cat-file: snapshot pack indexes before enumerating all objects

The previous commit made "git cat-file --batch-all-objects" error out
when a concurrent repack removes a pack mid-walk.  That is correct but
pessimistic on servers that repack often.

Reduce how often it triggers by opening every pack index up front,
before the walk.  An index mmap survives unlink() of the .idx and pack
fd pressure (close_one_pack() closes only the pack fd), so the object
set is fixed once the indexes are open.  For --batch-all-objects this
adds no work: the walk opens every index anyway.

Like f6b2625 (fsck: snapshot default refs before object walk,
2026-01-09), this narrows rather than closes the race; the residue is
still caught by the previous commit rather than silently dropped.

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

Author: newren

builtin/cat-file.c

@@ -1017,6 +1017,23 @@ static int batch_one_object_oi(const struct object_id *oid,
 	return payload->callback(oid, NULL, 0, payload->payload);
 }
 
+/*
+ * Open every pack index up front so the enumeration's object set is
+ * fixed: an index mmap survives unlink() of the .idx and pack fd
+ * pressure (close_one_pack() closes only the pack fd).  This narrows
+ * the race with concurrent repacks, like f6b262581a88 (fsck: snapshot
+ * default refs before object walk, 2026-01-09).
+ */
+static int snapshot_pack_indexes(void)
+{
+	struct packed_git *p;
+
+	repo_for_each_pack(the_repository, p)
+		if (open_pack_index(p))
+			return -1;
+	return 0;
+}
+
 static int batch_each_object(struct batch_options *opt,
 			     for_each_object_fn callback,
 			     unsigned flags,
@@ -1084,6 +1101,11 @@ static int batch_objects(struct batch_options *opt)
 
 		disable_replace_refs();
 
+		if (snapshot_pack_indexes()) {
+			strbuf_release(&output);
+			return error(_("unable to enumerate all objects"));
+		}
+
 		cb.opt = opt;
 		cb.expand = &data;
 		cb.scratch = &output;