From 24be189728e28ee52be1a2b301597fc7b08d1db2 Mon Sep 17 00:00:00 2001
From: Andrew Mayorov <encube.ul@gmail.com>
Date: Wed, 29 May 2024 12:49:53 +0200
Subject: [PATCH] fix(topic): respect special topic rules when intersecting

---
 apps/emqx/src/emqx_topic.erl        |  9 ++++++++-
 apps/emqx/test/emqx_topic_SUITE.erl | 16 ++++++++++++++++
 2 files changed, 24 insertions(+), 1 deletion(-)

diff --git a/apps/emqx/src/emqx_topic.erl b/apps/emqx/src/emqx_topic.erl
index 9cd631508..dd3d4294e 100644
--- a/apps/emqx/src/emqx_topic.erl
+++ b/apps/emqx/src/emqx_topic.erl
@@ -111,11 +111,18 @@ match(_, _) ->
 -spec intersection(TopicOrFilter, TopicOrFilter) -> TopicOrFilter | false when
     TopicOrFilter :: emqx_types:topic().
 intersection(Topic1, Topic2) when is_binary(Topic1), is_binary(Topic2) ->
-    case intersection(words(Topic1), words(Topic2), []) of
+    case intersect_start(words(Topic1), words(Topic2)) of
         [] -> false;
         Intersection -> join(lists:reverse(Intersection))
     end.
 
+intersect_start([<<"$", _/bytes>> | _], [W | _]) when ?IS_WILDCARD(W) ->
+    [];
+intersect_start([W | _], [<<"$", _/bytes>> | _]) when ?IS_WILDCARD(W) ->
+    [];
+intersect_start(Words1, Words2) ->
+    intersection(Words1, Words2, []).
+
 intersection(Words1, ['#'], Acc) ->
     lists:reverse(Words1, Acc);
 intersection(['#'], Words2, Acc) ->
diff --git a/apps/emqx/test/emqx_topic_SUITE.erl b/apps/emqx/test/emqx_topic_SUITE.erl
index 6e0451591..5993fbc40 100644
--- a/apps/emqx/test/emqx_topic_SUITE.erl
+++ b/apps/emqx/test/emqx_topic_SUITE.erl
@@ -28,6 +28,7 @@
     [
         wildcard/1,
         match/2,
+        intersection/2,
         validate/1,
         prepend/2,
         join/1,
@@ -128,6 +129,21 @@ t_match_perf(_) ->
     true = match(Name, Filter),
     ok = bench('match/2', fun emqx_topic:match/2, [Name, Filter]).
 
+t_intersect(_) ->
+    <<"t/global/1/+">> = intersection(<<"t/global/#">>, <<"t/+/1/+">>),
+    <<"t/global/#">> = intersection(<<"t/global/#">>, <<"#">>),
+    <<"t/global/#">> = intersection(<<"t/global/#">>, <<"t/global/#">>),
+    <<"1/2/3/4/5">> = intersection(<<"1/+/3/+/5/#">>, <<"+/2/+/4/+">>),
+    <<"t/local/1">> = intersection(<<"t/local/1/#">>, <<"t/local/+">>),
+    false = intersection(<<"t/global/#">>, <<"t/local/+">>),
+    false = intersection(<<"t/local/1/+">>, <<"t/local/+">>).
+
+t_sys_intersect(_) ->
+    <<"$SYS/broker/+">> = intersection(<<"$SYS/broker/#">>, <<"$SYS/+/+">>),
+    <<"$SYS/broker">> = intersection(<<"$SYS/broker">>, <<"$SYS/+">>),
+    false = intersection(<<"$SYS/broker">>, <<"+/+">>),
+    false = intersection(<<"$SYS/broker">>, <<"#">>).
+
 t_validate(_) ->
     true = validate(<<"a/+/#">>),
     true = validate(<<"a/b/c/d">>),