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>

app/server.go

@@ -30,6 +30,7 @@ import (
 	"github.com/getsentry/sentry-go"
 	sentryhttp "github.com/getsentry/sentry-go/http"
 	"github.com/gorilla/mux"
+	"github.com/mailru/easygo/netpoll"
 	"github.com/pkg/errors"
 	"github.com/rs/cors"
 	"golang.org/x/crypto/acme/autocert"
@@ -102,8 +103,11 @@ type Server struct {
 
 	EmailService *EmailService
 
-	hubs     []*Hub
-	hashSeed maphash.Seed
+	hubs          []*Hub
+	hashSeed      maphash.Seed
+	poller        netpoll.Poller
+	webConnSema   chan struct{}
+	webConnSemaWg sync.WaitGroup
 
 	PushNotificationsHub   PushNotificationsHub
 	pushNotificationClient *http.Client // TODO: move this to it's own package
@@ -226,6 +230,16 @@ func NewServer(options ...Option) (*Server, error) {
 		mlog.Error("Could not initiate logging", mlog.Err(err))
 	}
 
+	// epoll/kqueue is not available on Windows.
+	if runtime.GOOS != "windows" {
+		poller, err := netpoll.New(nil)
+		if err != nil {
+			return nil, errors.Wrap(err, "failed to create a netpoll instance")
+		}
+		s.poller = poller
+		s.webConnSema = make(chan struct{}, runtime.NumCPU()*8) // numCPU * 8 is a good amount of concurrency.
+	}
+
 	// This is called after initLogging() to avoid a race condition.
 	mlog.Info("Server is initializing...", mlog.String("go_version", runtime.Version()))
 
@@ -259,9 +273,9 @@ func NewServer(options ...Option) (*Server, error) {
 	}
 
 	if *s.Config().ServiceSettings.EnableOpenTracing {
-		tracer, err := tracing.New()
-		if err != nil {
-			return nil, err
+		tracer, err2 := tracing.New()
+		if err2 != nil {
+			return nil, err2
 		}
 		s.tracer = tracer
 	}
@@ -1201,7 +1215,7 @@ func (a *App) OriginChecker() func(*http.Request) bool {
 
 		return utils.OriginChecker(allowed)
 	}
-	return nil
+	return utils.SameOriginChecker()
 }
 
 func (s *Server) checkPushNotificationServerUrl() {
@@ -1621,6 +1635,10 @@ func (s *Server) SetLog(l *mlog.Logger) {
 	s.Log = l
 }
 
+func (s *Server) Poller() netpoll.Poller {
+	return s.poller
+}
+
 func (a *App) GenerateSupportPacket() []model.FileData {
 	// If any errors we come across within this function, we will log it in a warning.txt file so that we know why certain files did not get produced if any
 	var warnings []string