netWritePosix

fn netWritePosix(
    fd: net.Socket.Handle,
    header: []const u8,
    data: []const []const u8,
    splat: usize,
) net.Stream.Writer.Error!usize {
    if (!have_networking) return error.NetworkDown;

    var iovecs: [max_iovecs_len]posix.iovec_const = undefined;
    var msg: posix.msghdr_const = .{
        .name = null,
        .namelen = 0,
        .iov = &iovecs,
        .iovlen = 0,
        .control = null,
        .controllen = 0,
        .flags = 0,
    };
    addBuf(&iovecs, &msg.iovlen, header);
    for (data[0 .. data.len - 1]) |bytes| addBuf(&iovecs, &msg.iovlen, bytes);
    const pattern = data[data.len - 1];

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

    const syscall: Syscall = try .start();
    while (true) {
        const rc = posix.system.sendmsg(fd, &msg, flags);
        switch (posix.errno(rc)) {
            .SUCCESS => {
                syscall.finish();
                return @intCast(rc);
            },
            .INTR => {
                try syscall.checkCancel();
                continue;
            },
            else => |e| {
                syscall.finish();
                switch (e) {
                    .ACCES => |err| return errnoBug(err),
                    .AGAIN => |err| return errnoBug(err),
                    .ALREADY => return error.FastOpenAlreadyInProgress,
                    .BADF => |err| return errnoBug(err), // File descriptor used after closed.
                    .CONNRESET => return error.ConnectionResetByPeer,
                    .DESTADDRREQ => |err| return errnoBug(err), // The socket is not connection-mode, and no peer address is set.
                    .FAULT => |err| return errnoBug(err), // An invalid user space address was specified for an argument.
                    .INVAL => |err| return errnoBug(err), // Invalid argument passed.
                    .ISCONN => |err| return errnoBug(err), // connection-mode socket was connected already but a recipient was specified
                    .MSGSIZE => |err| return errnoBug(err),
                    .NOBUFS => return error.SystemResources,
                    .NOMEM => return error.SystemResources,
                    .NOTSOCK => |err| return errnoBug(err), // The file descriptor sockfd does not refer to a socket.
                    .OPNOTSUPP => |err| return errnoBug(err), // Some bit in the flags argument is inappropriate for the socket type.
                    .PIPE => return error.SocketUnconnected,
                    .AFNOSUPPORT => return error.AddressFamilyUnsupported,
                    .HOSTUNREACH => return error.HostUnreachable,
                    .NETUNREACH => return error.NetworkUnreachable,
                    .NOTCONN => return error.SocketUnconnected,
                    .TIMEDOUT => return error.ConnectionTimedOut,
                    .NETDOWN => return error.NetworkDown,
                    else => |err| return posix.unexpectedErrno(err),
                }
            },
        }
    }
}