"rebase -i": support special-purpose editor to edit insn sheet

diff --git a/sha1_file.c b/sha1_file.c
index d5616dca0809bdb9fd6a7a1980b92ded9ae6e230..e002056b85ce89c089dd09c1300461d60d81dffa 100644 (file)
--- a/sha1_file.c
+++ b/sha1_file.c
@@ -380,7 +380,7 @@ void add_to_alternates_file(const char *reference)
 {
        struct lock_file *lock = xcalloc(1, sizeof(struct lock_file));
        int fd = hold_lock_file_for_append(lock, git_path("objects/info/alternates"), LOCK_DIE_ON_ERROR);
-       char *alt = mkpath("%s/objects\n", reference);
+       char *alt = mkpath("%s\n", reference);
        write_or_die(fd, alt, strlen(alt));
        if (commit_lock_file(lock))
                die("could not close alternates file");
@@ -1217,14 +1217,34 @@ static int experimental_loose_object(unsigned char *map)
        unsigned int word;
 
        /*
-        * Is it a zlib-compressed buffer? If so, the first byte
-        * must be 0x78 (15-bit window size, deflated), and the
-        * first 16-bit word is evenly divisible by 31. If so,
-        * we are looking at the official format, not the experimental
-        * one.
+        * We must determine if the buffer contains the standard
+        * zlib-deflated stream or the experimental format based
+        * on the in-pack object format. Compare the header byte
+        * for each format:
+        *
+        * RFC1950 zlib w/ deflate : 0www1000 : 0 <= www <= 7
+        * Experimental pack-based : Stttssss : ttt = 1,2,3,4
+        *
+        * If bit 7 is clear and bits 0-3 equal 8, the buffer MUST be
+        * in standard loose-object format, UNLESS it is a Git-pack
+        * format object *exactly* 8 bytes in size when inflated.
+        *
+        * However, RFC1950 also specifies that the 1st 16-bit word
+        * must be divisible by 31 - this checksum tells us our buffer
+        * is in the standard format, giving a false positive only if
+        * the 1st word of the Git-pack format object happens to be
+        * divisible by 31, ie:
+        *      ((byte0 * 256) + byte1) % 31 = 0
+        *   =>        0ttt10000www1000 % 31 = 0
+        *
+        * As it happens, this case can only arise for www=3 & ttt=1
+        * - ie, a Commit object, which would have to be 8 bytes in
+        * size. As no Commit can be that small, we find that the
+        * combination of these two criteria (bitmask & checksum)
+        * can always correctly determine the buffer format.
         */
        word = (map[0] << 8) + map[1];
-       if (map[0] == 0x78 && !(word % 31))
+       if ((map[0] & 0x8F) == 0x08 && !(word % 31))
                return 0;
        else
                return 1;