Re: [PATCH 0/2] recover from "failed to apply delta"

[Junio C Hamano <gitster@pobox.com>]

Jeff King <peff@peff.net> writes:

> On Thu, Jun 13, 2013 at 08:05:21PM -0400, Nicolas Pitre wrote:
>
>> > We already handle the case where we were not able to read
>> > the delta from disk. However, when we find that the delta we
>> > read does not apply, we simply die.  This case is harder to
>> > trigger, as corruption in the delta data itself would
>> > trigger a crc error from zlib.  However, a corruption that
>> > pointed us at the wrong delta base might cause it.
>>
>> That makes sense.
>> 
>> Could you produce a test case to go along with this change?
>
> Yes. I was a little worried I would have trouble doing it without
> relying on a lot of pack internals, but the infrastructure you set up in
> t5303 makes it relatively easy (and we do not have to make any
> assumptions that t5303 does not already make).
>
> Here is a re-roll; the first patch is a small cleanup in t5303 that is
> required for the new tests to work.

Heh, I was doing the same, but I cheated ;-)

diff --git a/t/t5303-pack-corruption-resilience.sh b/t/t5303-pack-corruption-resilience.sh
index 5b1250f..57436db 100755
--- a/t/t5303-pack-corruption-resilience.sh
+++ b/t/t5303-pack-corruption-resilience.sh
@@ -51,7 +51,7 @@ do_corrupt_object() {
     ofs=`git show-index < ${pack}.idx | grep $1 | cut -f1 -d" "` &&
     ofs=$(($ofs + $2)) &&
     chmod +w ${pack}.pack &&
-    dd of=${pack}.pack count=1 bs=1 conv=notrunc seek=$ofs &&
+    dd of=${pack}.pack count=${3-1} bs=1 conv=notrunc seek=$ofs &&
     test_must_fail git verify-pack ${pack}.pack
 }
 
@@ -276,6 +276,30 @@ test_expect_success \
      git cat-file blob $blob_3 > /dev/null'
 
 test_expect_success \
+    'corrupt delta-part of a packed object, fall back to loose' \
+    'create_new_pack &&
+     path=$(echo "$blob_3" | sed -e "s|^\(..\)|\1/|") &&
+     cat ".git/objects/$path" >saved &&
+     git prune-packed &&
+
+     dd if=${pack}.idx bs=1 count=20 skip=1032 >blob1-bin &&
+     dd if=${pack}.pack bs=1 count=20 skip=2233 >blob3-delta-base-bin &&
+
+     # At the beginning of the REF_DELTA representation of $blob_3,
+     # write 20-byte base object name for $blob_1, instead of $blob_2.
+     # The binary representation of object name for $blob_1 is found
+     # at offset 4 + 4 + 256*4 = 1032 for 20 bytes.
+     dd if=${pack}.idx bs=1 count=20 skip=1032 | do_corrupt_object $blob_3 2 20 &&
+     test_must_fail git cat-file blob $blob_3 >/dev/null &&
+
+     # Resurrect the loose object for $blob_3
+     mkdir -p .git/objects/$(echo "$path" | sed -e "s|^\(..\).*|\1|") &&
+     cat saved >".git/objects/$path" &&
+
+     git cat-file blob $blob_3 >/dev/null
+'
+
+test_expect_success \
     'corrupting header to have too small output buffer fails unpack' \
     'create_new_pack &&
      git prune-packed &&