diff options
Diffstat (limited to 'sha1_file.c')
| -rw-r--r-- | sha1_file.c | 32 |
1 files changed, 26 insertions, 6 deletions
diff --git a/sha1_file.c b/sha1_file.c index 697f4a43c5..475d215c14 100644 --- a/sha1_file.c +++ b/sha1_file.c @@ -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; |