fileWriteFilePositional

fn fileWriteFilePositional(
    userdata: ?*anyopaque,
    file: File,
    header: []const u8,
    file_reader: *File.Reader,
    limit: Io.Limit,
    offset: u64,
) File.WriteFilePositionalError!usize {
    const ev: *Evented = @ptrCast(@alignCast(userdata));
    const reader_buffered = file_reader.interface.buffered();
    if (reader_buffered.len >= @backingInt(limit)) {
        const n = try fileWritePositional(
            ev,
            file,
            header,
            &.{limit.slice(reader_buffered)},
            1,
            offset,
        );
        file_reader.interface.toss(n -| header.len);
        return n;
    }
    const out_fd = 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) {
                const n = try fileWritePositional(
                    ev,
                    file,
                    header,
                    &.{limit.slice(reader_buffered)},
                    1,
                    offset,
                );
                file_reader.interface.toss(n -| header.len);
                return n;
            } else {
                return error.EndOfStream;
            }
        }
    }

    if (@atomicLoad(UseFcopyfile, &ev.use_fcopyfile, .monotonic) == .disabled)
        return error.Unimplemented;
    if (file_reader.pos != 0) return error.Unimplemented;
    if (offset != 0) return error.Unimplemented;
    if (limit != .unlimited) return error.Unimplemented;
    const size = file_reader.getSize() catch return error.Unimplemented;
    if (header.len != 0 or reader_buffered.len != 0) {
        const n = try fileWritePositional(
            ev,
            file,
            header,
            &.{limit.slice(reader_buffered)},
            1,
            offset,
        );
        file_reader.interface.toss(n -| header.len);
        return n;
    }
    while (true) {
        const rc = c.fcopyfile(in_fd, out_fd, null, .{ .DATA = true });
        switch (c.errno(rc)) {
            .SUCCESS => break,
            .INTR => {},
            .OPNOTSUPP => {
                // Give calling code chance to observe before trying
                // something else.
                @atomicStore(UseFcopyfile, &ev.use_fcopyfile, .disabled, .monotonic);
                return 0;
            },
            else => |e| {
                assert(error.Unexpected == switch (e) {
                    .NOMEM => return error.SystemResources,
                    .INVAL => |err| errnoBug(err),
                    else => |err| unexpectedErrno(err),
                });
                return 0;
            },
        }
    }
    file_reader.pos = size;
    return size;
}