[master] 5c2a682aa sml: Optimize freebehind (transient) memory usage

[Nils Goroll]

commit 5c2a682aa39af1c61e860f3a62a204d200ac3211
Author: Nils Goroll <nils.goroll at uplex.de>
Date:   Thu Sep 26 12:06:03 2024 +0200

    sml: Optimize freebehind (transient) memory usage
    
    Previously, sml_iterator() could not free the last segment while iterating,
    because it could have turned out to be over-allocated at any time and be
    removed.
    
    With the change from the previous commit, we can now lift this limitation and
    take a reference to also the last segment.
    
    This is relevant because, due to of the race between the fetch and iterator site
    of a transient operation, the "is last segment" condition can happen at any
    point while an object is busy.

diff --git a/bin/varnishd/storage/storage_simple.c b/bin/varnishd/storage/storage_simple.c
index 74657365a..53eb305de 100644
--- a/bin/varnishd/storage/storage_simple.c
+++ b/bin/varnishd/storage/storage_simple.c
@@ -395,15 +395,14 @@ sml_iterator(struct worker *wrk, struct objcore *oc,
 			assert(nl > 0);
 			sl += st->len;
 			st = VTAILQ_PREV(st, storagehead, list);
-			if (VTAILQ_PREV(st, storagehead, list) != NULL) {
-				if (final && checkpoint != NULL) {
-					VTAILQ_REMOVE(&obj->list,
-					    checkpoint, list);
-					sml_stv_free(stv, checkpoint);
-				}
-				checkpoint = st;
-				checkpoint_len = sl;
+			if (final && checkpoint != NULL) {
+				VTAILQ_REMOVE(&obj->list, checkpoint, list);
+				if (checkpoint == boc->stevedore_priv)
+					boc->stevedore_priv = trim_once;
+				sml_stv_free(stv, checkpoint);
 			}
+			checkpoint = st;
+			checkpoint_len = sl;
 		}
 		CHECK_OBJ_NOTNULL(obj, OBJECT_MAGIC);
 		CHECK_OBJ_NOTNULL(st, STORAGE_MAGIC);