builtin/repack.c: avoid making cruft packs preferred

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>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

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);
 

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