pstree: Sanitize pstree preparation code

We have 3 calls for that, two called directly from cr-restore,
one -- from one the former. Clean this mess and write short
comments on each stage.
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>

cr-restore.c

@@ -1250,9 +1250,6 @@ int cr_restore_tasks(pid_t pid, struct cr_options *opts)
 	if (prepare_pstree() < 0)
 		return -1;
 
-	if (prepare_pstree_ids() < 0)
-		return -1;
-
 	if (crtools_prepare_shared() < 0)
 		return -1;
 

include/pstree.h

@@ -55,7 +55,6 @@ extern struct pstree_item *pstree_item_next(struct pstree_item *item);
 
 extern bool restore_before_setsid(struct pstree_item *child);
 extern int prepare_pstree(void);
-extern int prepare_pstree_ids(void);
 
 extern int dump_pstree(struct pstree_item *root_item);
 

pstree.c

@@ -175,7 +175,7 @@ static int prepare_pstree_for_shell_job(struct pstree_item *root)
 	return 0;
 }
 
-int prepare_pstree(void)
+static int read_pstree_image(void)
 {
 	int ret = 0, i, ps_fd;
 	struct pstree_item *pi, *parent = NULL;
@@ -261,14 +261,12 @@ int prepare_pstree(void)
 
 		pstree_entry__free_unpacked(e, NULL);
 	}
-
-	ret = prepare_pstree_for_shell_job(root_item);
 err:
 	close(ps_fd);
 	return ret;
 }
 
-int prepare_pstree_ids(void)
+static int prepare_pstree_ids(void)
 {
 	struct pstree_item *item, *child, *helper, *tmp;
 	LIST_HEAD(helpers);
@@ -427,6 +425,27 @@ int prepare_pstree_ids(void)
 	return 0;
 }
 
+int prepare_pstree(void)
+{
+	int ret;
+
+	ret = read_pstree_image();
+	if (!ret)
+		/*
+		 * Shell job may inherit sid/pgid from the current
+		 * shell, not from image. Set things up for this.
+		 */
+		ret = prepare_pstree_for_shell_job(root_item);
+	if (!ret)
+		/*
+		 * Session/Group leaders might be dead. Need to fix
+		 * pstree with properly injected helper tasks.
+		 */
+		ret = prepare_pstree_ids();
+
+	return ret;
+}
+
 bool restore_before_setsid(struct pstree_item *child)
 {
 	int csid = child->born_sid == -1 ? child->sid : child->born_sid;