MM-21012: Revamp websocket implementation (#16620)

* MM-21012: Revamp websocket implementation

We replace the old gorilla/websocket implementation with the
gobwas/ws library. The gorilla library was in maintenance mode
and had a high level API due to which we cannot use that for
situations where a large number of concurrent connections needs
to be supported.

The ws library is a very low-level library that allows us
to work with raw net.Conns. We make several improvements:

- We completely remove the reader goroutines, and instead
replace them with a manual epoll implementation which sends off
messages to be read when it receives any data on the connection.
This lets us scale to a much larger number of connections.
- The reader buffer is eliminated, because we directly read
from the connection now.

https://mattermost.atlassian.net/browse/MM-21012

```release-notes
Improved the websocket implementation by using epoll manually
to read from a websocket. As a result, the number of goroutines
is expected to go down by half.
```

* fix tests

* fix shadowing errors

* final changes

* windows support!

* Remove pointer to waitgroup

* Fix edge case

* Trigger CI

* Trigger CI

Co-authored-by: Mattermod <mattermod@users.noreply.github.com>

api4/websocket.go

@@ -5,8 +5,10 @@ package api4
 
 import (
 	"net/http"
+	"runtime"
+	"time"
 
-	"github.com/gorilla/websocket"
+	"github.com/gobwas/ws"
 
 	"github.com/mattermost/mattermost-server/v5/model"
 )
@@ -17,23 +19,30 @@ func (api *API) InitWebSocket() {
 }
 
 func connectWebSocket(c *Context, w http.ResponseWriter, r *http.Request) {
-	upgrader := websocket.Upgrader{
-		ReadBufferSize:  model.SOCKET_MAX_MESSAGE_SIZE_KB,
-		WriteBufferSize: model.SOCKET_MAX_MESSAGE_SIZE_KB,
-		CheckOrigin:     c.App.OriginChecker(),
+	fn := c.App.OriginChecker()
+	if fn != nil && !fn(r) {
+		c.Err = model.NewAppError("origin_check", "api.web_socket.connect.check_origin.app_error", nil, "", http.StatusBadRequest)
+		return
 	}
 
-	ws, err := upgrader.Upgrade(w, r, nil)
+	upgrader := ws.HTTPUpgrader{
+		Timeout: 5 * time.Second,
+	}
+
+	conn, _, _, err := upgrader.Upgrade(r, w)
 	if err != nil {
 		c.Err = model.NewAppError("connect", "api.web_socket.connect.upgrade.app_error", nil, "", http.StatusInternalServerError)
 		return
 	}
 
-	wc := c.App.NewWebConn(ws, *c.App.Session(), c.App.T, "")
-
+	wc := c.App.NewWebConn(conn, *c.App.Session(), c.App.T, "")
 	if c.App.Session().UserId != "" {
 		c.App.HubRegister(wc)
 	}
 
-	wc.Pump()
+	if runtime.GOOS == "windows" {
+		wc.BlockingPump()
+	} else {
+		go wc.Pump()
+	}
 }