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/web_broadcast_hooks_test.go

@@ -0,0 +1,114 @@
+// Copyright (c) 2015-present Mattermost, Inc. All Rights Reserved.
+// See LICENSE.txt for license information.
+
+package app
+
+import (
+	"testing"
+
+	"github.com/mattermost/mattermost/server/public/model"
+	"github.com/mattermost/mattermost/server/v8/channels/app/platform"
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+)
+
+func TestAddMentionsHook_Process(t *testing.T) {
+	hook := &addMentionsBroadcastHook{}
+
+	userID := model.NewId()
+	otherUserID := model.NewId()
+
+	webConn := &platform.WebConn{
+		UserId: userID,
+	}
+
+	t.Run("should add a mentions entry for the current user", func(t *testing.T) {
+		msg := platform.MakeHookedWebSocketEvent(model.NewWebSocketEvent(model.WebsocketEventPosted, "", "", "", nil, ""))
+
+		require.Nil(t, msg.Event().GetData()["mentions"])
+
+		hook.Process(msg, webConn, map[string]any{
+			"mentions": model.StringArray{userID},
+		})
+
+		assert.Equal(t, `["`+userID+`"]`, msg.Event().GetData()["mentions"])
+		assert.Nil(t, msg.Event().GetData()["followers"])
+	})
+
+	t.Run("should not add a mentions entry for another user", func(t *testing.T) {
+		msg := platform.MakeHookedWebSocketEvent(model.NewWebSocketEvent(model.WebsocketEventPosted, "", "", "", nil, ""))
+
+		require.Nil(t, msg.Event().GetData()["mentions"])
+
+		hook.Process(msg, webConn, map[string]any{
+			"mentions": model.StringArray{otherUserID},
+		})
+
+		assert.Nil(t, msg.Event().GetData()["mentions"])
+	})
+}
+
+func TestAddFollowersHook_Process(t *testing.T) {
+	hook := &addFollowersBroadcastHook{}
+
+	userID := model.NewId()
+	otherUserID := model.NewId()
+
+	webConn := &platform.WebConn{
+		UserId: userID,
+	}
+
+	t.Run("should add a followers entry for the current user", func(t *testing.T) {
+		msg := platform.MakeHookedWebSocketEvent(model.NewWebSocketEvent(model.WebsocketEventPosted, "", "", "", nil, ""))
+
+		require.Nil(t, msg.Event().GetData()["followers"])
+
+		hook.Process(msg, webConn, map[string]any{
+			"followers": model.StringArray{userID},
+		})
+
+		assert.Equal(t, `["`+userID+`"]`, msg.Event().GetData()["followers"])
+	})
+
+	t.Run("should not add a followers entry for another user", func(t *testing.T) {
+		msg := platform.MakeHookedWebSocketEvent(model.NewWebSocketEvent(model.WebsocketEventPosted, "", "", "", nil, ""))
+
+		require.Nil(t, msg.Event().GetData()["followers"])
+
+		hook.Process(msg, webConn, map[string]any{
+			"followers": model.StringArray{otherUserID},
+		})
+
+		assert.Nil(t, msg.Event().GetData()["followers"])
+	})
+}
+
+func TestAddMentionsAndAddFollowersHooks(t *testing.T) {
+	addMentionsHook := &addMentionsBroadcastHook{}
+	addFollowersHook := &addFollowersBroadcastHook{}
+
+	userID := model.NewId()
+
+	webConn := &platform.WebConn{
+		UserId: userID,
+	}
+
+	msg := platform.MakeHookedWebSocketEvent(model.NewWebSocketEvent(model.WebsocketEventPosted, "", "", "", nil, ""))
+
+	originalData := msg.Event().GetData()
+
+	require.Nil(t, originalData["mentions"])
+	require.Nil(t, originalData["followers"])
+
+	addMentionsHook.Process(msg, webConn, map[string]any{
+		"mentions": model.StringArray{userID},
+	})
+	addFollowersHook.Process(msg, webConn, map[string]any{
+		"followers": model.StringArray{userID},
+	})
+
+	t.Run("should be able to add both mentions and followers to a single event", func(t *testing.T) {
+		assert.Equal(t, `["`+userID+`"]`, msg.Event().GetData()["followers"])
+		assert.Equal(t, `["`+userID+`"]`, msg.Event().GetData()["mentions"])
+	})
+}