MM-33893: Disable TCP_NO_DELAY for websocket connections (#17129)

* MM-33893: Disable TCP_NO_DELAY for websocket connections

In very large installations, websocket messages cause too much
traffic congestion by sending too small packets and thereby cause
a drop in throughput.

To counter this, we disable the TCP_NO_DELAY flag for websocket
connections. This has shown to give noticeable improvements in
load tests.

We wrap this in a feature flag for now to let it soak in Community
first.

```release-note
NONE
```

https://mattermost.atlassian.net/browse/MM-33893

* fix gorilla specific conn

app/web_conn.go

@@ -7,6 +7,7 @@ import (
 	"bytes"
 	"encoding/json"
 	"fmt"
+	"net"
 	"net/http"
 	"sync"
 	"sync/atomic"
@@ -61,6 +62,17 @@ func (a *App) NewWebConn(ws *websocket.Conn, session model.Session, t i18n.Trans
 		})
 	}
 
+	if a.srv.Config().FeatureFlags.WebSocketDelay {
+		// Disable TCP_NO_DELAY for higher throughput
+		tcpConn, ok := ws.UnderlyingConn().(*net.TCPConn)
+		if ok {
+			err := tcpConn.SetNoDelay(false)
+			if err != nil {
+				mlog.Warn("Error in setting NoDelay socket opts", mlog.Err(err))
+			}
+		}
+	}
+
 	wc := &WebConn{
 		App:                a,
 		send:               make(chan model.WebSocketMessage, sendQueueSize),

model/feature_flags.go

@@ -23,6 +23,8 @@ type FeatureFlags struct {
 	PluginIncidentManagement string `plugin_id:"com.mattermost.plugin-incident-management"`
 	// Toggle on and off support for Files search
 	FilesSearch bool
+	// Feature flag to control setting the TCP_NO_DELAY setting for websockets.
+	WebSocketDelay bool
 }
 
 func (f *FeatureFlags) SetDefaults() {
@@ -32,6 +34,7 @@ func (f *FeatureFlags) SetDefaults() {
 	f.CollapsedThreads = false
 	f.FilesSearch = false
 	f.PluginIncidentManagement = "1.6.0"
+	f.WebSocketDelay = false
 }
 
 func (f *FeatureFlags) Plugins() map[string]string {