TUN-8861: Add session limiter to TCP session manager

## Summary
In order to make cloudflared behavior more predictable and
prevent an exhaustion of resources, we have decided to add
session limits that can be configured by the user. This commit
adds the session limiter to the HTTP/TCP handling path.
For now the limiter is set to run only in unlimited mode.

connection/connection_test.go

@@ -2,14 +2,18 @@ package connection
 
 import (
 	"context"
+	"crypto/rand"
 	"fmt"
 	"io"
-	"math/rand"
+	"math/big"
 	"net/http"
 	"time"
 
+	pkgerrors "github.com/pkg/errors"
 	"github.com/rs/zerolog"
 
+	cfdsession "github.com/cloudflare/cloudflared/session"
+
 	"github.com/cloudflare/cloudflared/stream"
 	"github.com/cloudflare/cloudflared/tracing"
 	tunnelpogs "github.com/cloudflare/cloudflared/tunnelrpc/pogs"
@@ -77,7 +81,7 @@ func (moc *mockOriginProxy) ProxyHTTP(
 			return wsFlakyEndpoint(w, req)
 		default:
 			originRespEndpoint(w, http.StatusNotFound, []byte("ws endpoint not found"))
-			return fmt.Errorf("Unknwon websocket endpoint %s", req.URL.Path)
+			return fmt.Errorf("unknown websocket endpoint %s", req.URL.Path)
 		}
 	}
 	switch req.URL.Path {
@@ -95,7 +99,6 @@ func (moc *mockOriginProxy) ProxyHTTP(
 		originRespEndpoint(w, http.StatusNotFound, []byte("page not found"))
 	}
 	return nil
-
 }
 
 func (moc *mockOriginProxy) ProxyTCP(
@@ -103,6 +106,10 @@ func (moc *mockOriginProxy) ProxyTCP(
 	rwa ReadWriteAcker,
 	r *TCPRequest,
 ) error {
+	if r.CfTraceID == "flow-rate-limited" {
+		return pkgerrors.Wrap(cfdsession.ErrTooManyActiveSessions, "tcp flow rate limited")
+	}
+
 	return nil
 }
 
@@ -178,7 +185,8 @@ func wsFlakyEndpoint(w ResponseWriter, r *http.Request) error {
 
 	wsConn := websocket.NewConn(wsCtx, NewHTTPResponseReadWriterAcker(w, w.(http.Flusher), r), &log)
 
-	closedAfter := time.Millisecond * time.Duration(rand.Intn(50))
+	rInt, _ := rand.Int(rand.Reader, big.NewInt(50))
+	closedAfter := time.Millisecond * time.Duration(rInt.Int64())
 	originConn := &flakyConn{closeAt: time.Now().Add(closedAfter)}
 	stream.Pipe(wsConn, originConn, &log)
 	cancel()