models/server: fix one possible error when frps accept too many user connections in a short time

src/frp/cmd/frps/control.go

@@ -243,7 +243,7 @@ func doLogin(req *msg.ControlReq, c *conn.Conn) (ret int64, info string) {
 			return
 		}
 		// the connection will close after join over
-		s.RecvNewWorkConn(c)
+		s.RegisterNewWorkConn(c)
 	} else {
 		info = fmt.Sprintf("Unsupport login message type [%d]", req.Type)
 		log.Warn("Unsupport login message type [%d]", req.Type)

src/frp/models/server/server.go

@@ -15,7 +15,7 @@
 package server
 
 import (
-	"container/list"
+	"fmt"
 	"sync"
 	"time"
 
@@ -42,15 +42,13 @@ type ProxyServer struct {
 	listeners    []Listener      // accept new connection from remote users
 	ctlMsgChan   chan int64      // every time accept a new user conn, put "1" to the channel
 	workConnChan chan *conn.Conn // get new work conns from control goroutine
-	userConnList *list.List      // store user conns
 	mutex        sync.Mutex
 }
 
 func (p *ProxyServer) Init() {
 	p.Status = consts.Idle
-	p.workConnChan = make(chan *conn.Conn)
+	p.workConnChan = make(chan *conn.Conn, 100)
 	p.ctlMsgChan = make(chan int64)
-	p.userConnList = list.New()
 	p.listeners = make([]Listener, 0)
 }
 
@@ -96,74 +94,34 @@ func (p *ProxyServer) Start() (err error) {
 				}
 				log.Debug("ProxyName [%s], get one new user conn [%s]", p.Name, c.GetRemoteAddr())
 
-				// insert into list
-				p.Lock()
 				if p.Status != consts.Working {
 					log.Debug("ProxyName [%s] is not working, new user conn close", p.Name)
 					c.Close()
-					p.Unlock()
 					return
 				}
-				p.userConnList.PushBack(c)
-				p.Unlock()
 
-				// put msg to control conn
-				p.ctlMsgChan <- 1
-
-				// set timeout
-				time.AfterFunc(time.Duration(UserConnTimeout)*time.Second, func() {
-					p.Lock()
-					element := p.userConnList.Front()
-					p.Unlock()
-					if element == nil {
+				// start another goroutine for join two conns from frpc and user
+				go func() {
+					workConn, err := p.getWorkConn()
+					if err != nil {
 						return
 					}
 
-					userConn := element.Value.(*conn.Conn)
-					if userConn == c {
-						log.Warn("ProxyName [%s], user conn [%s] timeout", p.Name, c.GetRemoteAddr())
-						userConn.Close()
+					userConn := c
+					// msg will transfer to another without modifying
+					// l means local, r means remote
+					log.Debug("Join two connections, (l[%s] r[%s]) (l[%s] r[%s])", workConn.GetLocalAddr(), workConn.GetRemoteAddr(),
+						userConn.GetLocalAddr(), userConn.GetRemoteAddr())
+
+					if p.UseEncryption {
+						go conn.JoinMore(userConn, workConn, p.AuthToken)
+					} else {
+						go conn.Join(userConn, workConn)
 					}
-				})
+				}()
 			}
 		}(listener)
 	}
-
-	// start another goroutine for join two conns from frpc and user
-	go func() {
-		for {
-			workConn, ok := <-p.workConnChan
-			if !ok {
-				return
-			}
-
-			p.Lock()
-			element := p.userConnList.Front()
-
-			var userConn *conn.Conn
-			if element != nil {
-				userConn = element.Value.(*conn.Conn)
-				p.userConnList.Remove(element)
-			} else {
-				workConn.Close()
-				p.Unlock()
-				continue
-			}
-			p.Unlock()
-
-			// msg will transfer to another without modifying
-			// l means local, r means remote
-			log.Debug("Join two connections, (l[%s] r[%s]) (l[%s] r[%s])", workConn.GetLocalAddr(), workConn.GetRemoteAddr(),
-				userConn.GetLocalAddr(), userConn.GetRemoteAddr())
-
-			if p.UseEncryption {
-				go conn.JoinMore(userConn, workConn, p.AuthToken)
-			} else {
-				go conn.Join(userConn, workConn)
-			}
-		}
-	}()
-
 	return nil
 }
 
@@ -180,7 +138,6 @@ func (p *ProxyServer) Close() {
 		}
 		close(p.ctlMsgChan)
 		close(p.workConnChan)
-		p.userConnList = list.New()
 	}
 	p.Unlock()
 }
@@ -195,6 +152,40 @@ func (p *ProxyServer) WaitUserConn() (closeFlag bool) {
 	return
 }
 
-func (p *ProxyServer) RecvNewWorkConn(c *conn.Conn) {
+func (p *ProxyServer) RegisterNewWorkConn(c *conn.Conn) {
 	p.workConnChan <- c
 }
+
+// when frps get one user connection, we get one work connection from the pool and return it
+// if no workConn available in the pool, send message to frpc to get one or more
+// and wait until it is available
+// return an error if wait timeout
+func (p *ProxyServer) getWorkConn() (workConn *conn.Conn, err error) {
+	var ok bool
+
+	// get a work connection from the pool
+	select {
+	case workConn, ok = <-p.workConnChan:
+		if !ok {
+			err = fmt.Errorf("ProxyName [%s], no work connections available, control is closing", p.Name)
+			return
+		}
+	default:
+		// no work connections available in the poll, send message to frpc to get one
+		p.ctlMsgChan <- 1
+
+		select {
+		case workConn, ok = <-p.workConnChan:
+			if !ok {
+				err = fmt.Errorf("ProxyName [%s], no work connections available, control is closing", p.Name)
+				return
+			}
+
+		case <-time.After(time.Duration(UserConnTimeout) * time.Second):
+			log.Warn("ProxyName [%s], timeout trying to get work connection", p.Name)
+			err = fmt.Errorf("ProxyName [%s], timeout trying to get work connection", p.Name)
+			return
+		}
+	}
+	return
+}