[PATCH v2 5/5] repack: mark non-MIDX packs above the split as excluded-open

[Taylor Blau <me@ttaylorr.com>]

In 5ee86c273bf (repack: exclude cruft pack(s) from the MIDX where
possible, 2025-06-23), geometric repacking learned to exclude cruft
packs from the MIDX when 'repack.midxMustContainCruft' is set to
'false'.

This works because packs generated with '--stdin-packs=follow' rescue
any once-unreachable objects that later become reachable, making the
resulting packs closed under reachability without needing the cruft pack
in the MIDX.

However, packs above the geometric split that were not part of the
previous MIDX may not have full object closure.  When such packs are
marked as excluded-closed ('^'), pack-objects treats them as a
reachability boundary and does not traverse through them during the
follow pass, potentially leaving the resulting pack without full
closure.

Fix this by marking packs above the geometric split that were not in the
previous MIDX as excluded-open ('!') instead of excluded-closed ('^').
This causes pack-objects to walk through their commits during the follow
pass, rescuing any reachable objects not present in the closed-excluded
packs.

Note that MIDXs which were generated prior to this change and are
unlucky enough to not be closed under reachability may still exhibit
this bug, as we treat all MIDX'd packs as closed. That is true in an
overwhelming number of cases, since in order to have a non-closed MIDX
you would have to:

 - Generate a pack via an earlier geometric repack that is not closed
   under reachability.

 - Store that pack in the MIDX.

 - Avoid picking any commits to receive reachability bitmaps which
   happen to reach objects from which the missing objects are reachable.

In the extremely rare chance that all of the above should happen, an
all-into-one repack will resolve the issue.

Unfortunately, there is no perfect way to determine whether a MIDX'd
pack is closed outside of ensuring that there is a '1' bit in at least
one bitmap for every bit position corresponding to objects in that pack.
While this is possible to do, this approach would treat MIDX'd packs as
open in cases where there is at least one object that is not reachable
from the subset of commits selected for bitmapping.

Signed-off-by: Taylor Blau <me@ttaylorr.com>
---
 builtin/repack.c        | 19 +++++++++++++++++--
 t/t7704-repack-cruft.sh |  2 +-
 2 files changed, 18 insertions(+), 3 deletions(-)

diff --git a/builtin/repack.c b/builtin/repack.c
index f6bb04bef72..4c5a82c2c8d 100644
--- a/builtin/repack.c
+++ b/builtin/repack.c
@@ -369,8 +369,23 @@ int cmd_repack(int argc,
 		 */
 		for (i = 0; i < geometry.split; i++)
 			fprintf(in, "%s\n", pack_basename(geometry.pack[i]));
-		for (i = geometry.split; i < geometry.pack_nr; i++)
-			fprintf(in, "^%s\n", pack_basename(geometry.pack[i]));
+		for (i = geometry.split; i < geometry.pack_nr; i++) {
+			const char *basename = pack_basename(geometry.pack[i]);
+			char marker = '^';
+
+			if (!midx_must_contain_cruft &&
+			    !string_list_has_string(&existing.midx_packs,
+						    basename)) {
+				/*
+				 * Assume non-MIDX'd packs are not
+				 * necessarily closed under
+				 * reachability.
+				 */
+				marker = '!';
+			}
+
+			fprintf(in, "%c%s\n", marker, basename);
+		}
 		fclose(in);
 	}
 
diff --git a/t/t7704-repack-cruft.sh b/t/t7704-repack-cruft.sh
index 77133395b5d..9e03b04315d 100755
--- a/t/t7704-repack-cruft.sh
+++ b/t/t7704-repack-cruft.sh
@@ -869,7 +869,7 @@ test_expect_success 'repack --write-midx includes cruft when already geometric'
 	)
 '
 
-test_expect_failure 'repack rescues once-cruft objects above geometric split' '
+test_expect_success 'repack rescues once-cruft objects above geometric split' '
 	git config repack.midxMustContainCruft false &&
 
 	test_commit reachable &&
-- 
2.53.0.614.g164f3b634ec