repack: do not rebuild packs on --dry-run

"git repack --drop-filtered --dry-run" is documented to list the
objects that would be dropped "without rebuilding any pack or
deleting anything", but it does both.

This is a bug in cmd_repack(): after printing the candidates, the
--dry-run block falls through into the regular repack code.
repack_promisor_objects() writes a new promisor pack, and with -d,
existing_packs_remove_redundant() deletes the old packs.

The existing guard only skips the implied "delete_redundant = 1", so
it does not stop an explicit -d, and it does not stop the new pack
from being written either.

This has been broken since --dry-run was introduced in 1e746b00aa
(builtin/repack: add --drop-filtered and --dry-run options). The
--dry-run block never returned early, so a dry run has always gone
on to do a regular repack. It was not broken by a later change.

Fix it by returning right after the candidates are listed, and add a
test that checks the pack directory is unchanged with and without -d.

A dry run can no longer get past that point, so the two "!dry_run"
checks that tried to keep it from deleting or dropping anything are
now redundant.

Reported-by: Coy Geek <coygeek@gmail.com>
Helped-by: D. Ben Knoble <ben.knoble@gmail.com>
Helped-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Siddharth Shrimali <r.siddharth.shrimali@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/repack.c

@@ -351,10 +351,9 @@ int cmd_repack(int argc,
 		/*
 		 * Dropping objects means rebuilding the promisor packs
 		 * without them and then removing the old packs, so the
-		 * redundant packs must be deleted. Imply -d on a real run.
+		 * redundant packs must be deleted. Imply -d.
 		 */
-		if (!dry_run)
-			delete_redundant = 1;
+		delete_redundant = 1;
 
 		ret = enumerate_promisor_blobs(repo, &po_args.filter_options, &drop_oids);
 
@@ -391,6 +390,13 @@ int cmd_repack(int argc,
 			oidset_iter_init(&drop_oids, &iter);
 			while ((oid = oidset_iter_next(&iter)))
 				printf("%s\n", oid_to_hex(oid));
+
+			/*
+			 * Only list the candidates to be dropped; skip
+			 * the actual repack
+			 */
+			ret = 0;
+			goto cleanup;
 		}
 	}
 
@@ -505,7 +511,7 @@ int cmd_repack(int argc,
 
 	if (pack_everything & ALL_INTO_ONE) {
 		repack_promisor_objects(repo, &po_args, &names, packtmp,
-			(drop_filtered && !dry_run) ? &drop_oids : NULL);
+			drop_filtered ? &drop_oids : NULL);
 
 		if (existing_packs_has_non_kept(&existing) &&
 		    delete_redundant &&

t/t7706-repack-drop-filtered.sh

@@ -135,6 +135,25 @@ test_expect_success '--dry-run does not remove the filtered objects' '
 	git -C repo cat-file -e "$BIG"
 '
 
+test_expect_success '--dry-run leaves the pack directory untouched' '
+	BIG=$(cat big_oid) &&
+	packdir=repo/.git/objects/pack &&
+
+	for opt in "" -d
+	do
+		ls $packdir >before &&
+
+		git -C repo -c repack.writeBitmaps=false \
+			repack --drop-filtered --filter=blob:limit=1k \
+			--dry-run -a $opt >out &&
+
+		ls $packdir >after &&
+		test_cmp before after &&
+		test_grep "$BIG" out &&
+		git -C repo cat-file -e "$BIG" || return 1
+	done
+'
+
 test_expect_success '--drop-filtered removes the promisor blob locally' '
 	BIG=$(cat big_oid) &&
 	SMALL=$(cat small_oid) &&