connectd: implement @ correctly.

dev_blackhole_fd was a hack, and doesn't work well now we are async
(it worked for sync comms in per-peer daemons, but now we could sneak
through a read before we get to the next write).

So, make explicit flags and use them.  This is much easier now we
have all peer comms in one place.

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

common/dev_disconnect.c

@@ -105,56 +105,4 @@ void dev_sabotage_fd(int fd, bool close_fd)
 	dup2(fds[1], fd);
 	close(fds[1]);
 }
-
-/* Replace fd with blackhole until dev_disconnect file is truncated. */
-void dev_blackhole_fd(int fd)
-{
-	int fds[2];
-	int i;
-	struct stat st;
-
-	int maxfd;
-
-	if (socketpair(AF_UNIX, SOCK_STREAM, 0, fds) != 0)
-		err(1, "dev_blackhole_fd: creating socketpair");
-
-	switch (fork()) {
-	case -1:
-		err(1, "dev_blackhole_fd: forking");
-	case 0:
-		/* Close everything but the dev_disconnect_fd, the socket
-		 * which is pretending to be the peer, and stderr.
-		 * The "correct" way to do this would be to move the
-		 * fds we want to preserve to the low end (0, 1, 2...)
-		 * of the fd space and then just do a single closefrom
-		 * call, but dup2 could fail with ENFILE (which is a
-		 * *system*-level error, i.e. the entire system has too
-		 * many processes with open files) and we have no
-		 * convenient way to inform the parent of the error.
-		 * So loop until we reach whichever is higher of fds[0]
-		 * or dev_disconnect_fd, and *then* closefrom after that.
-		 */
-		maxfd = (fds[0] > dev_disconnect_fd) ? fds[0] :
-						       dev_disconnect_fd ;
-		for (i = 0; i < maxfd; i++)
-			if (i != fds[0]
-			    && i != dev_disconnect_fd
-			    && i != STDERR_FILENO)
-				close(i);
-		closefrom(maxfd + 1);
-
-		/* Close once dev_disconnect file is truncated. */
-		for (;;) {
-			if (fstat(dev_disconnect_fd, &st) != 0)
-				err(1, "fstat of dev_disconnect_fd failed");
-			if (st.st_size == 0)
-				_exit(0);
-			sleep(1);
-		}
-	}
-
-	close(fds[0]);
-	dup2(fds[1], fd);
-	close(fds[1]);
-}
 #endif