[PATCH V2] allow RAID5 to grow to RAID6 with a backup_file

[Nigel Croxon <ncroxon@redhat.com>]

This problem came in, as the user did not specify a full path with
the backup_file option when growing an RAID5 array to RAID6.
When the full path is specified, the symbolic link is created
properly (/run/mdadm/backup_file-mdX). But the code did not support
the symbolic link when looking for the backup_file. Added two
checks for symlink.

This addresses https://www.spinics.net/lists/raid/msg48910.html
and numerous customer reported problems.

V2:
- Removed unneeded break; in both case-statements
- Returned the error checking on call to lstat

Signed-off-by: Nigel Croxon <ncroxon@redhat.com>
---
 Grow.c | 21 +++++++++++++++++++--
 1 file changed, 19 insertions(+), 2 deletions(-)

diff --git a/Grow.c b/Grow.c
index 764374f..53245d7 100644
--- a/Grow.c
+++ b/Grow.c
@@ -1135,6 +1135,15 @@ int reshape_open_backup_file(char *backup_file,
 	unsigned int dev;
 	int i;
 
+	if (lstat(backup_file, &stb) != -1) {
+		switch (stb.st_mode & S_IFMT) {
+		case S_IFLNK:
+			return 1;
+		default:
+			break;
+		}
+	}
+
 	*fdlist = open(backup_file, O_RDWR|O_CREAT|(restart ? O_TRUNC : O_EXCL),
 		       S_IRUSR | S_IWUSR);
 	*offsets = 8 * 512;
@@ -5236,8 +5245,16 @@ char *locate_backup(char *name)
 	char *fl = make_backup(name);
 	struct stat stb;
 
-	if (stat(fl, &stb) == 0 && S_ISREG(stb.st_mode))
-		return fl;
+	if (lstat(fl, &stb) == 0) {
+		switch (stb.st_mode & S_IFMT) {
+		case S_IFLNK:
+			return fl;
+		case S_IFREG:
+			return fl;
+		default:
+			break;
+		}
+	}
 
 	free(fl);
 	return NULL;
-- 
2.20.1