sync: split init config (stream) and synchronisation (seqpacket) pipes

We have different requirements for the initial configuration and
initWaiter pipe (just send netlink and JSON blobs with no complicated
handling needed for message coalescing) and the packet-based
synchronisation pipe.

Tests with switching everything to SOCK_SEQPACKET lead to endless issues
with runc hanging on start-up because random things would try to do
short reads (which SOCK_SEQPACKET will not allow and the Go stdlib
explicitly treats as a streaming source), so splitting it was the only
reasonable solution. Even doing somewhat dodgy tricks such as adding a
Read() wrapper which actually calls ReadPacket() and makes it seem like
a stream source doesn't work -- and is a bit too magical.

One upside is that doing it this way makes the difference between the
modes clearer -- INITPIPE is still used for initWaiter syncrhonisation
but aside from that all other synchronisation is done by SYNCPIPE.

Signed-off-by: Aleksa Sarai <cyphar@cyphar.com>

libcontainer/utils/utils_unix.go

@@ -90,7 +90,7 @@ func CloseExecFrom(minFd int) error {
 	return nil
 }
 
-// NewSockPair returns a new unix socket pair
+// NewSockPair returns a new SOCK_STREAM unix socket pair.
 func NewSockPair(name string) (parent, child *os.File, err error) {
 	fds, err := unix.Socketpair(unix.AF_LOCAL, unix.SOCK_STREAM|unix.SOCK_CLOEXEC, 0)
 	if err != nil {