cat-file: fall back to OID lookup when a pack disappears mid-stream

"git cat-file --batch-all-objects --unordered" reads each object from
the pack and offset the walk handed it, rather than looking it up
again.  If a concurrent repack removes that pack in between,
packed_object_info() calls use_pack(), which cannot reopen the unlinked
pack and dies with "packfile ... cannot be accessed" -- even though the
object still lives in the pack repack just wrote.

Guard the read with is_pack_valid(): if the pack can no longer be
opened, clear the pointer so we fall back to a normal object-database
lookup, which finds the object in whatever pack now holds it.
is_pack_valid() opens and pins the pack's fd, so once it succeeds the
pack stays readable even if it is unlinked an instant later.

Assisted-by: Claude Opus 4.8
Signed-off-by: Elijah Newren <newren@gmail.com>

Author: newren

builtin/cat-file.c

@@ -543,6 +543,15 @@ static void batch_object_write(const char *obj_name,
 		if (opt->objects_filter.choice == LOFC_BLOB_LIMIT)
 			data->info.sizep = &data->size;
 
+		/*
+		 * With "--batch-all-objects --unordered" we read straight
+		 * from the pack the walk handed us.  If a concurrent repack
+		 * removed it, fall back to a normal lookup; is_pack_valid()
+		 * pins the pack's fd, so a passing check stays valid even if
+		 * the pack is unlinked right after.
+		 */
+		if (pack && !is_pack_valid(pack))
+			pack = NULL;
 		if (pack)
 			ret = packed_object_info(NULL, pack, offset, &data->info);
 		else