fileWritePositional

fn fileWritePositional(
    userdata: ?*anyopaque,
    file: File,
    header: []const u8,
    data: []const []const u8,
    splat: usize,
    offset: u64,
) File.WritePositionalError!usize {
    const ev: *Evented = @ptrCast(@alignCast(userdata));
    _ = ev;
    var iovecs: [max_iovecs_len]iovec_const = undefined;
    var iovlen: iovlen_t = 0;
    var remaining: Io.Limit = .unlimited;
    addBuf(true, &iovecs, &iovlen, &remaining, header);
    for (data[0 .. data.len - 1]) |bytes| addBuf(true, &iovecs, &iovlen, &remaining, bytes);
    const pattern = data[data.len - 1];
    var backup_buffer: [splat_buffer_size]u8 = undefined;
    if (iovecs.len - iovlen != 0 and remaining != .nothing) switch (splat) {
        0 => {},
        1 => addBuf(true, &iovecs, &iovlen, &remaining, pattern),
        else => switch (pattern.len) {
            0 => {},
            1 => {
                const splat_buffer = &backup_buffer;
                const memset_len = @min(splat_buffer.len, splat);
                const buf = splat_buffer[0..memset_len];
                @memset(buf, pattern[0]);
                addBuf(true, &iovecs, &iovlen, &remaining, buf);
                var remaining_splat = splat - buf.len;
                while (remaining_splat > splat_buffer.len and iovecs.len - iovlen != 0 and remaining != .nothing) {
                    assert(buf.len == splat_buffer.len);
                    addBuf(true, &iovecs, &iovlen, &remaining, splat_buffer);
                    remaining_splat -= splat_buffer.len;
                }
                addBuf(true, &iovecs, &iovlen, &remaining, splat_buffer[0..@min(remaining_splat, splat_buffer.len)]);
            },
            else => for (0..@min(splat, iovecs.len - iovlen)) |_| {
                if (remaining == .nothing) break;
                addBuf(true, &iovecs, &iovlen, &remaining, pattern);
            },
        },
    };
    if (iovlen == 0) return 0;
    while (true) {
        const rc = c.pwritev(file.handle, &iovecs, iovlen, @bitCast(offset));
        switch (c.errno(rc)) {
            .SUCCESS => return @intCast(rc),
            .INTR => {},
            .INVAL => |err| return errnoBug(err),
            .FAULT => |err| return errnoBug(err),
            .DESTADDRREQ => |err| return errnoBug(err), // `connect` was never called.
            .CONNRESET => |err| return errnoBug(err), // Not a socket handle.
            .BADF => return error.NotOpenForWriting,
            .AGAIN => return error.WouldBlock,
            .DQUOT => return error.DiskQuota,
            .FBIG => return error.FileTooBig,
            .IO => return error.InputOutput,
            .NOSPC => return error.NoSpaceLeft,
            .PERM => return error.PermissionDenied,
            .PIPE => return error.BrokenPipe,
            .BUSY => return error.DeviceBusy,
            .TXTBSY => return error.FileBusy,
            .NXIO => return error.Unseekable,
            .SPIPE => return error.Unseekable,
            .OVERFLOW => return error.Unseekable,
            else => |err| return unexpectedErrno(err),
        }
    }
}