sd-daemon: Replace SO_LINGER with shutdown() + recv()

Let's shutdown the write end and wait for EOF from the other side
before continuing to make sure that the receiver has received all
data we sent on the socket.

(cherry picked from commit 13b67b6)

Author: DaanDeMeyer

src/libsystemd/sd-daemon/sd-daemon.c

@@ -594,6 +594,19 @@ static int pid_notify_with_fds_internal(
                 }
         } while (!iovec_increment(msghdr.msg_iov, msghdr.msg_iovlen, n));
 
+        if (address.sockaddr.sa.sa_family == AF_VSOCK && IN_SET(type, SOCK_STREAM, SOCK_SEQPACKET)) {
+                /* For AF_VSOCK, we need to close the socket to signal the end of the message. */
+                if (shutdown(fd, SHUT_WR) < 0)
+                        return log_error_errno(errno, "Failed to shutdown notify socket: %m");
+
+                char buf[1];
+                n = recv(fd, buf, sizeof(buf), MSG_NOSIGNAL);
+                if (n > 0)
+                        return log_error_errno(errno, "Unexpectedly received data on notify socket: %m");
+                if (n < 0)
+                        return log_error_errno(errno, "Failed to wait for EOF on notify socket: %m");
+        }
+
         return 1;
 }