdOutput else posix.STDOUT_FILENO };
}

pub fn stderr() File {
    return .{ .handle = if (is_windows) windows.peb().ProcessParameters.hStdError else posix.STDERR_FILENO };
}

pub fn stdin() File {
    return .{ .handle = if (is_windows) windows.peb().ProcessParameters.hStdInput else posix.STDIN_FILENO };
}

/// Upon success, the stream is in an uninitialized state. To continue using it,
/// you must use the open() function.
pub fn close(self: File) void {
    if (is_windows) {
        windows.CloseHandle(self.handle);
    } else {
        posix.close(self.handle);
    }
}

pub const SyncError = posix.SyncError;

/// Blocks until all pending file contents and metadata modifications
/// for the file have been synchronized with the underlying filesystem.
///
/// Note that this does not ensure that metadata for the
/// directory containing the file has also reached disk.
pub fn sync(self: File) SyncError!void {
    return posix.fsync(self.handle);
}

/// Test whether the file refers to a terminal.
/// See also `getOrEnableAnsiEscapeSupport` and `supportsAnsiEscapeCodes`.
pub fn isTty(self: File) bool {
    return posix.isatty(self.handle);
}

pub fn isCygwinPty(file: File) bool {
    if (builtin.os.tag != .windows) return false;

    const handle = file.handle;

    // If this is a MSYS2/cygwin pty, then it will be a named pipe with a name in one of these formats:
    //   msys-[...]-ptyN-[...]
    //   cygwin-[...]-ptyN-[...]
    //
    // Example: msys-1888ae32e00d56aa-pty0-to-master

    // First, just check that the handle is a named pipe.
    // This allows us to avoid the more costly NtQueryInformationFile call
    // for handles that aren't named pipes.
    {
        var io_status: windows.IO_STATUS_BLOCK = undefined;
        var device_info: windows.FILE_FS_DEVICE_INFORMATION = undefined;
        const rc = windows.ntdll.NtQueryVolumeInformationFile(handle, &io_status, &device_info, @sizeOf(windows.FILE_FS_DEVICE_INFORMATION), .FileFsDeviceInformation);
        switch (rc) {
            .SUCCESS => {},
            else => return false,
        }
        if (device_info.DeviceType != windows.FILE_DEVICE_NAMED_PIPE) return false;
    }

    const name_bytes_offset = @offsetOf(windows.FILE_NAME_INFO, "FileName");
    // `NAME_MAX` UTF-16 code units (2 bytes each)
    // This buffer may not be long enough to handle *all* possible paths
    // (PATH_MAX_WIDE would be necessary for that), but because we only care
    // about certain paths and we know they must be within a reasonable length,
    // we can use this smaller buffer and just return false on any error from
    // NtQueryInformationFile.
    const num_name_bytes = windows.MAX_PATH * 2;
    var name_info_bytes align(@alignOf(windows.FILE_NAME_INFO)) = [_]u8{0} ** (name_bytes_offset + num_name_bytes);

    var io_status_block: windows.IO_STATUS_BLOCK = undefined;
    const rc = windows.ntdll.NtQueryInformationFile(handle, &io_status_block, &name_info_bytes, @intCast(name_info_bytes.len), .FileNameInformation);
    switch (rc) {
        .SUCCESS => {},
        .INVALID_PARAMETER => unreachable,
        else => return false,
    }

    const name_info: *const windows.FILE_NAME_INFO = @ptrCast(&name_info_bytes);
    const name_bytes = name_info_bytes[name_bytes_offset .. name_bytes_offset + name_info.FileNameLength];
    const name_wide = std.mem.bytesAsSlice(u16, name_bytes);
    // The name we get from NtQueryInformationFile will be prefixed with a '\', e.g. \msys-1888ae32e00d56aa-pty0-to-master
    return (std.mem.startsWith(u16, name_wide, &[_]u16{ '\\', 'm', 's', 'y', 's', '-' }) or
        std.mem.startsWith(u16, name_wide, &[_]u16{ '\\', 'c', 'y', 'g', 'w', 'i', 'n', '-' })) and
        std.mem.indexOf(u16, name_wide, &[_]u16{ '-', 'p', 't', 'y' }) != null;
}

/// Returns whether or not ANSI escape codes will be treated as such,
/// and attempts to enable support for ANSI escape codes if necessary
/// (on Windows).
///
/// Returns `true` if ANSI escape codes are supported or support was
/// successfully enabled. Returns false if ANSI escape codes are not
/// supported or support was unable to be enabled.
///
/// See also `supportsAnsiEscapeCodes`.
pub fn getOrEnableAnsiEscapeSupport(self: File) bool {
    if (builtin.os.tag == .windows) {
        var original_console_mode: windows.DWORD = 0;

        // For Windows Terminal, VT Sequences processing is enabled by default.
        if (windows.kernel32.GetConsoleMode(self.handle, &original_console_mode) != 0) {
            if (original_console_mode & windows.ENABLE_VIRTUAL_TERMINAL_PROCESSING != 0) return true;

            // For Windows Console, VT Sequences processing support was added in Windows 10 build 14361, but disabled by default.
            // https://devblogs.microsoft.com/commandline/tmux-support-arrives-for-bash-on-ubuntu-on-windows/
            //
            // Note: In Microsoft's example for enabling virtual terminal processing, it
            // shows attempting to enable `DISABLE_NEWLINE_AUTO_RETURN` as well:
            // https://learn.microsoft.com/en-us/windows/console/console-virtual-terminal-sequences#example-of-enabling-virtual-terminal-processing
            // This is avoided because in the old Windows Console, that flag causes \n (as opposed to \r\n)
            // to behave unexpectedly (the cursor moves down 1 row but remains on the same column).
            // Additionally, the default console mode in Windows Terminal does not have
            // `DISABLE_NEWLINE_AUTO_RETURN` set, so by only enabling `ENABLE_VIRTUAL_TERMINAL_PROCESSING`
            // we end up matching the mode of Windows Terminal.
            const requested_console_modes = windows.ENABLE_VIRTUAL_TERMINAL_PROCESSING;
            const console_mode = original_console_mode | requested_console_modes;
            if (windows.kernel32.SetConsoleMode(self.handle, console_mode) != 0) return true;
        }

        return self.isCygwinPty();
    }
    return self.supportsAnsiEscapeCodes();
}

/// Test whether ANSI escape codes will be treated as such without
/// attempting to enable support for ANSI escape codes.
///
/// See also `getOrEnableAnsiEscapeSupport`.
pub fn supportsAnsiEscapeCodes(self: File) bool {
    if (builtin.os.tag == .windows) {
        var console_mode: windows.DWORD = 0;
        if (windows.kernel32.GetConsoleMode(self.handle, &console_mode) != 0) {
            if (console_mode & windows.ENABLE_VIRTUAL_TERMINAL_PROCESSING != 0) return true;
        }

        return self.isCygwinPty();
    }
    if (builtin.os.tag == .wasi) {
        // WASI sanitizes stdout when fd is a tty so ANSI escape codes
        // will not be interpreted as actual cursor commands, and
        // stderr is always sanitized.
        return false;
    }
    if (self.isTty()) {
        if (self.handle == posix.STDOUT_FILENO or self.handle == posix.STDERR_FILENO) {
            if (posix.getenvZ("TERM")) |term| {
                if (std.mem.eql(u8, term, "dumb"))
                    return false;
            }
        }
        return true;
    }
    return false;
}

pub const SetEndPosError = posix.TruncateError;

/// Shrinks or expands the file.
/// The file offset after this call is left unchanged.
pub fn setEndPos(self: File, length: u64) SetEndPosError!void {
    try posix.ftruncate(self.handle, length);
}

pub const SeekError = posix.SeekError;

/// Repositions read/write file offset relative to the current offset.
/// TODO: integrate with async I/O
pub fn seekBy(self: File, offset: i64) SeekError!void {
    return posix.lseek_CUR(self.handle, offset);
}

/// Repositions read/write file offset relative to the end.
/// TODO: integrate with async I/O
pub fn seekFromEnd(self: File, offset: i64) SeekError!void {
    return posix.lseek_END(self.handle, offset);
}

/// Repositions read/write file offset relative to the beginning.
/// TODO: integrate with async I/O
pub fn seekTo(self: File, offset: u64) SeekError!void {
    return posix.lseek_SET(self.handle, offset);
}

pub const GetSeekPosError = posix.SeekError || StatError;

/// TODO: integrate with async I/O
pub fn getPos(self: File) GetSeekPosError!u64 {
    return posix.lseek_CUR_get(self.handle);
}

pub const GetEndPosError = std.os.windows.GetFileSizeError || StatError;

/// TODO: integrate with async I/O
pub fn getEndPos(self: File) GetEndPosError!u64 {
    if (builtin.os.tag == .windows) {
        return windows.GetFileSizeEx(self.handle);
    }
    return (try self.stat()).size;
}

pub const ModeError = StatError;

/// TODO: integrate with async I/O
pub fn mode(self: File) ModeError!Mode {
    if (builtin.os.tag == .windows) {
        return 0;
    }
    return (try self.stat()).mode;
}

pub const Stat = Io.File.Stat;

pub const StatError = posix.FStatError;

/// Returns `Stat` containing basic information about the `File`.
pub fn stat(self: File) StatError!Stat {
    var threaded: Io.Threaded = .init_single_threaded;
    const io = threaded.ioBasic();
    return Io.File.stat(.{ .handle = self.handle }, io);
}

pub const ChmodError = posix.FChmodError;

/// Changes the mode of the file.
/// The process must have the correct privileges in order to do this
/// successfully, or must have the effective user ID matching the owner
/// of the file.
pub fn chmod(self: File, new_mode: Mode) ChmodError!void {
    try posix.fchmod(self.handle, new_mode);
}

pub const ChownError = posix.FChownError;

/// Changes the owner and group of the file.
/// The process must have the correct privileges in order to do this
/// successfully. The group may be changed by the owner of the file to
/// any group of which the owner is a member. If the owner or group is
/// specified as `null`, the ID is not changed.
pub fn chown(self: File, owner: ?Uid, group: ?Gid) ChownError!void {
    try posix.fchown(self.handle, owner, group);
}

/// Cross-platform representation of permissions on a file.
/// The `readonly` and `setReadonly` are the only methods available across all platforms.
/// Platform-specific functionality is available through the `inner` field.
pub const Permissions = struct {
    /// You may use the `inner` field to use platform-specific functionality
    inner: switch (builtin.os.tag) {
        .windows => PermissionsWindows,
        else => PermissionsUnix,
    },

    const Self = @This();

    /// Returns `true` if permissions represent an unwritable file.
    /// On Unix, `true` is returned only if no class has write permissions.
    pub fn readOnly(self: Self) bool {
        return self.inner.readOnly();
    }

    /// Sets whether write permissions are provided.
    /// On Unix, this affects *all* classes. If this is undesired, use `unixSet`.
    /// This method *DOES NOT* set permissions on the filesystem: use `File.setPermissions(permissions)`
    pub fn setReadOnly(self: *Self, read_only: bool) void {
        self.inner.setReadOnly(read_only);
    }
};

pub const PermissionsWindows = struct {
    attributes: windows.DWORD,

    const Self = @This();

    /// Returns `true` if permissions represent an unwritable file.
    pub fn readOnly(self: Self) bool {
        return self.attributes & windows.FILE_ATTRIBUTE_READONLY != 0;
    }

    /// Sets whether write permissions are provided.
    /// This method *DOES NOT* set permissions on the filesystem: use `File.setPermissions(permissions)`
    pub fn setReadOnly(self: *Self, read_only: bool) void {
        if (read_only) {
            self.attributes |= windows.FILE_ATTRIBUTE_READONLY;
        } else {
            self.attributes &= ~@as(windows.DWORD, windows.FILE_ATTRIBUTE_READONLY);
        }
    }
};

pub const PermissionsUnix = struct {
    mode: Mode,

    const Self = @This();

    /// Returns `true` if permissions represent an unwritable file.
    /// `true` is returned only if no class has write permissions.
    pub fn readOnly(self: Self) bool {
        return self.mode & 0o222 == 0;
    }

    /// Sets whether write permissions are provided.
    /// This affects *all* classes. If this is undesired, use `unixSet`.
    /// This method *DOES NOT* set permissions on the filesystem: use `File.setPermissions(permissions)`
    pub fn setReadOnly(self: *Self, read_only: bool) void {
        if (read_only) {
            self.mode &= ~@as(Mode, 0o222);
        } else {
            self.mode |= @as(Mode, 0o222);
        }
    }

    pub const Class = enum(u2) {
        user = 2,
        group = 1,
        other = 0,
    };

    pub const Permission = enum(u3) {
        read = 0o4,
        write = 0o2,
        execute = 0o1,
    };

    /// Returns `true` if the chosen class has the selected permission.
    /// This method is only available on Unix platforms.
    pub fn unixHas(self: Self, class: Class, permission: Permission) bool {
        const mask = @as(Mode, @intFromEnum(permission)) << @as(u3, @intFromEnum(class)) * 3;
        return self.mode & mask != 0;
    }

    /// Sets the permissions for the chosen class. Any permissions set to `null` are left unchanged.
    /// This method *DOES NOT* set permissions on the filesystem: use `File.setPermissions(permissions)`
    pub fn unixSet(self: *Self, class: Class, permissions: struct {
        read: ?bool = null,
        write: ?bool = null,
        execute: ?bool = null,
    }) void {
        const shift = @as(u3, @intFromEnum(class)) * 3;
        if (permissions.read) |r| {
            if (r) {
                self.mode |= @as(Mode, 0o4) << shift;
            } else {
                self.mode &= ~(@as(Mode, 0o4) << shift);
            }
        }
        if (permissions.write) |w| {
            if (w) {
                self.mode |= @as(Mode, 0o2) << shift;
            } else {
                self.mode &= ~(@as(Mode, 0o2) << shift);
            }
        }
        if (permissions.execute) |x| {
            if (x) {
                self.mode |= @as(Mode, 0o1) << shift;
            } else {
                self.mode &= ~(@as(Mode, 0o1) << shift);
            }
        }
    }

    /// Returns a `Permissions` struct representing the permissions from the passed mode.
    pub fn unixNew(new_mode: Mode) Self {
        return Self{
            .mode = new_mode,
        };
    }
};

pub const SetPermissionsError = ChmodError;

/// Sets permissions according to the provided `Permissions` struct.
/// This method is *NOT* available on WASI
pub fn setPermissions(self: File, permissions: Permissions) SetPermissionsError!void {
    switch (builtin.os.tag) {
        .windows => {
            var io_status_block: windows.IO_STATUS_BLOCK = undefined;
            var info = windows.FILE_BASIC_INFORMATION{
                .CreationTime = 0,
                .LastAccessTime = 0,
                .LastWriteTime = 0,
                .ChangeTime = 0,
                .FileAttributes = permissions.inner.attributes,
            };
            const rc = windows.ntdll.NtSetInformationFile(
                self.handle,
                &io_status_block,
                &info,
                @sizeOf(windows.FILE_BASIC_INFORMATION),
                .FileBasicInformation,
            );
            switch (rc) {
                .SUCCESS => return,
                .INVALID_HANDLE => unreachable,
                .ACCESS_DENIED => return error.AccessDenied,
                else => return windows.unexpectedStatus(rc),
            }
        },
        .wasi => @compileError("Unsupported OS"), // Wasi filesystem does not *yet* support chmod
        else => {
            try self.chmod(permissions.inner.mode);
        },
    }
}

pub const UpdateTimesError = posix.FutimensError || windows.SetFileTimeError;

/// The underlying file system may have a different granularity than nanoseconds,
/// and therefore this function cannot guarantee any precision will be stored.
/// Further, the maximum value is limited by the system ABI. When a value is provided
/// that exceeds this range, the value is clamped to the maximum.
/// TODO: integrate with async I/O
pub fn updateTimes(
    self: File,
    /// access timestamp in nanoseconds
    atime: Io.Timestamp,
    /// last modification timestamp in nanoseconds
    mtime: Io.Timestamp,
) UpdateTimesError!void {
    if (builtin.os.tag == .windows) {
        const atime_ft = windows.nanoSecondsToFileTime(atime);
        const mtime_ft = windows.nanoSecondsToFileTime(mtime);
        return windows.SetFileTime(self.handle, null, &atime_ft, &mtime_ft);
    }
    const times = [2]posix.timespec{
        posix.timespec{
            .sec = math.cast(isize, @divFloor(atime.nanoseconds, std.time.ns_per_s)) orelse maxInt(isize),
            .nsec = math.cast(isize, @mod(atime.nanoseconds, std.time.ns_per_s)) orelse maxInt(isize),
        },
        posix.timespec{
            .sec = math.cast(isize, @divFloor(mtime.nanoseconds, std.time.ns_per_s)) orelse maxInt(isize),
            .nsec = math.cast(isize, @mod(mtime.nanoseconds, std.time.ns_per_s)) orelse maxInt(isize),
        },
    };
    try posix.futimens(self.handle, &times);
}

pub const ReadError = posix.ReadError;
pub const PReadError = posix.PReadError;

pub fn read(self: File, buffer: []u8) ReadError!usize {
    if (is_windows) {
        return windows.ReadFile(self.handle, buffer, null);
    }

    return posix.read(self.handle, buffer);
}

/// On Windows, this function currently does alter the file pointer.
/// https://github.com/ziglang/zig/issues/12783
pub fn pread(self: File, buffer: []u8, offset: u64) PReadError!usize {
    if (is_windows) {
        return windows.ReadFile(self.handle, buffer, offset);
    }

    return posix.pread(self.handle, buffer, offset);
}

/// Deprecated in favor of `Reader`.
pub fn preadAll(self: File, buffer: []u8, offset: u64) PReadError!usize {
    var index: usize = 0;
    while (index != buffer.len) {
        const amt = try self.pread(buffer[index..], offset + index);
        if (amt == 0) break;
        index += amt;
    }
    return index;
}

/// See https://github.com/ziglang/zig/issues/7699
pub fn readv(self: File, iovecs: []const posix.iovec) ReadError!usize {
    if (is_windows) {
        if (iovecs.len == 0) return 0;
        const first = iovecs[0];
        return windows.ReadFile(self.handle, first.base[0..first.len], null);
    }

    return posix.readv(self.handle, iovecs);
}

/// See https://github.com/ziglang/zig/issues/7699
/// On Windows, this function currently does alter the file pointer.
/// https://github.com/ziglang/zig/issues/12783
pub fn preadv(self: File, iovecs: []const posix.iovec, offset: u64) PReadError!usize {
    if (is_windows) {
        if (iovecs.len == 0) return 0;
        const first = iovecs[0];
        return windows.ReadFile(self.handle, first.base[0..first.len], offset);
    }

    return posix.preadv(self.handle, iovecs, offset);
}

pub const WriteError = posix.WriteError;
pub const PWriteError = posix.PWriteError;

pub fn write(self: File, bytes: []const u8) WriteError!usize {
    if (is_windows) {
        return windows.WriteFile(self.handle, bytes, null);
    }

    return posix.write(self.handle, bytes);
}

pub fn writeAll(self: File, bytes: []const u8) WriteError!void {
    var index: usize = 0;
    while (index < bytes.len) {
        index += try self.write(bytes[index..]);
    }
}

/// Deprecated in favor of `Writer`.
pub fn pwriteAll(self: File, bytes: []const u8, offset: u64) PWriteError!void {
    var index: usize = 0;
    while (index < bytes.len) {
        index += try self.pwrite(bytes[index..], offset + index);
    }
}

/// On Windows, this function currently does alter the file pointer.
/// https://github.com/ziglang/zig/issues/12783
pub fn pwrite(self: File, bytes: []const u8, offset: u64) PWriteError!usize {
    if (is_windows) {
        return windows.WriteFile(self.handle, bytes, offset);
    }

    return posix.pwrite(self.handle, bytes, offset);
}

/// See https://github.com/ziglang/zig/issues/7699
pub fn writev(self: File, iovecs: []const posix.iovec_const) WriteError!usize {
    if (is_windows) {
        // TODO improve this to use WriteFileScatter
        if (iovecs.len == 0) return 0;
        const first = iovecs[0];
        return windows.WriteFile(self.handle, first.base[0..first.len], null);
    }

    return posix.writev(self.handle, iovecs);
}

/// See https://github.com/ziglang/zig/issues/7699
/// On Windows, this function currently does alter the file pointer.
/// https://github.com/ziglang/zig/issues/12783
pub fn pwritev(self: File, iovecs: []posix.iovec_const, offset: u64) PWriteError!usize {
    if (is_windows) {
        if (iovecs.len == 0) return 0;
        const first = iovecs[0];
        return windows.WriteFile(self.handle, first.base[0..first.len], offset);
    }

    return posix.pwritev(self.handle, iovecs, offset);
}

/// Deprecated in favor of `Writer`.
pub const CopyRangeError = posix.CopyFileRangeError;

/// Deprecated in favor of `Writer`.
pub fn copyRange(in: File, in_offset: u64, out: File, out_offset: u64, len: u64) CopyRangeError!u64 {
    const adjusted_len = math.cast(usize, len) orelse maxInt(usize);
    const result = try posix.copy_file_range(in.handle, in_offset, out.handle, out_offset, adjusted_len, 0);
    return result;
}

/// Deprecated in favor of `Writer`.
pub fn copyRangeAll(in: File, in_offset: u64, out: File, out_offset: u64, len: u64) CopyRangeError!u64 {
    var total_bytes_copied: u64 = 0;
    var in_off = in_offset;
    var out_off = out_offset;
    while (total_bytes_copied < len) {
        const amt_copied = try copyRange(in, in_off, out, out_off, len - total_bytes_copied);
        if (amt_copied == 0) return total_bytes_copied;
        total_bytes_copied += amt_copied;
        in_off += amt_copied;
        out_off += amt_copied;
    }
    return total_bytes_copied;
}

/// Deprecated in favor of `Io.File.Reader`.
pub const Reader = Io.File.Reader;

pub const Writer = struct {
    file: File,
    err: ?WriteError = null,
    mode: Writer.Mode = .positional,
    /// Tracks the true seek position in the file. To obtain the logical
    /// position, add the buffer size to this value.
    pos: u64 = 0,
    sendfile_err: ?SendfileError = null,
    copy_file_range_err: ?CopyFileRangeError = null,
    fcopyfile_err: ?FcopyfileError = null,
    seek_err: ?Writer.SeekError = null,
    interface: Io.Writer,

    pub const Mode = Reader.Mode;

    pub const SendfileError = error{
        UnsupportedOperation,
        SystemResources,
        InputOutput,
        BrokenPipe,
        WouldBlock,
        Unexpected,
    };

    pub const CopyFileRangeError = std.os.freebsd.CopyFileRangeError || std.os.linux.wrapped.CopyFileRangeError;

    pub const FcopyfileError = error{
        OperationNotSupported,
        OutOfMemory,
        Unexpected,
    };

    pub const SeekError = File.SeekError;

    /// Number of slices to store on the stack, when trying to send as many byte
    /// vectors through the underlying write calls as possible.
    const max_buffers_len = 16;

    pub fn init(file: File, buffer: []u8) Writer {
        return .{
            .file = file,
            .interface = initInterface(buffer),
            .mode = .positional,
        };
    }

    /// Positional is more threadsafe, since the global seek position is not
    /// affected, but when such syscalls are not available, preemptively
    /// initializing in streaming mode will skip a failed syscall.
    pub fn initStreaming(file: File, buffer: []u8) Writer {
        return .{
            .file = file,
            .interface = initInterface(buffer),
            .mode = .streaming,
        };
    }

    pub fn initInterface(buffer: []u8) Io.Writer {
        return .{
            .vtable = &.{
                .drain = drain,
                .sendFile = sendFile,
            },
            .buffer = buffer,
        };
    }

    /// TODO when this logic moves from fs.File to Io.File the io parameter should be deleted
    pub fn moveToReader(w: *Writer, io: Io) Reader {
        defer w.* = undefined;
        return .{
            .io = io,
            .file = .{ .handle = w.file.handle },
            .mode = w.mode,
            .pos = w.pos,
            .interface = Reader.initInterface(w.interface.buffer),
            .seek_err = w.seek_err,
        };
    }

    pub fn drain(io_w: *Io.Writer, data: []const []const u8, splat: usize) Io.Writer.Error!usize {
        const w: *Writer = @alignCast(@fieldParentPtr("interface", io_w));
        const handle = w.file.handle;
        const buffered = io_w.buffered();
        if (is_windows) switch (w.mode) {
            .positional, .positional_reading => {
                if (buffered.len != 0) {
                    const n = windows.WriteFile(handle, buffered, w.pos) catch |err| {
                        w.err = err;
                        return error.WriteFailed;
                    };
                    w.pos += n;
                    return io_w.consume(n);
                }
                for (data[0 .. data.len - 1]) |buf| {
                    if (buf.len == 0) continue;
                    const n = windows.WriteFile(handle, buf, w.pos) catch |err| {
                        w.err = err;
                        return error.WriteFailed;
                    };
                    w.pos += n;
                    return io_w.consume(n);
                }
                const pattern = data[data.len - 1];
                if (pattern.len == 0 or splat == 0) return 0;
                const n = windows.WriteFile(handle, pattern, w.pos) catch |err| {
                    w.err = err;
                    return error.WriteFailed;
                };
                w.pos += n;
                return io_w.consume(n);
            },
            .streaming, .streaming_reading => {
                if (buffered.len != 0) {
                    const n = windows.WriteFile(handle, buffered, null) catch |err| {
                        w.err = err;
                        return error.WriteFailed;
                    };
                    w.pos += n;
                    return io_w.consume(n);
                }
                for (data[0 .. data.len - 1]) |buf| {
                    if (buf.len == 0) continue;
                    const n = windows.WriteFile(handle, buf, null) catch |err| {
                        w.err = err;
                        return error.WriteFailed;
                    };
                    w.pos += n;
                    return io_w.consume(n);
                }
                const pattern = data[data.len - 1];
                if (pattern.len == 0 or splat == 0) return 0;
                const n = windows.WriteFile(handle, pattern, null) catch |err| {
                    w.err = err;
                    return error.WriteFailed;
                };
                w.pos += n;
                return io_w.consume(n);
            },
            .failure => return error.WriteFailed,
        };
        var iovecs: [max_buffers_len]std.posix.iovec_const = undefined;
        var len: usize = 0;
        if (buffered.len > 0) {
            iovecs[len] = .{ .base = buffered.ptr, .len = buffered.len };
            len += 1;
        }
        for (data[0 .. data.len - 1]) |d| {
            if (d.len == 0) continue;
            iovecs[len] = .{ .base = d.ptr, .len = d.len };
            len += 1;
            if (iovecs.len - len == 0) break;
        }
        const pattern = data[data.len - 1];
        if (iovecs.len - len != 0) switch (splat) {
            0 => {},
            1 => if (pattern.len != 0) {
                iovecs[len] = .{ .base = pattern.ptr, .len = pattern.len };
                len += 1;
            },
            else => switch (pattern.len) {
                0 => {},
                1 => {
                    const splat_buffer_candidate = io_w.buffer[io_w.end..];
                    var backup_buffer: [64]u8 = undefined;
                    const splat_buffer = if (splat_buffer_candidate.len >= backup_buffer.len)
                        splat_buffer_candidate
                    else
                        &backup_buffer;
                    const memset_len = @min(splat_buffer.len, splat);
                    const buf = splat_buffer[0..memset_len];
                    @memset(buf, pattern[0]);
                    iovecs[len] = .{ .base = buf.ptr, .len = buf.len };
                    len += 1;
                    var remaining_splat = splat - buf.len;
                    while (remaining_splat > splat_buffer.len and iovecs.len - len != 0) {
                        assert(buf.len == splat_buffer.len);
                        iovecs[len] = .{ .base = splat_buffer.ptr, .len = splat_buffer.len };
                        len += 1;
                        remaining_splat -= splat_buffer.len;
                    }
                    if (remaining_splat > 0 and iovecs.len - len != 0) {
                        iovecs[len] = .{ .base = splat_buffer.ptr, .len = remaining_splat };
                        len += 1;
                    }
                },
                else => for (0..splat) |_| {
                    iovecs[len] = .{ .base = pattern.ptr, .len = pattern.len };
                    len += 1;
                    if (iovecs.len - len == 0) break;
                },
            },
        };
        if (len == 0) return 0;
        switch (w.mode) {
            .positional, .positional_reading => {
                const n = std.posix.pwritev(handle, iovecs[0..len], w.pos) catch |err| switch (err) {
                    error.Unseekable => {
                        w.mode = w.mode.toStreaming();
                        const pos = w.pos;
                        if (pos != 0) {
                            w.pos = 0;
                            w.seekTo(@intCast(pos)) catch {
                                w.mode = .failure;
                                return error.WriteFailed;
                            };
                        }
                        return 0;
                    },
                    else => |e| {
                        w.err = e;
                        return error.WriteFailed;
                    },
                };
                w.pos += n;
                return io_w.consume(n);
            },
            .streaming, .streaming_reading => {
                const n = std.posix.writev(handle, iovecs[0..len]) catch |err| {
                    w.err = err;
                    return error.WriteFailed;
                };
                w.pos += n;
                return io_w.consume(n);
            },
            .failure => return error.WriteFailed,
        }
    }

    pub fn sendFile(
        io_w: *Io.Writer,
        file_reader: *Io.File.Reader,
        limit: Io.Limit,
    ) Io.Writer.FileError!usize {
        const reader_buffered = file_reader.interface.buffered();
        if (reader_buffered.len >= @intFromEnum(limit))
            return sendFileBuffered(io_w, file_reader, limit.slice(reader_buffered));
        const writer_buffered = io_w.buffered();
        const file_limit = @intFromEnum(limit) - reader_buffered.len;
        const w: *Writer = @alignCast(@fieldParentPtr("interface", io_w));
        const out_fd = w.file.handle;
        const in_fd = file_reader.file.handle;

        if (file_reader.size) |size| {
            if (size - file_reader.pos == 0) {
                if (reader_buffered.len != 0) {
                    return sendFileBuffered(io_w, file_reader, reader_buffered);
                } else {
                    return error.EndOfStream;
                }
            }
        }

        if (native_os == .freebsd and w.mode == .streaming) sf: {
            // Try using sendfile on FreeBSD.
            if (w.sendfile_err != null) break :sf;
            const offset = std.math.cast(std.c.off_t, file_reader.pos) orelse break :sf;
            var hdtr_data: std.c.sf_hdtr = undefined;
            var headers: [2]posix.iovec_const = undefined;
            var headers_i: u8 = 0;
            if (writer_buffered.len != 0) {
                headers[headers_i] = .{ .base = writer_buffered.ptr, .len = writer_buffered.len };
                headers_i += 1;
            }
            if (reader_buffered.len != 0) {
                headers[headers_i] = .{ .base = reader_buffered.ptr, .len = reader_buffered.len };
                headers_i += 1;
            }
            const hdtr: ?*std.c.sf_hdtr = if (headers_i == 0) null else b: {
                hdtr_data = .{
                    .headers = &headers,
                    .hdr_cnt = headers_i,
                    .trailers = null,
                    .trl_cnt = 0,
                };
                break :b &hdtr_data;
            };
            var sbytes: std.c.off_t = undefined;
            const nbytes: usize = @min(file_limit, maxInt(usize));
            const flags = 0;
            switch (posix.errno(std.c.sendfile(in_fd, out_fd, offset, nbytes, hdtr, &sbytes, flags))) {
                .SUCCESS, .INTR => {},
                .INVAL, .OPNOTSUPP, .NOTSOCK, .NOSYS => w.sendfile_err = error.UnsupportedOperation,
                .BADF => if (builtin.mode == .Debug) @panic("race condition") else {
                    w.sendfile_err = error.Unexpected;
                },
                .FAULT => if (builtin.mode == .Debug) @panic("segmentation fault") else {
                    w.sendfile_err = error.Unexpected;
                },
                .NOTCONN => w.sendfile_err = error.BrokenPipe,
                .AGAIN, .BUSY => if (sbytes == 0) {
                    w.sendfile_err = error.WouldBlock;
                },
                .IO => w.sendfile_err = error.InputOutput,
                .PIPE => w.sendfile_err = error.BrokenPipe,
                .NOBUFS => w.sendfile_err = error.SystemResources,
                else => |err| w.sendfile_err = posix.unexpectedErrno(err),
            }
            if (w.sendfile_err != null) {
                // Give calling code chance to observe the error before trying
                // something else.
                return 0;
            }
            if (sbytes == 0) {
                file_reader.size = file_reader.pos;
                return error.EndOfStream;
            }
            const consumed = io_w.consume(@intCast(sbytes));
            file_reader.seekBy(@intCast(consumed)) catch return error.ReadFailed;
            return consumed;
        }

        if (native_os.isDarwin() and w.mode == .streaming) sf: {
            // Try using sendfile on macOS.
            if (w.sendfile_err != null) break :sf;
            const offset = std.math.cast(std.c.off_t, file_reader.pos) orelse break :sf;
            var hdtr_data: std.c.sf_hdtr = undefined;
            var headers: [2]posix.iovec_const = undefined;
            var headers_i: u8 = 0;
            if (writer_buffered.len != 0) {
                headers[headers_i] = .{ .base = writer_buffered.ptr, .len = writer_buffered.len };
                headers_i += 1;
            }
            if (reader_buffered.len != 0) {
                headers[headers_i] = .{ .base = reader_buffered.ptr, .len = reader_buffered.len };
                headers_i += 1;
            }
            const hdtr: ?*std.c.sf_hdtr = if (headers_i == 0) null else b: {
                hdtr_data = .{
                    .headers = &headers,
                    .hdr_cnt = headers_i,
                    .trailers = null,
                    .trl_cnt = 0,
                };
                break :b &hdtr_data;
            };
            const max_count = maxInt(i32); // Avoid EINVAL.
            var len: std.c.off_t = @min(file_limit, max_count);
            const flags = 0;
            switch (posix.errno(std.c.sendfile(in_fd, out_fd, offset, &len, hdtr, flags))) {
                .SUCCESS, .INTR => {},
                .OPNOTSUPP, .NOTSOCK, .NOSYS => w.sendfile_err = error.UnsupportedOperation,
                .BADF => if (builtin.mode == .Debug) @panic("race condition") else {
                    w.sendfile_err = error.Unexpected;
                },
                .FAULT => if (builtin.mode == .Debug) @panic("segmentation fault") else {
                    w.sendfile_err = error.Unexpected;
                },
                .INVAL => if (builtin.mode == .Debug) @panic("invalid API usage") else {
                    w.sendfile_err = error.Unexpected;
                },
                .NOTCONN => w.sendfile_err = error.BrokenPipe,
                .AGAIN => if (len == 0) {
                    w.sendfile_err = error.WouldBlock;
                },
                .IO => w.sendfile_err = error.InputOutput,
                .PIPE => w.sendfile_err = error.BrokenPipe,
                else => |err| w.sendfile_err = posix.unexpectedErrno(err),
            }
            if (w.sendfile_err != null) {
                // Give calling code chance to observe the error before trying
                // something else.
                return 0;
            }
            if (len == 0) {
                file_reader.size = file_reader.pos;
                return error.EndOfStream;
            }
            const consumed = io_w.consume(@bitCast(len));
            file_reader.seekBy(@intCast(consumed)) catch return error.ReadFailed;
            return consumed;
        }

        if (native_os == .linux and w.mode == .streaming) sf: {
            // Try using sendfile on Linux.
            if (w.sendfile_err != null) break :sf;
            // Linux sendfile does not support headers.
            if (writer_buffered.len != 0 or reader_buffered.len != 0)
                return sendFileBuffered(io_w, file_reader, reader_buffered);
            const max_count = 0x7ffff000; // Avoid EINVAL.
            var off: std.os.linux.off_t = undefined;
            const off_ptr: ?*std.os.linux.off_t, const count: usize = switch (file_reader.mode) {
                .positional => o: {
                    const size = file_reader.getSize() catch return 0;
                    off = std.math.cast(std.os.linux.off_t, file_reader.pos) orelse return error.ReadFailed;
                    break :o .{ &off, @min(@intFromEnum(limit), size - file_reader.pos, max_count) };
                },
                .streaming => .{ null, limit.minInt(max_count) },
                .streaming_reading, .positional_reading => break :sf,
                .failure => return error.ReadFailed,
            };
            const n = std.os.linux.wrapped.sendfile(out_fd, in_fd, off_ptr, count) catch |err| switch (err) {
                error.Unseekable => {
                    file_reader.mode = file_reader.mode.toStreaming();
                    const pos = file_reader.pos;
                    if (pos != 0) {
                        file_reader.pos = 0;
                        file_reader.seekBy(@intCast(pos)) catch {
                            file_reader.mode = .failure;
                            return error.ReadFailed;
                        };
                    }
                    return 0;
                },
                else => |e| {
                    w.sendfile_err = e;
                    return 0;
                },
            };
            if (n == 0) {
                file_reader.size = file_reader.pos;
                return error.EndOfStream;
            }
            file_reader.pos += n;
            w.pos += n;
            return n;
        }

        const copy_file_range = switch (native_os) {
            .freebsd => std.os.freebsd.copy_file_range,
            .linux => std.os.linux.wrapped.copy_file_range,
            else => {},
        };
        if (@TypeOf(copy_file_range) != void) cfr: {
            if (w.copy_file_range_err != null) break :cfr;
            if (writer_buffered.len != 0 or reader_buffered.len != 0)
                return sendFileBuffered(io_w, file_reader, reader_buffered);
            var off_in: i64 = undefined;
            var off_out: i64 = undefined;
            const off_in_ptr: ?*i64 = switch (file_reader.mode) {
                .positional_reading, .streaming_reading => return error.Unimplemented,
                .positional => p: {
                    off_in = @intCast(file_reader.pos);
                    break :p &off_in;
                },
                .streaming => null,
                .failure => return error.WriteFailed,
            };
            const off_out_ptr: ?*i64 = switch (w.mode) {
                .positional_reading, .streaming_reading => return error.Unimplemented,
                .positional => p: {
                    off_out = @intCast(w.pos);
                    break :p &off_out;
                },
                .streaming => null,
                .failure => return error.WriteFailed,
            };
            const n = copy_file_range(in_fd, off_in_ptr, out_fd, off_out_ptr, @intFromEnum(limit), 0) catch |err| {
                w.copy_file_range_err = err;
                return 0;
            };
            if (n == 0) {
                file_reader.size = file_reader.pos;
                return error.EndOfStream;
            }
            file_reader.pos += n;
            w.pos += n;
            return n;
        }

        if (builtin.os.tag.isDarwin()) fcf: {
            if (w.fcopyfile_err != null) break :fcf;
            if (file_reader.pos != 0) break :fcf;
            if (w.pos != 0) break :fcf;
            if (limit != .unlimited) break :fcf;
            const size = file_reader.getSize() catch break :fcf;
            if (writer_buffered.len != 0 or reader_buffered.len != 0)
                return sendFileBuffered(io_w, file_reader, reader_buffered);
            const rc = std.c.fcopyfile(in_fd, out_fd, null, .{ .DATA = true });
            switch (posix.errno(rc)) {
                .SUCCESS => {},
                .INVAL => if (builtin.mode == .Debug) @panic("invalid API usage") else {
                    w.fcopyfile_err = error.Unexpected;
                    return 0;
                },
                .NOMEM => {
                    w.fcopyfile_err = error.OutOfMemory;
                    return 0;
                },
                .OPNOTSUPP => {
                    w.fcopyfile_err = error.OperationNotSupported;
                    return 0;
                },
                else => |err| {
                    w.fcopyfile_err = posix.unexpectedErrno(err);
                    return 0;
                },
            }
            file_reader.pos = size;
            w.pos = size;
            return size;
        }

        return error.Unimplemented;
    }

    fn sendFileBuffered(
        io_w: *Io.Writer,
        file_reader: *Io.File.Reader,
        reader_buffered: []const u8,
    ) Io.Writer.FileError!usize {
        const n = try drain(io_w, &.{reader_buffered}, 1);
        file_reader.seekBy(@intCast(n)) catch return error.ReadFailed;
        return n;
    }

    pub fn seekTo(w: *Writer, offset: u64) (Writer.SeekError || Io.Writer.Error)!void {
        try w.interface.flush();
        try seekToUnbuffered(w, offset);
    }

    /// Asserts that no data is currently buffered.
    pub fn seekToUnbuffered(w: *Writer, offset: u64) Writer.SeekError!void {
        assert(w.interface.buffered().len == 0);
        switch (w.mode) {
            .positional, .positional_reading => {
                w.pos = offset;
            },
            .streaming, .streaming_reading => {
                if (w.seek_err) |err| return err;
                posix.lseek_SET(w.file.handle, offset) catch |err| {
                    w.seek_err = err;
                    return err;
                };
                w.pos = offset;
            },
            .failure => return w.seek_err.?,
        }
    }

    pub const EndError = SetEndPosError || Io.Writer.Error;

    /// Flushes any buffered data and sets the end position of the file.
    ///
    /// If not overwriting existing contents, then calling `interface.flush`
    /// directly is sufficient.
    ///
    /// Flush failure is handled by setting `err` so that it can be handled
    /// along with other write failures.
    pub fn end(w: *Writer) EndError!void {
        try w.interface.flush();
        switch (w.mode) {
            .positional,
            .positional_reading,
            => w.file.setEndPos(w.pos) catch |err| switch (err) {
                error.NonResizable => return,
                else => |e| return e,
            },

            .streaming,
            .streaming_reading,
            .failure,
            => {},
        }
    }
};

/// Defaults to positional reading; falls back to streaming.
///
/// Positional is more threadsafe, since the global seek position is not
/// affected.
pub fn reader(file: File, io: Io, buffer: []u8) Reader {
    return .init(.{ .handle = file.handle }, io, buffer);
}

/// Positional is more threadsafe, since the global seek position is not
/// affected, but when such syscalls are not available, preemptively
/// initializing in streaming mode skips a failed syscall.
pub fn readerStreaming(file: File, io: Io, buffer: []u8) Reader {
    return .initStreaming(.{ .handle = file.handle }, io, buffer);
}

/// Defaults to positional reading; falls back to streaming.
///
/// Positional is more threadsafe, since the global seek position is not
/// affected.
pub fn writer(file: File, buffer: []u8) Writer {
    return .init(file, buffer);
}

/// Positional is more threadsafe, since the global seek position is not
/// affected, but when such syscalls are not available, preemptively
/// initializing in streaming mode will skip a failed syscall.
pub fn writerStreaming(file: File, buffer: []u8) Writer {
    return .initStreaming(file, buffer);
}

const range_off: windows.LARGE_INTEGER = 0;
const range_len: windows.LARGE_INTEGER = 1;

pub const LockError = error{
    SystemResources,
    FileLocksNotSupported,
} || posix.UnexpectedError;

/// Blocks when an incompatible lock is held by another process.
/// A process may hold only one type of lock (shared or exclusive) on
/// a file. When a process terminates in any way, the lock is released.
///
/// Assumes the file is unlocked.
///
/// TODO: integrate with async I/O
pub fn lock(file: File, l: Lock) LockError!void {
    if (is_windows) {
        var io_status_block: windows.IO_STATUS_BLOCK = undefined;
        const exclusive = switch (l) {
            .none => return,
            .shared => false,
            .exclusive => true,
        };
        return windows.LockFile(
            file.handle,
            null,
            null,
            null,
            &io_status_block,
            &range_off,
            &range_len,
            null,
            windows.FALSE, // non-blocking=false
            @intFromBool(exclusive),
        ) catch |err| switch (err) {
            error.WouldBlock => unreachable, // non-blocking=false
            else => |e| return e,
        };
    } else {
        return posix.flock(file.handle, switch (l) {
            .none => posix.LOCK.UN,
            .shared => posix.LOCK.SH,
            .exclusive => posix.LOCK.EX,
        }) catch |err| switch (err) {
            error.WouldBlock => unreachable, // non-blocking=false
            else => |e| return e,
        };
    }
}

/// Assumes the file is locked.
pub fn unlock(file: File) void {
    if (is_windows) {
        var io_status_block: windows.IO_STATUS_BLOCK = undefined;
        return windows.UnlockFile(
            file.handle,
            &io_status_block,
            &range_off,
            &range_len,
            null,
        ) catch |err| switch (err) {
            error.RangeNotLocked => unreachable, // Function assumes unlocked.
            error.Unexpected => unreachable, // Resource deallocation must succeed.
        };
    } else {
        return posix.flock(file.handle, posix.LOCK.UN) catch |err| switch (err) {
            error.WouldBlock => unreachable, // unlocking can't block
            error.SystemResources => unreachable, // We are deallocating resources.
            error.FileLocksNotSupported => unreachable, // We already got the lock.
            error.Unexpected => unreachable, // Resource deallocation must succeed.
        };
    }
}

/// Attempts to obtain a lock, returning `true` if the lock is
/// obtained, and `false` if there was an existing incompatible lock held.
/// A process may hold only one type of lock (shared or exclusive) on
/// a file. When a process terminates in any way, the lock is released.
///
/// Assumes the file is unlocked.
///
/// TODO: integrate with async I/O
pub fn tryLock(file: File, l: Lock) LockError!bool {
    if (is_windows) {
        var io_status_block: windows.IO_STATUS_BLOCK = undefined;
        const exclusive = switch (l) {
            .none => return,
            .shared => false,
            .exclusive => true,
        };
        windows.LockFile(
            file.handle,
            null,
            null,
            null,
            &io_status_block,
            &range_off,
            &range_len,
            null,
            windows.TRUE, // non-blocking=true
            @intFromBool(exclusive),
        ) catch |err| switch (err) {
            error.WouldBlock => return false,
            else => |e| return e,
        };
    } else {
        posix.flock(file.handle, switch (l) {
            .none => posix.LOCK.UN,
            .shared => posix.LOCK.SH | posix.LOCK.NB,
            .exclusive => posix.LOCK.EX | posix.LOCK.NB,
        }) catch |err| switch (err) {
            error.WouldBlock => return false,
            else => |e| return e,
        };
    }
    return true;
}

/// Assumes the file is already locked in exclusive mode.
/// Atomically modifies the lock to be in shared mode, without releasing it.
///
/// TODO: integrate with async I/O
pub fn downgradeLock(file: File) LockError!void {
    if (is_windows) {
        // On Windows it works like a semaphore + exclusivity flag. To implement this
        // function, we first obtain another lock in shared mode. This changes the
        // exclusivity flag, but increments the semaphore to 2. So we follow up with
        // an NtUnlockFile which decrements the semaphore but does not modify the
        // exclusivity flag.
        var io_status_block: windows.IO_STATUS_BLOCK = undefined;
        windows.LockFile(
            file.handle,
            null,
            null,
            null,
            &io_status_block,
            &range_off,
            &range_len,
            null,
            windows.TRUE, // non-blocking=true
            windows.FALSE, // exclusive=false
        ) catch |err| switch (err) {
            error.WouldBlock => unreachable, // File was not locked in exclusive mode.
            else => |e| return e,
        };
        return windows.UnlockFile(
            file.handle,
            &io_status_block,
            &range_off,
            &range_len,
            null,
        ) catch |err| switch (err) {
            error.RangeNotLocked => unreachable, // File was not locked.
            error.Unexpected => unreachable, // Resource deallocation must succeed.
        };
    } else {
        return posix.flock(file.handle, posix.LOCK.SH | posix.LOCK.NB) catch |err| switch (err) {
            error.WouldBlock => unreachable, // File was not locked in exclusive mode.
            else => |e| return e,
        };
    }
}

pub fn adaptToNewApi(file: File) Io.File {
    return .{ .handle = file.handle };
}

pub fn adaptFromNewApi(file: Io.File) File {
    return .{ .handle = file.handle };
}