Subject: Inspecting a corrupt git object

From: Magnus Bäck <[email-masked]>
Date: Wed, 4 Aug 2010 11:25:30 +0200

We recently discovered a git tree object corruption in one of our
busiest gits on the master server. From what I can tell "git cat-file -p"
output looked just fine, but "git gc" complained loudly about the object
being corrupt. I had the same git cloned on my machine and found (after
unpacking the packfiles) that my object was different from the one on
the server. Same size and everything, but the second byte (and only the
second byte) differed between good and bad object.

$ head -n 5 /tmp/hexdump_corrupt.txt
00000000  78 9c 2b 29 4a 4d 55 30  32 36 62 30 34 30 30 33 |x.+)JMU026b04003|
00000010  31 51 70 cc 4b 29 ca cf  4c d1 cb cd 66 a8 38 dd |1Qp.K)..L...f.8.|
00000020  76 77 82 ba af da a1 66  06 b9 b4 03 66 9d 27 18 |vw.....f....f.'.|
00000030  93 ec 50 55 f9 26 e6 65  a6 a5 16 97 e8 55 e4 e6 |..PU.&.e.....U..|
00000040  30 d8 98 fe a9 93 98 cc  be 24 a4 ac 93 3b 43 b7 |0........$...;C.|
$ head -n 5 /tmp/hexdump_okay.txt
00000000  78 01 2b 29 4a 4d 55 30  32 36 62 30 34 30 30 33 |x.+)JMU026b04003|
00000010  31 51 70 cc 4b 29 ca cf  4c d1 cb cd 66 a8 38 dd |1Qp.K)..L...f.8.|
00000020  76 77 82 ba af da a1 66  06 b9 b4 03 66 9d 27 18 |vw.....f....f.'.|
00000030  93 ec 50 55 f9 26 e6 65  a6 a5 16 97 e8 55 e4 e6 |..PU.&.e.....U..|
00000040  30 d8 98 fe a9 93 98 cc  be 24 a4 ac 93 3b 43 b7 |0........$...;C.|

From what I gather from the community book and Pro Git, a git object
file is a deflated representation of the object type as a string, the
payload size, a null byte, and the payload. Is there a standard tool for
inflating the file back so that I can inspect what the actual difference
between these two are? Short of writing a tool utilizing zlib, at least.

Any other ideas why we would see such a difference? Hardware
malfunction or memory corruption I guess, but something else?
I can supply the actual object files if necessary.

-- 
Magnus Bäck                      Opinions are my own and do not necessarily
SW Configuration Manager         represent the ones of my employer, etc.
Sony Ericsson