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read-cache: force_verify_index_checksum

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Teach git to skip verification of the SHA1-1 checksum at the end of
the index file in verify_hdr() which is called from read_index()
unless the "force_verify_index_checksum" global variable is set.

Teach fsck to force this verification.

The checksum verification is for detecting disk corruption, and for
small projects, the time it takes to compute SHA-1 is not that
significant, but for gigantic repositories this calculation adds
significant time to every command.

These effect can be seen using t/perf/p0002-read-cache.sh:

Test                                          HEAD~1            HEAD
--------------------------------------------------------------------------------------
0002.1: read_cache/discard_cache 1000 times   0.66(0.44+0.20)   0.30(0.27+0.02) -54.5%

Signed-off-by: Jeff Hostetler <jeffhost@microsoft.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
This commit is contained in:
Jeff Hostetler
2017-04-14 20:32:21 +00:00
committed by Junio C Hamano
parent e1104a5ee5
commit a33fc72fe9
4 changed files with 23 additions and 0 deletions
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13
t/t1450-fsck.sh
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@ -689,4 +689,17 @@ test_expect_success 'bogus head does not fallback to all heads' '
! grep $blob out
'
test_expect_success 'detect corrupt index file in fsck' '
cp .git/index .git/index.backup &&
test_when_finished "mv .git/index.backup .git/index" &&
echo zzzzzzzz >zzzzzzzz &&
git add zzzzzzzz &&
sed -e "s/zzzzzzzz/yyyyyyyy/" .git/index >.git/index.yyy &&
mv .git/index.yyy .git/index &&
# Confirm that fsck detects invalid checksum
test_must_fail git fsck --cache &&
# Confirm that status no longer complains about invalid checksum
git status
'
test_done