vhost-device-vsock doesn't support half-open states

[allisonkarlitskaya]

When I use qemu's native vsock support and the guest connects to a socket on the host and then does a partial shutdown (SHUT_WR) of that socket, that information is visible from the host side: attempting to read() from the socket will return 0.

When using vhost-device-vsock, this doesn't happen. Looking at the code there is nowhere that calls shutdown() and no mention of SHUT_WR. Being able to read EOF from the reader side of a socket and then still reply on the writer side is an important part of many protocols.

In particular this makes it very difficult to implement systemd's vmm.notify_socket: the protocol there is:

• receive a connection
• read to EOF
• close the writer end from your side

but the "read to EOF" part will hang forever because both sides are still holding the socket open.

The relevant systemd code is here: https://github.com/systemd/systemd/blob/8685f8edd17a5e465bf05d733ac22c010c464938/src/libsystemd/sd-daemon/sd-daemon.c#L603

Note that they do SHUT_WR and then a read() to wait for the socket to be closed from the other side.

[stefano-garzarella]

@allisonkarlitskaya with "native vsock support" do you mean QEMU using the device emulated by the host kernel (vhost-vsock kernel module), right?

Yeah, I think this device can do something similar, so when receiving the VSOCK_OP_RST can half-close also the unix socket.
I guess here:

vhost-device/vhost-device-vsock/src/thread_backend.rs

Line 381 in a0261d6

if pkt.op() == VSOCK_OP_RST {

we should call something like conn.stream.shutdown() with the right params. (Maybe
StreamType needs to be extended, but should be doable).

Do you have time to try to implement this?