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2013-11-21drop support for "experimental" loose objectsLibravatar Jeff King1-0/+0
In git v1.4.3, we introduced a new loose object format that encoded some object information outside of the zlib stream. Ultimately the format was dropped in v1.5.3, but we kept the reading side around to help people migrate objects. Each time we open a loose object, we use a heuristic to check whether it is in the normal loose format, or the experimental one. This heuristic is robust in the face of valid data, but it tends to treat corrupted or garbage data as an experimental object. With the regular format, we would notice quickly that zlib's crc does not check out and complain. With the experimental object, we are likely to extract a nonsensical object size and try to allocate a huge buffer, resulting in xmalloc calling "die". This latter behavior is much worse, for two reasons. One, git reports an allocation error when the real error is corruption. And two, the program dies unconditionally, so you cannot even run fsck (which would otherwise ignore the broken object and keep going). We could try to improve the heuristic to err on the side of normal objects in the face of corruption, but there is really little point. The experimental format is long-dead, and was never enabled by default to begin with. We can instead simply remove it. The only affected repository would be one that explicitly set core.legacyheaders in 2007, and then never repacked in the intervening 6 years. Signed-off-by: Jeff King <peff@peff.net> Signed-off-by: Junio C Hamano <gitster@pobox.com>
2011-08-11Tolerate zlib deflation with window size < 32KbLibravatar Roberto Tyley1-0/+0
Git currently reports loose objects as 'corrupt' if they've been deflated using a window size less than 32Kb, because the experimental_loose_object() function doesn't recognise the header byte as a zlib header. This patch makes the function tolerant of all valid window sizes (15-bit to 8-bit) - but doesn't sacrifice it's accuracy in distingushing the standard loose-object format from the experimental (now abandoned) format. On memory constrained systems zlib may use a much smaller window size - working on Agit, I found that Android uses a 4KB window; giving a header byte of 0x48, not 0x78. Consequently all loose objects generated appear 'corrupt', which is why Agit is a read-only Git client at this time - I don't want my client to generate Git repos that other clients treat as broken :( This patch makes Git tolerant of different deflate settings - it might appear that it changes experimental_loose_object() to the point where it could incorrectly identify the experimental format as the standard one, but the two criteria (bitmask & checksum) can only give a false result for an experimental object where both of the following are true: 1) object size is exactly 8 bytes when uncompressed (bitmask) 2) [single-byte in-pack git type&size header] * 256 + [1st byte of the following zlib header] % 31 = 0 (checksum) As it happens, for all possible combinations of valid object type (1-4) and window bits (0-7), the only time when the checksum will be divisible by 31 is for 0x1838 - ie object type *1*, a Commit - which, due the fields all Commit objects must contain, could never be as small as 8 bytes in size. Given this, the combination of the two criteria (bitmask & checksum) always correctly determines the buffer format, and is more tolerant than the previous version. The alternative to this patch is simply removing support for the experimental format, which I am also totally cool with. References: Android uses a 4KB window for deflation: http://android.git.kernel.org/?p=platform/libcore.git;a=blob;f=luni/src/main/native/java_util_zip_Deflater.cpp;h=c0b2feff196e63a7b85d97cf9ae5bb2583409c28;hb=refs/heads/gingerbread#l53 Code snippet searching for false positives with the zlib checksum: https://gist.github.com/1118177 Signed-off-by: Roberto Tyley <roberto.tyley@guardian.co.uk> Signed-off-by: Junio C Hamano <gitster@pobox.com>