MM-54238 Add WebSocket broadcast hook and don't broadcast when other users were mentioned (#24641)

* MM-54238 Initial implementation

* MM-54238 Move websocket hook into app package

* MM-54238 Add tests for mentions in posted websocket messages

* Fix styling

* Fix other styling

* Idiomatic ID naming for new code

* Fix more styles

* Separate hooks to add mentions and followers

* Improved error handling for invalid types in hooks

* Rename HasChanges to ShouldProcess

* Pass broadcast hooks through hubStart

* Add test helper for asserting json unmarshaling

* Fix missing arguments in tests

* Ensure broadcast hooks are sent across the cluster and not to users

* Ensure tests actually cover following a post

* Fix code broken by merge

* Go vet again...

* Deep copy event before processing it with hooks

* Replace RemoveBroadcastHooks with WithoutBroadcastHooks

* Address feedback

* Add helper to fix type information for hook args

* Wrap WebSocketEvent and simplify BroadcastHook

* Address feedback

* Address feedback

server/channels/app/platform/web_hub_test.go

@@ -4,6 +4,7 @@
 package platform
 
 import (
+	"bytes"
 	"encoding/json"
 	"net"
 	"net/http"
@@ -69,7 +70,7 @@ func TestHubStopWithMultipleConnections(t *testing.T) {
 	})
 	require.NoError(t, err)
 
-	th.Service.Start()
+	th.Service.Start(nil)
 	wc1 := registerDummyWebConn(t, th, s.Listener.Addr(), session)
 	wc2 := registerDummyWebConn(t, th, s.Listener.Addr(), session)
 	wc3 := registerDummyWebConn(t, th, s.Listener.Addr(), session)
@@ -93,7 +94,7 @@ func TestHubStopRaceCondition(t *testing.T) {
 	})
 	require.NoError(t, err)
 
-	th.Service.Start()
+	th.Service.Start(nil)
 	wc1 := registerDummyWebConn(t, th, s.Listener.Addr(), session)
 	defer wc1.Close()
 
@@ -476,7 +477,7 @@ func TestHubIsRegistered(t *testing.T) {
 	s := httptest.NewServer(dummyWebsocketHandler(t))
 	defer s.Close()
 
-	th.Service.Start()
+	th.Service.Start(nil)
 	wc1 := registerDummyWebConn(t, th, s.Listener.Addr(), session)
 	wc2 := registerDummyWebConn(t, th, s.Listener.Addr(), session)
 	wc3 := registerDummyWebConn(t, th, s.Listener.Addr(), session)
@@ -583,7 +584,7 @@ func BenchmarkGetHubForUserId(b *testing.B) {
 	th := Setup(b).InitBasic()
 	defer th.TearDown()
 
-	th.Service.Start()
+	th.Service.Start(nil)
 
 	b.ResetTimer()
 	for i := 0; i < b.N; i++ {
@@ -618,3 +619,54 @@ func TestClusterBroadcast(t *testing.T) {
 	require.NoError(t, err)
 	require.Equal(t, clusterEvent.Broadcast, broadcast)
 }
+
+func TestClusterBroadcastHooks(t *testing.T) {
+	t.Run("should send broadcast hook information across cluster", func(t *testing.T) {
+		testCluster := &testlib.FakeClusterInterface{}
+
+		th := SetupWithCluster(t, testCluster)
+		defer th.TearDown()
+
+		hookID := broadcastTest
+		hookArgs := map[string]any{
+			"makes_changes": true,
+		}
+
+		event := model.NewWebSocketEvent(model.WebsocketEventPosted, "", "", "", nil, "")
+		event.GetBroadcast().AddHook(hookID, hookArgs)
+
+		th.Service.Publish(event)
+
+		received, err := model.WebSocketEventFromJSON(bytes.NewReader(testCluster.GetMessages()[0].Data))
+
+		require.NoError(t, err)
+		assert.Equal(t, []string{hookID}, received.GetBroadcast().BroadcastHooks)
+		assert.Equal(t, []map[string]any{hookArgs}, received.GetBroadcast().BroadcastHookArgs)
+	})
+
+	t.Run("should not preserve type information for args", func(t *testing.T) {
+		// This behaviour isn't ideal, but this test confirms that it hasn't changed
+		testCluster := &testlib.FakeClusterInterface{}
+
+		th := SetupWithCluster(t, testCluster)
+		defer th.TearDown()
+
+		hookID := "test_broadcast_hook_with_args"
+		hookArgs := map[string]any{
+			"user":  &model.User{Id: "user1"},
+			"array": []string{"a", "b", "c"},
+		}
+
+		event := model.NewWebSocketEvent(model.WebsocketEventPosted, "", "", "", nil, "")
+		event.GetBroadcast().AddHook(hookID, hookArgs)
+
+		th.Service.Publish(event)
+
+		received, err := model.WebSocketEventFromJSON(bytes.NewReader(testCluster.GetMessages()[0].Data))
+
+		require.NoError(t, err)
+		assert.Equal(t, []string{hookID}, received.GetBroadcast().BroadcastHooks)
+		assert.IsType(t, map[string]any{}, received.GetBroadcast().BroadcastHookArgs[0]["user"])
+		assert.IsType(t, []any{}, received.GetBroadcast().BroadcastHookArgs[0]["array"])
+	})
+}