[PATCH V2 16/23] mdrestore: Detect metadump v1 magic before reading the header

[Chandan Babu R <chandan.babu@oracle.com>]

In order to support both v1 and v2 versions of metadump, mdrestore will have
to detect the format in which the metadump file has been stored on the disk
and then read the ondisk structures accordingly. In a step in that direction,
this commit splits the work of reading the metadump header from disk into two
parts
1. Read the first 4 bytes containing the metadump magic code.
2. Read the remaining part of the header.

A future commit will take appropriate action based on the value of the magic
code.

Signed-off-by: Chandan Babu R <chandan.babu@oracle.com>
---
 mdrestore/xfs_mdrestore.c | 17 ++++++++++++++---
 1 file changed, 14 insertions(+), 3 deletions(-)

diff --git a/mdrestore/xfs_mdrestore.c b/mdrestore/xfs_mdrestore.c
index 564630f7..2a9527b9 100644
--- a/mdrestore/xfs_mdrestore.c
+++ b/mdrestore/xfs_mdrestore.c
@@ -198,6 +198,7 @@ main(
 	int		open_flags;
 	struct stat	statbuf;
 	int		is_target_file;
+	uint32_t	magic;
 	struct xfs_metablock	mb;
 
 	mdrestore.show_progress = false;
@@ -245,10 +246,20 @@ main(
 			fatal("cannot open source dump file\n");
 	}
 
-	if (fread(&mb, sizeof(mb), 1, src_f) != 1)
-		fatal("error reading from metadump file\n");
-	if (mb.mb_magic != cpu_to_be32(XFS_MD_MAGIC_V1))
+	if (fread(&magic, sizeof(magic), 1, src_f) != 1)
+		fatal("Unable to read metadump magic from metadump file\n");
+
+	switch (be32_to_cpu(magic)) {
+	case XFS_MD_MAGIC_V1:
+		mb.mb_magic = cpu_to_be32(XFS_MD_MAGIC_V1);
+		if (fread((uint8_t *)&mb + sizeof(mb.mb_magic),
+			sizeof(mb) - sizeof(mb.mb_magic), 1, src_f) != 1)
+			fatal("error reading from metadump file\n");
+		break;
+	default:
 		fatal("specified file is not a metadata dump\n");
+		break;
+	}
 
 	if (mdrestore.show_info) {
 		if (mb.mb_info & XFS_METADUMP_INFO_FLAGS) {
-- 
2.39.1