[PATCH v2] builtin/repack.c: avoid making cruft packs preferred
- From
Taylor Blau <me@ttaylorr.com>
- Date
- Oct 3, 2023, 21:54 UTC
- Message-ID
- <035393935108d02aaf8927189b05102f4f74f340.1696370003.git.me@ttaylorr.com>
- In-Reply-To
- <19d9aae08eab05c6b5dda4c2090236b1c3f62998.1696349955.git.me@ttaylorr.com>
When doing a `--geometric` repack, we make sure that the preferred pack (if writing a MIDX) is the largest pack that we *didn't* repack. That has the effect of keeping the preferred pack in sync with the pack containing a majority of the repository's reachable objects.
But if the repository happens to double in size, we'll repack everything. Here we don't specify any `--preferred-pack`, and instead let the MIDX code choose.
In the past, that worked fine, since there would only be one pack to choose from: the one we just wrote. But it's no longer necessarily the case that there is one pack to choose from. It's possible that the repository also has a cruft pack, too.
If the cruft pack happens to come earlier in lexical order (and has an earlier mtime than any non-cruft pack), we'll pick that pack as preferred. This makes it impossible to reuse chunks of the reachable pack verbatim from pack-objects, so is sub-optimal.
Luckily, this is a somewhat rare circumstance to be in, since we would have to repack the entire repository during a `--geometric` repack, and the cruft pack would have to sort ahead of the pack we just created.
Signed-off-by: Taylor Blau <me@ttaylorr.com> --- A small re-roll of the original patch, removing a few paragraphs from the proposed log message that are irrelevant and impossible to produce in today's git.git.
For more details, see <ZRyNHRf+tQwV+T6P@nand.local>.
Range-diff against v1:
1: 19d9aae08e ! 1: 0353939351 builtin/repack.c: avoid making cruft packs preferred
@@ Commit message
have to repack the entire repository during a `--geometric` repack, and
the cruft pack would have to sort ahead of the pack we just created. - Note that this behavior is usually just a performance regression. But
- it's possible it could be a correctness issue.
-
- Suppose an object was duplicated among the cruft and non-cruft pack. The
- MIDX will pick the one from the pack with the lowest mtime, which will
- always be the cruft one. But if the non-cruft pack happens to sort
- earlier in lexical order, we'll treat that one as preferred, but not all
- duplicates will be resolved in favor of that pack.
-
- So if we happened to have an object which appears in both packs
- (e.g., due to a cruft object being freshened, causing it to appear
- loose, and then repacking it via the `--geometric` repack) it's possible
- the duplicate would be picked from the non-preferred pack.
-
Signed-off-by: Taylor Blau <me@ttaylorr.com>## builtin/repack.c ##
builtin/repack.c | 47 ++++++++++++++++++++++++++++++++++++++++- t/t7704-repack-cruft.sh | 39 ++++++++++++++++++++++++++++++++++ 2 files changed, 85 insertions(+), 1 deletion(-)
diff --git a/builtin/repack.c b/builtin/repack.c index 04770b15fe..a1a893d952 100644 --- a/builtin/repack.c +++ b/builtin/repack.c @@ -355,6 +355,18 @@ static struct generated_pack_data *populate_pack_exts(const char *name) return data; } +static int has_pack_ext(const struct generated_pack_data *data, + const char *ext) +{ + int i; + for (i = 0; i < ARRAY_SIZE(exts); i++) { + if (strcmp(exts[i].name, ext)) + continue; + return !!data->tempfiles[i]; + } + BUG("unknown pack extension: '%s'", ext); +} + static void repack_promisor_objects(const struct pack_objects_args *args, struct string_list *names) { @@ -772,6 +784,7 @@ static void midx_included_packs(struct string_list *include, static int write_midx_included_packs(struct string_list *include, struct pack_geometry *geometry, + struct string_list *names, const char *refs_snapshot, int show_progress, int write_bitmaps) { @@ -801,6 +814,38 @@ static int write_midx_included_packs(struct string_list *include, if (preferred) strvec_pushf(&cmd.args, "--preferred-pack=%s", pack_basename(preferred)); + else if (names->nr) { + /* The largest pack was repacked, meaning that either + * one or two packs exist depending on whether the + * repository has a cruft pack or not. + * + * Select the non-cruft one as preferred to encourage + * pack-reuse among packs containing reachable objects + * over unreachable ones. + * + * (Note we could write multiple packs here if + * `--max-pack-size` was given, but any one of them + * will suffice, so pick the first one.) + */ + for_each_string_list_item(item, names) { + struct generated_pack_data *data = item->util; + if (has_pack_ext(data, ".mtimes")) + continue; + + strvec_pushf(&cmd.args, "--preferred-pack=pack-%s.pack", + item->string); + break; + } + } else { + /* + * No packs were kept, and no packs were written. The + * only thing remaining are .keep packs (unless + * --pack-kept-objects was given). + * + * Set the `--preferred-pack` arbitrarily here. + */ + ; + } if (refs_snapshot) strvec_pushf(&cmd.args, "--refs-snapshot=%s", refs_snapshot); @@ -1360,7 +1405,7 @@ int cmd_repack(int argc, const char **argv, const char *prefix) struct string_list include = STRING_LIST_INIT_NODUP; midx_included_packs(&include, &existing, &names, &geometry); - ret = write_midx_included_packs(&include, &geometry, + ret = write_midx_included_packs(&include, &geometry, &names, refs_snapshot ? get_tempfile_path(refs_snapshot) : NULL, show_progress, write_bitmaps > 0); diff --git a/t/t7704-repack-cruft.sh b/t/t7704-repack-cruft.sh index dc86ca8269..be3735dff0 100755 --- a/t/t7704-repack-cruft.sh +++ b/t/t7704-repack-cruft.sh @@ -372,4 +372,43 @@ test_expect_success '--max-cruft-size ignores non-local packs' ' ) ' +test_expect_success 'reachable packs are preferred over cruft ones' ' + repo="cruft-preferred-packs" && + git init "$repo" && + ( + cd "$repo" && + + # This test needs to exercise careful control over when a MIDX + # is and is not written. Unset the corresponding TEST variable + # accordingly. + sane_unset GIT_TEST_MULTI_PACK_INDEX && + + test_commit base && + test_commit --no-tag cruft && + + non_cruft="$(echo base | git pack-objects --revs $packdir/pack)" && + # Write a cruft pack which both (a) sorts ahead of the non-cruft + # pack in lexical order, and (b) has an older mtime to appease + # the MIDX preferred pack selection routine. + cruft="$(echo pack-$non_cruft.pack | git pack-objects --cruft $packdir/pack-A)" && + test-tool chmtime -1000 $packdir/pack-A-$cruft.pack && + + test_commit other && + git repack -d && + + git repack --geometric 2 -d --write-midx --write-bitmap-index && + + # After repacking, there are two packs left: one reachable one + # (which is the result of combining both of the existing two + # non-cruft packs), and one cruft pack. + find .git/objects/pack -type f -name "*.pack" >packs && + test_line_count = 2 packs && + + # Make sure that the pack we just wrote is marked as preferred, + # not the cruft one. + pack="$(test-tool read-midx --preferred-pack $objdir)" && + test_path_is_missing "$packdir/$(basename "$pack" ".idx").mtimes" + ) +' + test_done -- 2.42.0.341.g9d86323b73.dirty