fix: Don't send conversation.member-join on conversation reset to existing members

changelog.d/3-bug-fixes/WPB-22820

@@ -0,0 +1 @@
+Fixed a spurious `conversation.member-join` event being sent when an MLS 1:1 conversation is reset and re-established. Previously, re-establishing the group after a reset re-entered the 1:1 creation code path and reported both participants as newly joining, even though membership was unchanged, causing clients to show an incorrect "X added you" notification for a conversation they were already in.

integration/test/Test/MLS/Reset.hs

@@ -19,7 +19,7 @@ module Test.MLS.Reset where
 
 import API.Galley
 import MLS.Util
-import Notifications (isConvResetNotif)
+import Notifications (isConvResetNotif, isWelcomeNotif)
 import SetupHelpers
 import Testlib.Prelude
 import Testlib.VersionedFed
@@ -114,6 +114,42 @@ testResetOne2OneConversation = do
   conv'' %. "epoch" `shouldMatchInt` 1
   conv'' %. "group_id" `shouldMatch` (conv' %. "group_id")
 
+-- | Regression test for WPB-22820: after an MLS 1:1 conversation is reset
+-- (epoch back to 0, group id rotated, membership unchanged) and
+-- re-established, the backend must not report either participant as newly
+-- joining the conversation, since both were already members before the reset.
+testResetOne2OneNoSpuriousMemberJoin :: (HasCallStack) => App ()
+testResetOne2OneNoSpuriousMemberJoin = do
+  [alice, bob] <- createAndConnectUsers [OwnDomain, OwnDomain]
+  [aliceClient, bobClient] <- traverse (createMLSClient def) [alice, bob]
+  void . for [aliceClient, bobClient] $ \cid -> replicateM 2 $ uploadNewKeyPackage def cid
+  conv <- getMLSOne2OneConversation alice bob >>= getJSON 200
+  convId <- objConvId (conv %. "conversation")
+
+  -- establish the 1:1 conversation at epoch 1
+  resetOne2OneGroup def aliceClient conv
+  void $ createAddCommit aliceClient convId [bob] >>= sendAndConsumeCommitBundle
+
+  -- reset back to epoch 0 (group id rotates, membership stays: alice + bob)
+  conv' <- resetMLSConversation aliceClient (conv %. "conversation")
+  convId' <- objConvId conv'
+  resetOne2OneGroupGeneric def aliceClient conv' (conv %. "public_keys")
+
+  withWebSocket alice $ \wsAlice ->
+    withWebSocket bob $ \wsBob -> do
+      void $ createAddCommit aliceClient convId' [bob] >>= sendAndConsumeCommitBundle
+
+      -- Bob rejoins the rotated group (legitimate welcome), but since he was
+      -- already a member of this 1:1 conversation, he must not additionally be
+      -- told (via member-join) that he was just added.
+      void $ awaitMatch isWelcomeNotif wsBob
+      mEvent <- awaitAnyEvent 2 wsBob
+      for_ mEvent $ \n -> n %. "payload.0.type" `shouldNotMatch` "conversation.member-join"
+
+      -- Alice issued the commit and isn't newly joining anything either;
+      -- So, we expect no event for her.
+      awaitAnyEvent 2 wsAlice `shouldMatch` (Nothing :: Maybe Value)
+
 testResetMixedConversation :: (HasCallStack) => Domain -> App ()
 testResetMixedConversation domain = do
   -- create mixed conversation

libs/wire-subsystems/src/Wire/ConversationSubsystem/MLS/Commit/InternalCommit.hs

@@ -197,29 +197,41 @@ processInternalCommit senderIdentity con lConvOrSub ciphersuite ciphersuiteUpdat
                         ( mlsProtocolError
                             "The first commit in a 1-1 conversation should add exactly 1 other user"
                         )
-                  -- notify otherUser about being added to this 1-1 conversation
+                  -- Only emit a member-join if someone is actually newly
+                  -- joining. A reset leaves membership intact but re-enters
+                  -- this branch (epoch back to 0), so on re-establish both
+                  -- users are already members -- emitting a join then would be
+                  -- a spurious "X added you" for a conversation they're
+                  -- already in.
                   let bm = convBotsAndMembers conv
-                  members <-
-                    note
+                      newMembers =
+                        filter
+                          (flip Set.notMember (existingMembers lconv))
+                          (bmQualifiedMembers lconv bm)
+                  void
+                    . note
                       ( InternalErrorWithDescription
                           "Unexpected empty member list in MLS 1-1 conversation"
                       )
-                      $ nonEmpty (bmQualifiedMembers lconv bm)
-                  update <-
-                    sendConversationActionNotifications
-                      SConversationJoinTag
-                      senderUser
-                      False
-                      con
-                      lconv
-                      bm
-                      ConversationJoin
-                        { users = members,
-                          role = roleNameWireMember,
-                          joinType = def
-                        }
-                      def
-                  pure [update]
+                    $ nonEmpty (bmQualifiedMembers lconv bm)
+                  case nonEmpty newMembers of
+                    Nothing -> pure []
+                    Just membersNE -> do
+                      update <-
+                        sendConversationActionNotifications
+                          SConversationJoinTag
+                          senderUser
+                          False
+                          con
+                          lconv
+                          bm
+                          ConversationJoin
+                            { users = membersNE,
+                              role = roleNameWireMember,
+                              joinType = def
+                            }
+                          def
+                      pure [update]
             SubConv _ _ -> pure []
             Conv _ -> do
               -- remove users from the conversation and send events