packfile: refactor misleading code when unusing pack windows

The function `unuse_one_window()` is responsible for unmapping one of
the packfile windows, which is done when we have exceeded the allowed
number of window.

The function receives a `struct packed_git` as input, which serves as an
additional packfile that should be considered to be closed. If not
given, we seemingly skip that and instead go through all of the
repository's packfiles. The conditional that checks whether we have a
packfile though does not make much sense anymore, as we dereference the
packfile regardless of whether or not it is a `NULL` pointer to derive
the repository's packfile store.

The function was originally introduced via f0e17e8 (pack: move
release_pack_memory(), 2017-08-18), and here we indeed had a caller that
passed a `NULL` pointer. That caller was later removed via 9827d4c
(packfile: drop release_pack_memory(), 2019-08-12), so starting with
that commit we always pass a `struct packed_git`. In 9c5ce06
(packfile: use `repository` from `packed_git` directly, 2024-12-03) we
then inadvertently started to rely on the fact that the pointer is never
`NULL` because we use it now to identify the repository.

Arguably, it didn't really make sense in the first place that the caller
provides a packfile, as the selected window would have been overridden
anyway by the subsequent loop over all packfiles if there was an older
window. So the overall logic is quite misleading overall. The only case
where it _could_ make a difference is when there were two packfiles with
the same `last_used` value, but that case doesn't ever happen because
the `pack_used_ctr` is strictly increasing.

Refactor the code so that we instead pass in the object database to
help make the code less misleading.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Author: pks-t

packfile.c

@@ -355,16 +355,15 @@ static void scan_windows(struct packed_git *p,
 	}
 }
 
-static int unuse_one_window(struct packed_git *current)
+static int unuse_one_window(struct object_database *odb)
 {
 	struct packfile_list_entry *e;
 	struct packed_git *lru_p = NULL;
 	struct pack_window *lru_w = NULL, *lru_l = NULL;
 
-	if (current)
-		scan_windows(current, &lru_p, &lru_w, &lru_l);
-	for (e = current->repo->objects->packfiles->packs.head; e; e = e->next)
+	for (e = odb->packfiles->packs.head; e; e = e->next)
 		scan_windows(e->pack, &lru_p, &lru_w, &lru_l);
+
 	if (lru_p) {
 		munmap(lru_w->base, lru_w->len);
 		pack_mapped -= lru_w->len;
@@ -740,8 +739,8 @@ unsigned char *use_pack(struct packed_git *p,
 			win->len = (size_t)len;
 			pack_mapped += win->len;
 
-			while (settings->packed_git_limit < pack_mapped
-				&& unuse_one_window(p))
+			while (settings->packed_git_limit < pack_mapped &&
+			       unuse_one_window(p->repo->objects))
 				; /* nothing */
 			win->base = xmmap_gently(NULL, win->len,
 				PROT_READ, MAP_PRIVATE,