Document pack .idx file format upgrade strategy.

diff --git a/sha1_file.c b/sha1_file.c
index 1b1c0f7b4dc814764ead5ba1af77070ed381110c..0b705455627213e6e6dda89495dd0c01ac1cf12d 100644 (file)
--- a/sha1_file.c
+++ b/sha1_file.c
@@ -456,12 +456,23 @@ static int check_packed_git_idx(const char *path, unsigned long *idx_size_,
 
        /* check index map */
        if (idx_size < 4*256 + 20 + 20)
-               return error("index file too small");
+               return error("index file %s is too small", path);
+
+       /* a future index format would start with this, as older git
+        * binaries would fail the non-monotonic index check below.
+        * give a nicer warning to the user if we can.
+        */
+       if (index[0] == htonl(PACK_IDX_SIGNATURE))
+               return error("index file %s is a newer version"
+                       " and is not supported by this binary"
+                       " (try upgrading GIT to a newer version)",
+                       path);
+
        nr = 0;
        for (i = 0; i < 256; i++) {
                unsigned int n = ntohl(index[i]);
                if (n < nr)
-                       return error("non-monotonic index");
+                       return error("non-monotonic index %s", path);
                nr = n;
        }
 
@@ -473,7 +484,7 @@ static int check_packed_git_idx(const char *path, unsigned long *idx_size_,
         *  - 20-byte SHA1 file checksum
         */
        if (idx_size != 4*256 + nr * 24 + 20 + 20)
-               return error("wrong index file size");
+               return error("wrong index file size in %s", path);
 
        return 0;
 }