[Libguestfs] nbdcpy: from scratch nbdcopy using io_uring

[Abhay Raj Singh]

> However 6 is probably too low - in nbdcopy we use 64.
I kept it low just to verify the system works in the way I intended it to.

> So you can
> open multiple TCP connections on each side and issue multiple
> commands in flight on each of those connections.
I will look into this, so we open multiple sockets hence get multiple
socket_fds which we can read from and write to?

> The code looks very minimal at the moment.  I'm not very familiar with
> the fmt:: class.
It's just for formatting and output like printf.

>Does it successfully make a handshake to ‘nbdkit -o’
> yet?
I read in the protocol document
https://github.com/NetworkBlockDevice/nbd/blob/master/doc/proto.md#oldstyle-negotiation
that handshake is just about the server sending data to the client on
accept,
I open a socket and connect then read the handshake, and with the data,
then populate NbdConnection variables. All this is done currently in the
constructor of NbdConnection synchronously. So I guess the answer
is yes.

Current data I receive after handshake from source and destination ports
Copying from port 1234 to port 2345.
┌──────────────────────────────┐
│            source            │
└──────────────────────────────┘
NBDMAGIC (0x4e42444d41474943)
0x420281861253
4096
0b0000000000000000
0b0000110101101101
┌──────────────────────────────┐
│         destination          │
└──────────────────────────────┘
NBDMAGIC (0x4e42444d41474943)
0x420281861253
8192
0b0000000000000000
0b0000110101101101

and then the program ends, destructors call disconnecting code.

command to run nbkit server
nbdkit data ' ( 0x55 0xAA )*2048 '  -o -f -p 1234

> I notice that although you create an io_uring you're not using it.
> But I guess that's still to come.
I am making it now done with the NBD read request submission
should be ready soon.

Thanks and regards,
Abhay
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