files: Formalize fd restore priorities

There are places when we have to select which fd to ->open
and which to ->receive. To avoid deadlocks we sort them in
an ascending order on { pid, fd } pair.

Make this idea more formal by introducing an explicit function
doing this check and call it where appropriate (pipe.c master
selection is also simplified to fit new ... API).
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>

include/files.h

@@ -43,6 +43,12 @@ struct fdinfo_list_entry {
 	FdinfoEntry		*fe;
 };
 
+/* reports whether fd_a takes prio over fd_b */
+static inline int fdinfo_rst_prio(struct fdinfo_list_entry *fd_a, struct fdinfo_list_entry *fd_b)
+{
+	return (fd_a->pid < fd_b->pid) || ((fd_a->pid == fd_b->pid) && (fd_a->fe->fd < fd_b->fe->fd));
+}
+
 struct file_desc_ops {
 	unsigned int		type;
 	int			(*open)(struct file_desc *d);

pipes.c

@@ -112,7 +112,6 @@ void mark_pipe_master(void)
 	while (1) {
 		struct fdinfo_list_entry *fle;
 		struct pipe_info *pi, *pic, *p;
-		int fd, pid;
 
 		if (list_empty(&pipes))
 			break;
@@ -125,18 +124,15 @@ void mark_pipe_master(void)
 
 		fle = file_master(&pi->d);
 		p = pi;
-		fd = fle->fe->fd;
-		pid = fle->pid;
 
 		list_for_each_entry(pic, &pi->pipe_list, pipe_list) {
-			list_move(&pic->list, &head);
+			struct fdinfo_list_entry *f;
 
-			fle = file_master(&pic->d);
-			if (fle->pid < pid ||
-			    (pid == fle->pid && fle->fe->fd < fd)) {
+			list_move(&pic->list, &head);
+			f = file_master(&pic->d);
+			if (fdinfo_rst_prio(f, fle)) {
 				p = pic;
-				fd = fle->fe->fd;
-				pid = fle->pid;
+				fle = f;
 			}
 
 			show_saved_pipe_fds(pic);

sk-unix.c

@@ -762,9 +762,7 @@ int resolve_unix_peers(void)
 		fle = file_master(&ui->d);
 		fle_peer = file_master(&peer->d);
 
-		if ((fle->pid < fle_peer->pid) ||
-				(fle->pid == fle_peer->pid &&
-				 fle->fe->fd < fle_peer->fe->fd)) {
+		if (fdinfo_rst_prio(fle, fle_peer)) {
 			ui->flags |= USK_PAIR_MASTER;
 			peer->flags |= USK_PAIR_SLAVE;
 		} else {

tty.c

@@ -670,8 +670,7 @@ static void tty_setup_orphan_slavery(void)
 			 * waiting for each other.
 			 */
 			b = file_master(&peer->d);
-			if (a->pid > b->pid ||
-			    (a->pid == b->pid && a->fe->fd > b->fe->fd)) {
+			if (fdinfo_rst_prio(b, a)) {
 				a = b;
 				m = peer;
 			}