Overlooked a better solution as @PersDep kindly suggested. Sending the fixed version. 
 
Subject: [PATCH] Memtx_tuple_delete used heap after free
Struct of type tuple_format is being passed as
an argument to tuple_format_unref where it might
be freed. On such occasion any further references
to format fields should not take place.
 
Closes #4658
---
Issue:
https://github.com/tarantool/tarantool/issues/4658
Branch:
https://github.com/tarantool/tarantool/compare/eljashm/gh-4658-heap-use-after-free
 
 src/box/memtx_engine.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)
 
diff --git a/src/box/memtx_engine.c b/src/box/memtx_engine.c
index 23ccc4703..4da80824a 100644
--- a/src/box/memtx_engine.c
+++ b/src/box/memtx_engine.c
@@ -1177,7 +1177,6 @@ memtx_tuple_delete(struct tuple_format *format, struct tuple *tuple)
     struct memtx_engine *memtx = (struct memtx_engine *)format->engine;
     say_debug("%s(%p)", __func__, tuple);
     assert(tuple->refs == 0);
-    tuple_format_unref(format);
     struct memtx_tuple *memtx_tuple =
         container_of(tuple, struct memtx_tuple, base);
     size_t total = tuple_size(tuple) + offsetof(struct memtx_tuple, base);
@@ -1187,6 +1186,7 @@ memtx_tuple_delete(struct tuple_format *format, struct tuple *tuple)
         smfree(&memtx->alloc, memtx_tuple, total);
     else
         smfree_delayed(&memtx->alloc, memtx_tuple, total);
+    tuple_format_unref(format);
 }
 
 void
-- 
2.20.1 (Apple Git-117)
  
>Суббота, 30 ноября 2019, 0:39 +03:00 от Maria <maria.khaydich@tarantool.org>:
> 
>Struct of type tuple_format is being passed as
>an argument to tuple_format_unref where it might
>be freed. On such occasion any further references
>to format fields should not take place.
>
>Closes #4658
>---
>Issue:
>https://github.com/tarantool/tarantool/issues/4658
>Branch:
>https://github.com/tarantool/tarantool/compare/eljashm/gh-4658-heap-use-after-free
>
> src/box/memtx_engine.c | 4 ++--
> 1 file changed, 2 insertions(+), 2 deletions(-)
>
>diff --git a/src/box/memtx_engine.c b/src/box/memtx_engine.c
>index 23ccc4703..bdce4ac32 100644
>--- a/src/box/memtx_engine.c
>+++ b/src/box/memtx_engine.c
>@@ -1177,13 +1177,13 @@ memtx_tuple_delete(struct tuple_format *format, struct tuple *tuple)
>  struct memtx_engine *memtx = (struct memtx_engine *)format->engine;
>  say_debug("%s(%p)", __func__, tuple);
>  assert(tuple->refs == 0);
>+ bool is_temp = format->is_temporary;
>  tuple_format_unref(format);
>  struct memtx_tuple *memtx_tuple =
>  container_of(tuple, struct memtx_tuple, base);
>  size_t total = tuple_size(tuple) + offsetof(struct memtx_tuple, base);
>  if (memtx->alloc.free_mode != SMALL_DELAYED_FREE ||
>- memtx_tuple->version == memtx->snapshot_version ||
>- format->is_temporary)
>+ memtx_tuple->version == memtx->snapshot_version || is_temp)
>  smfree(&memtx->alloc, memtx_tuple, total);
>  else
>  smfree_delayed(&memtx->alloc, memtx_tuple, total);
>--
>2.20.1 (Apple Git-117)
>  
 
 
--
Maria Khaydich