From 655d31726a84dc73c4a0a08288c3cc4bd2c66ef2 Mon Sep 17 00:00:00 2001
From: Tobias Lindahl <tobias@happihacking.com>
Date: Tue, 19 Oct 2021 10:49:17 +0200
Subject: [PATCH] feat(persistent_session): add early removal of delivered
 message

---
 apps/emqx/src/emqx_persistent_session.erl | 9 ++++++++-
 1 file changed, 8 insertions(+), 1 deletion(-)

diff --git a/apps/emqx/src/emqx_persistent_session.erl b/apps/emqx/src/emqx_persistent_session.erl
index 3b3db3eab..5db7aa81a 100644
--- a/apps/emqx/src/emqx_persistent_session.erl
+++ b/apps/emqx/src/emqx_persistent_session.erl
@@ -345,7 +345,14 @@ mark_as_delivered(SessionID, List) ->
 
 do_mark_as_delivered(SessionID, [{deliver, STopic, Msg}|Left]) ->
     MsgID = emqx_message:id(Msg),
-    put_session_message({SessionID, MsgID, STopic, ?DELIVERED}),
+    case next_session_message({SessionID, MsgID, STopic, ?ABANDONED}) of
+        {SessionID, MsgID, STopic, ?UNDELIVERED} = Key ->
+            %% We can safely delete this entry
+            %% instead of marking it as delivered.
+            delete_session_message(Key);
+        _ ->
+            put_session_message({SessionID, MsgID, STopic, ?DELIVERED})
+    end,
     do_mark_as_delivered(SessionID, Left);
 do_mark_as_delivered(_SessionID, []) ->
     ok.