git/t/t6501-freshen-objects.sh
Jeff King 2ba582ba4c prune: save reachable-from-recent objects with bitmaps
We pass our prune expiration to mark_reachable_objects(), which will
traverse not only the reachable objects, but consider any recent ones as
tips for reachability; see d3038d22f9 (prune: keep objects reachable
from recent objects, 2014-10-15) for details.

However, this interacts badly with the bitmap code path added in
fde67d6896 (prune: use bitmaps for reachability traversal, 2019-02-13).
If we hit the bitmap-optimized path, we return immediately to avoid the
regular traversal, accidentally skipping the "also traverse recent"
code.

Instead, we should do an if-else for the bitmap versus regular
traversal, and then follow up with the "recent" traversal in either
case. This reuses the "rev_info" for a bitmap and then a regular
traversal, but that should work OK (the bitmap code clears the pending
array in the usual way, just like a regular traversal would).

Note that I dropped the comment above the regular traversal here.  It
has little explanatory value, and makes the if-else logic much harder to
read.

Here are a few variants that I rejected:

  - it seems like both the reachability and recent traversals could be
    done in a single traversal. This was rejected by d3038d22f9 (prune:
    keep objects reachable from recent objects, 2014-10-15), though the
    balance may be different when using bitmaps. However, there's a
    subtle correctness issue, too: we use revs->ignore_missing_links for
    the recent traversal, but not the reachability one.

  - we could try using bitmaps for the recent traversal, too, which
    could possibly improve performance. But it would require some fixes
    in the bitmap code, which uses ignore_missing_links for its own
    purposes. Plus it would probably not help all that much in practice.
    We use the reachable tips to generate bitmaps, so those objects are
    likely not covered by bitmaps (unless they just became unreachable).
    And in general, we expect the set of unreachable objects to be much
    smaller anyway, so there's less to gain.

The test in t5304 detects the bug and confirms the fix.

I also beefed up the tests in t6501, which covers the mtime-checking
code more thoroughly, to handle the bitmap case (in addition to just
"loose" and "packed" cases). Interestingly, this test doesn't actually
detect the bug, because it is running "git gc", and not "prune"
directly. And "gc" will call "repack" first, which does not suffer the
same bug. So the old-but-reachable-from-recent objects get scooped up
into the new pack along with the actually-recent objects, which gives
both a recent mtime. But it seemed prudent to get more coverage of the
bitmap case for related code.

Reported-by: David Emett <dave@sp4m.net>
Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
2021-04-29 10:38:25 +09:00

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#!/bin/sh
#
# This test covers the handling of objects which might have old
# mtimes in the filesystem (because they were used previously)
# and are just now becoming referenced again.
#
# We're going to do two things that are a little bit "fake" to
# help make our simulation easier:
#
# 1. We'll turn off reflogs. You can still run into
# problems with reflogs on, but your objects
# don't get pruned until both the reflog expiration
# has passed on their references, _and_ they are out
# of prune's expiration period. Dropping reflogs
# means we only have to deal with one variable in our tests,
# but the results generalize.
#
# 2. We'll use a temporary index file to create our
# works-in-progress. Most workflows would mention
# referenced objects in the index, which prune takes
# into account. However, many operations don't. For
# example, a partial commit with "git commit foo"
# will use a temporary index. Or they may not need
# an index at all (e.g., creating a new commit
# to refer to an existing tree).
test_description='check pruning of dependent objects'
GIT_TEST_DEFAULT_INITIAL_BRANCH_NAME=main
export GIT_TEST_DEFAULT_INITIAL_BRANCH_NAME
. ./test-lib.sh
# We care about reachability, so we do not want to use
# the normal test_commit, which creates extra tags.
add () {
echo "$1" >"$1" &&
git add "$1"
}
commit () {
test_tick &&
add "$1" &&
git commit -m "$1"
}
maybe_repack () {
case "$title" in
loose)
: skip repack
;;
repack)
git repack -ad
;;
bitmap)
git repack -adb
;;
*)
echo >&2 "unknown test type in maybe_repack"
return 1
;;
esac
}
for title in loose repack bitmap
do
test_expect_success "make repo completely empty ($title)" '
rm -rf .git &&
git init
'
test_expect_success "disable reflogs ($title)" '
git config core.logallrefupdates false &&
git reflog expire --expire=all --all
'
test_expect_success "setup basic history ($title)" '
commit base
'
test_expect_success "create and abandon some objects ($title)" '
git checkout -b experiment &&
commit abandon &&
maybe_repack &&
git checkout main &&
git branch -D experiment
'
test_expect_success "simulate time passing ($title)" '
test-tool chmtime --get -86400 $(find .git/objects -type f)
'
test_expect_success "start writing new commit with old blob ($title)" '
tree=$(
GIT_INDEX_FILE=index.tmp &&
export GIT_INDEX_FILE &&
git read-tree HEAD &&
add unrelated &&
add abandon &&
git write-tree
)
'
test_expect_success "simultaneous gc ($title)" '
git gc --prune=12.hours.ago
'
test_expect_success "finish writing out commit ($title)" '
commit=$(echo foo | git commit-tree -p HEAD $tree) &&
git update-ref HEAD $commit
'
# "abandon" blob should have been rescued by reference from new tree
test_expect_success "repository passes fsck ($title)" '
git fsck
'
test_expect_success "abandon objects again ($title)" '
git reset --hard HEAD^ &&
test-tool chmtime --get -86400 $(find .git/objects -type f)
'
test_expect_success "start writing new commit with same tree ($title)" '
tree=$(
GIT_INDEX_FILE=index.tmp &&
export GIT_INDEX_FILE &&
git read-tree HEAD &&
add abandon &&
add unrelated &&
git write-tree
)
'
test_expect_success "simultaneous gc ($title)" '
git gc --prune=12.hours.ago
'
# tree should have been refreshed by write-tree
test_expect_success "finish writing out commit ($title)" '
commit=$(echo foo | git commit-tree -p HEAD $tree) &&
git update-ref HEAD $commit
'
done
test_expect_success 'do not complain about existing broken links (commit)' '
cat >broken-commit <<-EOF &&
tree $(test_oid 001)
parent $(test_oid 002)
author whatever <whatever@example.com> 1234 -0000
committer whatever <whatever@example.com> 1234 -0000
some message
EOF
commit=$(git hash-object -t commit -w broken-commit) &&
git gc -q 2>stderr &&
verbose git cat-file -e $commit &&
test_must_be_empty stderr
'
test_expect_success 'do not complain about existing broken links (tree)' '
cat >broken-tree <<-EOF &&
100644 blob $(test_oid 003) foo
EOF
tree=$(git mktree --missing <broken-tree) &&
git gc -q 2>stderr &&
git cat-file -e $tree &&
test_must_be_empty stderr
'
test_expect_success 'do not complain about existing broken links (tag)' '
cat >broken-tag <<-EOF &&
object $(test_oid 004)
type commit
tag broken
tagger whatever <whatever@example.com> 1234 -0000
this is a broken tag
EOF
tag=$(git hash-object -t tag -w broken-tag) &&
git gc -q 2>stderr &&
git cat-file -e $tag &&
test_must_be_empty stderr
'
test_done