[PATCH] Verify we know how to read a pack before trying to using it.

[Shawn Pearce <spearce@spearce.org>]

If the pack format were to ever change or be extended in the future
there is no assurance that just because the pack file lives in
objects/pack and doesn't end in .idx that we can read and decompress
its contents properly.

If we encounter what we think is a pack file and it isn't or we don't
recognize its version then die and suggest to the user that they
upgrade to a newer version of GIT which can handle that pack file.

Signed-off-by: Shawn O. Pearce <spearce@spearce.org>
---
 sha1_file.c |   12 ++++++++++++
 1 files changed, 12 insertions(+), 0 deletions(-)

diff --git a/sha1_file.c b/sha1_file.c
index d6eb9b3..bb9fa8b 100644
--- a/sha1_file.c
+++ b/sha1_file.c
@@ -481,6 +481,7 @@ int use_packed_git(struct packed_git *p)
 		int fd;
 		struct stat st;
 		void *map;
+		struct pack_header *hdr;
 
 		pack_mapped += p->pack_size;
 		while (PACK_MAX_SZ < pack_mapped && unuse_one_packed_git())
@@ -500,6 +501,17 @@ int use_packed_git(struct packed_git *p)
 			die("packfile %s cannot be mapped.", p->pack_name);
 		p->pack_base = map;
 
+		/* Check if we understand this pack file.  If we don't we're
+		 * likely too old to handle it.
+		 */
+		hdr = map;
+		if (hdr->hdr_signature != htonl(PACK_SIGNATURE))
+			die("packfile %s isn't actually a pack.", p->pack_name);
+		if (!pack_version_ok(hdr->hdr_version))
+			die("packfile %s is version %i and not supported"
+				" (try upgrading GIT to a newer version)",
+				p->pack_name, ntohl(hdr->hdr_version));
+
 		/* Check if the pack file matches with the index file.
 		 * this is cheap.
 		 */
-- 
1.4.2.g0dcb