[PATCH v3] btrfs-progs: Return more meaningful value for btrfs_read_deve_super

[Qu Wenruo <quwenruo@cn.fujitsu.com>]

btrfs_read_dev_super() only returns 0 or -1, which doesn't really help,
caller won't know if it's caused by bad superblock or superblock out of
range.

Return -errno if pread64() return -1, and return -EOF if none or part of
the super is read out, and return what check_super() returned.

So caller can get -EIO to catch real corrupted super blocks.

Signed-off-by: Qu Wenruo <quwenruo@cn.fujitsu.com>
---
changelog:
v2:
  Use better return values from pread64() and check_super().
v3:
  Fix wrong comment
---
 disk-io.c | 14 ++++++++++----
 1 file changed, 10 insertions(+), 4 deletions(-)

diff --git a/disk-io.c b/disk-io.c
index 854c285..2052bb3 100644
--- a/disk-io.c
+++ b/disk-io.c
@@ -1590,14 +1590,20 @@ int btrfs_read_dev_super(int fd, struct btrfs_super_block *sb, u64 sb_bytenr,
 
 	if (sb_bytenr != BTRFS_SUPER_INFO_OFFSET) {
 		ret = pread64(fd, buf, BTRFS_SUPER_INFO_SIZE, sb_bytenr);
+		/* real error */
+		if (ret < 0)
+			return -errno;
+
+		/* Not large enough sb, return -ENOENT instead of normal -EIO */
 		if (ret < BTRFS_SUPER_INFO_SIZE)
-			return -1;
+			return -ENOENT;
 
 		if (btrfs_super_bytenr(buf) != sb_bytenr)
-			return -1;
+			return -EIO;
 
-		if (check_super(buf, sbflags))
-			return -1;
+		ret = check_super(buf, sbflags);
+		if (ret < 0)
+			return ret;
 		memcpy(sb, buf, BTRFS_SUPER_INFO_SIZE);
 		return 0;
 	}
-- 
2.10.0