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_broadcast_hook.go

@@ -0,0 +1,87 @@
+// Copyright (c) 2015-present Mattermost, Inc. All Rights Reserved.
+// See LICENSE.txt for license information.
+
+package platform
+
+import (
+	"github.com/mattermost/mattermost/server/public/model"
+	"github.com/mattermost/mattermost/server/public/shared/mlog"
+)
+
+type BroadcastHook interface {
+	// Process takes a WebSocket event and modifies it in some way. It is passed a HookedWebSocketEvent which allows
+	// safe modification of the event.
+	Process(msg *HookedWebSocketEvent, webConn *WebConn, args map[string]any) error
+}
+
+func (h *Hub) runBroadcastHooks(msg *model.WebSocketEvent, webConn *WebConn, hookIDs []string, hookArgs []map[string]any) *model.WebSocketEvent {
+	if len(hookIDs) == 0 {
+		return msg
+	}
+
+	hookedEvent := MakeHookedWebSocketEvent(msg)
+
+	for i, hookID := range hookIDs {
+		hook := h.broadcastHooks[hookID]
+		args := hookArgs[i]
+		if hook == nil {
+			mlog.Warn("runBroadcastHooks: Unable to find broadcast hook", mlog.String("hook_id", hookID))
+			continue
+		}
+
+		hook.Process(hookedEvent, webConn, args)
+	}
+
+	return hookedEvent.Event()
+}
+
+// HookedWebSocketEvent is a wrapper for model.WebSocketEvent that is intended to provide a similar interface, except
+// it ensures the original WebSocket event is not modified.
+type HookedWebSocketEvent struct {
+	original *model.WebSocketEvent
+	copy     *model.WebSocketEvent
+}
+
+func MakeHookedWebSocketEvent(event *model.WebSocketEvent) *HookedWebSocketEvent {
+	return &HookedWebSocketEvent{
+		original: event,
+	}
+}
+
+func (he *HookedWebSocketEvent) Add(key string, value any) {
+	he.copyIfNecessary()
+
+	he.copy.Add(key, value)
+}
+
+func (he *HookedWebSocketEvent) EventType() string {
+	if he.copy == nil {
+		return he.original.EventType()
+	}
+
+	return he.copy.EventType()
+}
+
+// Get returns a value from the WebSocket event data. You should never mutate a value returned by this method.
+func (he *HookedWebSocketEvent) Get(key string) any {
+	if he.copy == nil {
+		return he.original.GetData()[key]
+	}
+
+	return he.copy.GetData()[key]
+}
+
+// copyIfNecessary should be called by any mutative method to ensure that the copy is instantiated.
+func (he *HookedWebSocketEvent) copyIfNecessary() {
+	if he.copy == nil {
+		he.copy = he.original.RemovePrecomputedJSON()
+	}
+}
+
+func (he *HookedWebSocketEvent) Event() *model.WebSocketEvent {
+	if he.copy == nil {
+		return he.original
+	}
+
+	return he.copy
+}