Solo5-hvt reads on network devices returns SOLO5_R_AGAIN on EOF

[reynir]

When a hypercall-initiated read on a network device returns 0 (EOF) the tender returns SOLO5_R_AGAIN. In mirage-net-solo5 this results in listen retrying the read. If the file descriptor is really closed this will end in an infinite loop.

solo5/tenders/hvt/hvt_module_net.c

Lines 77 to 82 in bf29b8a

	ret = read(e->b.hostfd, HVT_CHECKED_GPA_P(hvt, rd->data, rd->len), rd->len);
	if ((ret == 0) ||
	(ret == -1 && errno == EAGAIN)) {
	rd->ret = SOLO5_R_AGAIN;
	return;
	}

The spt target does not do anything special in the case of read(2) returns 0 as far as I can tell.

It is unclear to me if a file descriptor for a tap device could ever return EOF. I observe this by abusing the file descriptor mechanism for passing network devices in https://github.com/reynir/mirage-cat