MM-62960: Support both webhub iteration scopes properly (#30224)

When Channel iteration mode is enabled, we need to ensure that
channel scoped events do not fall through to the connIndex.All()
condition. This is possible because there are multiple hubs
in a given system. So a single event will flow through all of them
and in some hubs, a channel scoped event might not have any connections.

In that case, we need to stop processing further.

https://mattermost.atlassian.net/browse/MM-62960

```release-note
NONE
```

server/channels/api4/post_test.go

@@ -3354,8 +3354,25 @@ func TestPermanentDeletePost(t *testing.T) {
 }
 
 func TestWebHubMembership(t *testing.T) {
-	th := Setup(t).InitBasic()
-	defer th.TearDown()
+	t.Run("WithChannelIteration", func(t *testing.T) {
+		th := SetupConfig(t, func(cfg *model.Config) {
+			*cfg.ServiceSettings.EnableWebHubChannelIteration = true
+		}).InitBasic()
+		defer th.TearDown()
+
+		_testWebHubMembership(th, t)
+	})
+
+	t.Run("WithoutChannelIteration", func(t *testing.T) {
+		th := Setup(t).InitBasic()
+		defer th.TearDown()
+
+		_testWebHubMembership(th, t)
+	})
+}
+
+func _testWebHubMembership(th *TestHelper, t *testing.T) {
+	t.Helper()
 
 	u1 := th.CreateUser()
 	th.LinkUserToTeam(u1, th.BasicTeam)

server/channels/app/platform/web_hub.go

@@ -682,6 +682,14 @@ func (h *Hub) Start() {
 					continue
 				}
 
+				// There are multiple hubs in a system. So while supporting both channel based iteration and the old
+				// method, there would be events scoped to a channel being sent to multiple hubs. And only one hub would
+				// have the targetConns. Therefore, we need to stop here if channel based iteration is enabled, and it's a
+				// channel-scoped event.
+				if channelID := msg.GetBroadcast().ChannelId; channelID != "" && *h.platform.Config().ServiceSettings.EnableWebHubChannelIteration {
+					continue
+				}
+
 				for webConn := range connIndex.All() {
 					broadcast(webConn)
 				}

server/channels/app/platform/web_hub_test.go

@@ -386,7 +386,11 @@ func TestHubConnIndex(t *testing.T) {
 
 		t.Run("ForChannel", func(t *testing.T) {
 			require.Len(t, connIndex.byChannelID, 1)
-			require.Equal(t, []*WebConn{wc1, wc2, wc3}, connIndex.ForChannel(th.BasicChannel.Id))
+			ids := make([]string, 0)
+			for _, c := range connIndex.ForChannel(th.BasicChannel.Id) {
+				ids = append(ids, c.GetConnectionID())
+			}
+			require.ElementsMatch(t, []string{wc1ID, wc2ID, wc3ID}, ids)
 			require.Len(t, connIndex.ForChannel("notexist"), 0)
 		})