Tolerate zlib deflation with window size < 32Kb

2011-08-11T20:02:47Z

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 Signed-off-by: Junio C Hamano

git/t/t1013/objects, branch v1.8.2.2

Tolerate zlib deflation with window size < 32Kb