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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>
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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>
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