fix: delay accept new connections if out of fds

net_listener.go

@@ -91,7 +91,15 @@ func (ln *listener) Accept() (net.Conn, error) {
 	// tcp
 	var fd, sa, err = syscall.Accept(ln.fd)
 	if err != nil {
-		if err == syscall.EAGAIN {
+		/* https://man7.org/linux/man-pages/man2/accept.2.html
+		EAGAIN or EWOULDBLOCK
+		  The socket is marked nonblocking and no connections are
+		  present to be accepted.  POSIX.1-2001 and POSIX.1-2008
+		  allow either error to be returned for this case, and do
+		  not require these constants to have the same value, so a
+		  portable application should check for both possibilities.
+		*/
+		if err == syscall.EAGAIN || err == syscall.EWOULDBLOCK {
 			return nil, nil
 		}
 		return nil, err

netpoll_server.go

@@ -22,6 +22,7 @@ import (
 	"errors"
 	"strings"
 	"sync"
+	"syscall"
 	"time"
 )
 
@@ -92,39 +93,86 @@ func (s *server) Close(ctx context.Context) error {
 func (s *server) OnRead(p Poll) error {
 	// accept socket
 	conn, err := s.ln.Accept()
-	if err != nil {
-		// shut down
-		if strings.Contains(err.Error(), "closed") {
-			s.operator.Control(PollDetach)
-			s.onQuit(err)
+	if err == nil {
+		if conn != nil {
+			s.onAccept(conn.(Conn))
+		}
+		// EAGAIN | EWOULDBLOCK if conn and err both nil
+		return nil
+	}
+	logger.Printf("NETPOLL: accept conn failed: %v", err)
+
+	// delay accept when too many open files
+	if isOutOfFdErr(err) {
+		// since we use Epoll LT, we have to detach listener fd from epoll first
+		// and re-register it when accept successfully or there is no available connection
+		cerr := s.operator.Control(PollDetach)
+		if cerr != nil {
+			logger.Printf("NETPOLL: detach listener fd failed: %v", cerr)
 			return err
 		}
-		logger.Println("NETPOLL: accept conn failed:", err.Error())
-		return err
+		go func() {
+			retryTimes := []time.Duration{0, 10, 50, 100, 200, 500, 1000} // ms
+			retryTimeIndex := 0
+			for {
+				if retryTimeIndex > 0 {
+					time.Sleep(retryTimes[retryTimeIndex] * time.Millisecond)
+				}
+				conn, err := s.ln.Accept()
+				if err == nil {
+					if conn == nil {
+						// recovery accept poll loop
+						s.operator.Control(PollReadable)
+						return
+					}
+					s.onAccept(conn.(Conn))
+					logger.Println("NETPOLL: re-accept conn success:", conn.RemoteAddr())
+					retryTimeIndex = 0
+					continue
+				}
+				if retryTimeIndex+1 < len(retryTimes) {
+					retryTimeIndex++
+				}
+				logger.Printf("NETPOLL: re-accept conn failed, err=[%s] and next retrytime=%dms", err.Error(), retryTimes[retryTimeIndex])
+			}
+		}()
 	}
-	if conn == nil {
-		return nil
+
+	// shut down
+	if strings.Contains(err.Error(), "closed") {
+		s.operator.Control(PollDetach)
+		s.onQuit(err)
+		return err
 	}
+
+	return err
+}
+
+// OnHup implements FDOperator.
+func (s *server) OnHup(p Poll) error {
+	s.onQuit(errors.New("listener close"))
+	return nil
+}
+
+func (s *server) onAccept(conn Conn) {
 	// store & register connection
-	var connection = &connection{}
-	connection.init(conn.(Conn), s.opts)
-	if !connection.IsActive() {
-		return nil
+	var nconn = new(connection)
+	nconn.init(conn, s.opts)
+	if !nconn.IsActive() {
+		return
 	}
-	var fd = conn.(Conn).Fd()
-	connection.AddCloseCallback(func(connection Connection) error {
+	var fd = conn.Fd()
+	nconn.AddCloseCallback(func(connection Connection) error {
 		s.connections.Delete(fd)
 		return nil
 	})
-	s.connections.Store(fd, connection)
+	s.connections.Store(fd, nconn)
 
 	// trigger onConnect asynchronously
-	connection.onConnect()
-	return nil
+	nconn.onConnect()
 }
 
-// OnHup implements FDOperator.
-func (s *server) OnHup(p Poll) error {
-	s.onQuit(errors.New("listener close"))
-	return nil
+func isOutOfFdErr(err error) bool {
+	se, ok := err.(syscall.Errno)
+	return ok && (se == syscall.EMFILE || se == syscall.ENFILE)
 }

netpoll_test.go

@@ -21,9 +21,11 @@ import (
 	"context"
 	"errors"
 	"math/rand"
+	"os"
 	"runtime"
 	"sync"
 	"sync/atomic"
+	"syscall"
 	"testing"
 	"time"
 )
@@ -507,6 +509,78 @@ func TestClientWriteAndClose(t *testing.T) {
 	MustNil(t, err)
 }
 
+func TestServerAcceptWhenTooManyOpenFiles(t *testing.T) {
+	if os.Getenv("N_LOCAL") == "" {
+		t.Skip("Only test for debug purpose")
+		return
+	}
+
+	var originalRlimit syscall.Rlimit
+	err := syscall.Getrlimit(syscall.RLIMIT_NOFILE, &originalRlimit)
+	MustNil(t, err)
+	t.Logf("Original RLimit: %v", originalRlimit)
+
+	rlimit := syscall.Rlimit{Cur: 32, Max: originalRlimit.Max}
+	err = syscall.Setrlimit(syscall.RLIMIT_NOFILE, &rlimit)
+	MustNil(t, err)
+	err = syscall.Getrlimit(syscall.RLIMIT_NOFILE, &rlimit)
+	MustNil(t, err)
+	t.Logf("New RLimit: %v", rlimit)
+	defer func() { // reset
+		err = syscall.Setrlimit(syscall.RLIMIT_NOFILE, &originalRlimit)
+		MustNil(t, err)
+	}()
+
+	var network, address = "tcp", ":18888"
+	var connected int32
+	var loop = newTestEventLoop(network, address,
+		func(ctx context.Context, connection Connection) error {
+			buf, err := connection.Reader().Next(connection.Reader().Len())
+			connection.Writer().WriteBinary(buf)
+			connection.Writer().Flush()
+			return err
+		},
+		WithOnConnect(func(ctx context.Context, connection Connection) context.Context {
+			atomic.AddInt32(&connected, 1)
+			t.Logf("Conn[%s] accpeted", connection.RemoteAddr())
+			return ctx
+		}),
+		WithOnDisconnect(func(ctx context.Context, connection Connection) {
+			t.Logf("Conn[%s] disconnected", connection.RemoteAddr())
+		}),
+	)
+	time.Sleep(time.Millisecond * 10)
+
+	// out of fds
+	files := make([]*os.File, 0)
+	for {
+		f, err := os.Open("/dev/null")
+		if err != nil {
+			Assert(t, isOutOfFdErr(errors.Unwrap(err)), err)
+			break
+		}
+		files = append(files, f)
+	}
+	go func() {
+		time.Sleep(time.Second * 10)
+		t.Logf("close all files")
+		for _, f := range files {
+			f.Close()
+		}
+	}()
+
+	// we should use telnet manually
+	var connections = 1
+	for atomic.LoadInt32(&connected) < int32(connections) {
+		t.Logf("connected=%d", atomic.LoadInt32(&connected))
+		time.Sleep(time.Second)
+	}
+	time.Sleep(time.Second * 10)
+
+	err = loop.Shutdown(context.Background())
+	MustNil(t, err)
+}
+
 func createTestListener(network, address string) (Listener, error) {
 	for {
 		ln, err := CreateListener(network, address)

test_conns.sh

@@ -0,0 +1,13 @@
+#!/usr/bin/env bash
+
+ip="$1"
+port="$2"
+conns="$3"
+timeout="$4"
+
+for i in $(seq 1 $conns);
+do
+  nc -v -w $timeout $ip $port < /dev/null &
+done
+
+wait