net/socket: learn to talk with a unix dgram socket

-net socket has a fd argument, and may be passed pre-opened sockets.

TCP sockets use framing.
UDP sockets have datagram boundaries.

When given a unix dgram socket, it will be able to read from it, but
will attempt to send on the dgram_dst, which is unset. The other end
will not receive the data.

Let's teach -net socket to recognize a UNIX DGRAM socket, and use the
regular send() command (without dgram_dst).

This makes running slirp out-of-process possible that
way (python pseudo-code):

a, b = socket.socketpair(socket.AF_UNIX, socket.SOCK_DGRAM)

subprocess.Popen('qemu -net socket,fd=%d -net user' % a.fileno(), shell=True)
subprocess.Popen('qemu ... -net nic -net socket,fd=%d' % b.fileno(), shell=True)

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Signed-off-by: Jason Wang <jasowang@redhat.com>

net/socket.c

@@ -119,9 +119,13 @@ static ssize_t net_socket_receive_dgram(NetClientState *nc, const uint8_t *buf,
     ssize_t ret;
 
     do {
-        ret = qemu_sendto(s->fd, buf, size, 0,
+        if (s->dgram_dst.sin_family != AF_UNIX) {
-                          (struct sockaddr *)&s->dgram_dst,
+            ret = qemu_sendto(s->fd, buf, size, 0,
-                          sizeof(s->dgram_dst));
+                              (struct sockaddr *)&s->dgram_dst,
+                              sizeof(s->dgram_dst));
+        } else {
+            ret = send(s->fd, buf, size, 0);
+        }
     } while (ret == -1 && errno == EINTR);
 
     if (ret == -1 && errno == EAGAIN) {
@@ -336,6 +340,15 @@ static NetSocketState *net_socket_fd_init_dgram(NetClientState *peer,
     int newfd;
     NetClientState *nc;
     NetSocketState *s;
+    SocketAddress *sa;
+    SocketAddressType sa_type;
+
+    sa = socket_local_address(fd, errp);
+    if (!sa) {
+        return NULL;
+    }
+    sa_type = sa->type;
+    qapi_free_SocketAddress(sa);
 
     /* fd passed: multicast: "learn" dgram_dst address from bound address and save it
      * Because this may be "shared" socket from a "master" process, datagrams would be recv()
@@ -379,8 +392,12 @@ static NetSocketState *net_socket_fd_init_dgram(NetClientState *peer,
                  "socket: fd=%d (cloned mcast=%s:%d)",
                  fd, inet_ntoa(saddr.sin_addr), ntohs(saddr.sin_port));
     } else {
+        if (sa_type == SOCKET_ADDRESS_TYPE_UNIX) {
+            s->dgram_dst.sin_family = AF_UNIX;
+        }
+
         snprintf(nc->info_str, sizeof(nc->info_str),
-                 "socket: fd=%d", fd);
+                 "socket: fd=%d %s", fd, SocketAddressType_str(sa_type));
     }
 
     return s;