thodg/libgit2/src

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  • Hash : c51065e3
    Author : Roberto Tyley
    Date : 2011-10-24T14:39:03

    Tolerate zlib deflation with window size < 32Kb libgit2 currently identifies loose objects as corrupt if they've been deflated using a window size less than 32Kb, because the is_zlib_compressed_data() function doesn't recognise the header byte as a zlib header. This patch makes the method 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. It's based on a patch which has been merged into C-Git master branch: https://github.com/git/git/commit/7f684a2aff636f44a506 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 by the Android platform appear 'corrupt' :( It might appear that this patch changes isStandardFormat() 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. 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=c0b2feff196e63a7b85d97cf9ae5bb258 Code snippet searching for false positives with the zlib checksum: https://gist.github.com/1118177 Change-Id: Ifd84cd2bd6b46f087c9984fb4cbd8309f483dec0

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