Re: [Libguestfs] [p2v PATCH 10/10] nbd.c: bind listening socket without AI_ADDRCONFIG

On 03/31/22 16:44, Richard W.M. Jones wrote: Oh I think understand it now. In wait_for_nbd_server_to_start(), we connect to our nbdkit server child process like any other nbdkit client would, and even read "NBDMAGIC" (common to both old and new style negotiation). The server (the child process) most likely places the next protocol words in its Send-Q as well (IHAVEOPT and handshake flags), and then attempts to read back the client flags. But the "client", wait_for_nbd_server_to_start(), doesn't even attempt reading past byte#7 from its Recv-Q: char magic[8]; /* NBDMAGIC */ size_t bytes_read = 0; ssize_t recvd; ... do { recvd = recv (sockfd, magic, sizeof magic - bytes_read, 0); ... } ... close (sockfd); so when we reach the close(), one of two cases can happen: - the client's Recv-Q has some bytes from the server's IHAVEOPT, and the handshake flags, and throws them away, or - the client's Recv-Q is now empty (it didn't have any bytes beyond NBDMAGIC), but the server's Send-Q is not empty. I think that, in either case, the close() will result in an RST for whatever the server tries to do next with the socket. (Much more likely in the second case, i.e. when the server's Send-Q was not empty at the time of the client's close(), but I think the same happens in the first case too, when the client simply throws away data from its Recv-Q.) Now the question is if we are bothered by this "RST" in the conversion log, as it's expected. I think wait_for_nbd_server_to_start() does add a little bit of safety. Namely, if the child goes away unexpectedly (because the execlp() fails, or the new image exits for whatever reason very soon), we find out about that (with a timeout in the worst case), and don't start up "ssh -R". With wait_for_nbd_server_to_start() removed, this "child failure" will only be visible by virt-v2v attempting to connect back via "ssh -R", and then (with ssh on the p2v box getting an RST), witnessing a similarly abrubpt EOF / RST from sshd on the conversion server. Ultimately virt-v2v will fail, and this will be visible on virt-p2v, but that's a detour. I agree that functionally, wait_for_nbd_server_to_start() is not necessary, as long as the child does not vanish. With the child present, there is always at least one file descriptor pointing to the file description pointing to the listening socket, so the p2v side of "ssh -R" will always have a socket to connect to. I'll append a patch anyway to the series, just for discussion's sake. Yes, the message disappears then. Thanks Laszlo