fileWritePositional

fn fileWritePositional(
    userdata: ?*anyopaque,
    file: File,
    header: []const u8,
    data: []const []const u8,
    splat: usize,
    offset: u64,
) File.WritePositionalError!usize {
    const t: *Threaded = @ptrCast(@alignCast(userdata));
    _ = t;

    if (is_windows) {
        if (header.len != 0) {
            return writeFilePositionalWindows(file, header, offset);
        }
        for (data[0 .. data.len - 1]) |buf| {
            if (buf.len == 0) continue;
            return writeFilePositionalWindows(file, buf, offset);
        }
        const pattern = data[data.len - 1];
        if (pattern.len == 0 or splat == 0) return 0;
        return writeFilePositionalWindows(file, pattern, offset);
    }

    var iovecs: [max_iovecs_len]posix.iovec_const = undefined;
    var iovlen: iovlen_t = 0;
    addBuf(&iovecs, &iovlen, header);
    for (data[0 .. data.len - 1]) |bytes| addBuf(&iovecs, &iovlen, bytes);
    const pattern = data[data.len - 1];

    var splat_backup_buffer: [splat_buffer_size]u8 = undefined;
    if (iovecs.len - iovlen != 0) switch (splat) {
        0 => {},
        1 => addBuf(&iovecs, &iovlen, pattern),
        else => switch (pattern.len) {
            0 => {},
            1 => {
                const splat_buffer = &splat_backup_buffer;
                const memset_len = @min(splat_buffer.len, splat);
                const buf = splat_buffer[0..memset_len];
                @memset(buf, pattern[0]);
                addBuf(&iovecs, &iovlen, buf);
                var remaining_splat = splat - buf.len;
                while (remaining_splat > splat_buffer.len and iovecs.len - iovlen != 0) {
                    assert(buf.len == splat_buffer.len);
                    addBuf(&iovecs, &iovlen, splat_buffer);
                    remaining_splat -= splat_buffer.len;
                }
                addBuf(&iovecs, &iovlen, splat_buffer[0..@min(remaining_splat, splat_buffer.len)]);
            },
            else => for (0..@min(splat, iovecs.len - iovlen)) |_| {
                addBuf(&iovecs, &iovlen, pattern);
            },
        },
    };

    if (iovlen == 0) return 0;

    if (native_os == .wasi and !builtin.link_libc) {
        var n_written: usize = undefined;
        const syscall: Syscall = try .start();
        while (true) {
            switch (std.os.wasi.fd_pwrite(file.handle, &iovecs, iovlen, offset, &n_written)) {
                .SUCCESS => {
                    syscall.finish();
                    return n_written;
                },
                .INTR => {
                    try syscall.checkCancel();
                    continue;
                },
                else => |e| {
                    syscall.finish();
                    switch (e) {
                        .INVAL => |err| return errnoBug(err),
                        .FAULT => |err| return errnoBug(err),
                        .AGAIN => |err| return errnoBug(err),
                        .BADF => return error.NotOpenForWriting,
                        .DESTADDRREQ => |err| return errnoBug(err), // `connect` was never called.
                        .DQUOT => return error.DiskQuota,
                        .FBIG => return error.FileTooBig,
                        .IO => return error.InputOutput,
                        .NOSPC => return error.NoSpaceLeft,
                        .PERM => return error.PermissionDenied,
                        .PIPE => return error.BrokenPipe,
                        .NOTCAPABLE => return error.AccessDenied,
                        .NXIO => return error.Unseekable,
                        .SPIPE => return error.Unseekable,
                        .OVERFLOW => return error.Unseekable,
                        else => |err| return posix.unexpectedErrno(err),
                    }
                },
            }
        }
    }

    const syscall: Syscall = try .start();
    while (true) {
        const rc = if (native_os == .haiku)
            posix.system.writev_pos(file.handle, @bitCast(offset), &iovecs, @intCast(iovlen))
        else
            pwritev_sym(file.handle, &iovecs, @intCast(iovlen), @bitCast(offset));
        switch (posix.errno(rc)) {
            .SUCCESS => {
                syscall.finish();
                return @intCast(rc);
            },
            .INTR => {
                try syscall.checkCancel();
                continue;
            },
            .INVAL => |err| return syscall.errnoBug(err),
            .FAULT => |err| return syscall.errnoBug(err),
            .DESTADDRREQ => |err| return syscall.errnoBug(err), // `connect` was never called.
            .CONNRESET => |err| return syscall.errnoBug(err), // Not a socket handle.
            .BADF => return syscall.fail(error.NotOpenForWriting),
            .AGAIN => return syscall.fail(error.WouldBlock),
            .DQUOT => return syscall.fail(error.DiskQuota),
            .FBIG => return syscall.fail(error.FileTooBig),
            .IO => return syscall.fail(error.InputOutput),
            .NOSPC => return syscall.fail(error.NoSpaceLeft),
            .PERM => return syscall.fail(error.PermissionDenied),
            .PIPE => return syscall.fail(error.BrokenPipe),
            .BUSY => return syscall.fail(error.DeviceBusy),
            .TXTBSY => return syscall.fail(error.FileBusy),
            .NXIO => return syscall.fail(error.Unseekable),
            .SPIPE => return syscall.fail(error.Unseekable),
            .OVERFLOW => return syscall.fail(error.Unseekable),
            else => |err| return syscall.unexpectedErrno(err),
        }
    }
}