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

Jeff King [off-list ref] writes:

On Thu, Jun 13, 2013 at 08:05:21PM -0400, Nicolas Pitre wrote:

quoted

quoted

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 &&

On Fri, Jun 14, 2013 at 02:51:35PM -0700, Junio C Hamano wrote:

Yeah, I almost did that, but then I realized that dd will simply read
all of its input, anyway.

 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 &&

I didn't want to bother coming up with the binary version of the
REF_DELTA sha1, so I used OFS_DELTA. :)

-Peff