From 87b8b93695200a501d5ecbb36f1dfc337f3404b9 Mon Sep 17 00:00:00 2001
From: Nicola Murino <nicola.murino@gmail.com>
Date: Thu, 24 Dec 2020 01:20:12 +0100
Subject: [PATCH] don't quit listener after a temporary accept error (#211)

* don't quit listener after a temporary accept error

Fixes #210

* fix lint issues

Co-authored-by: Florent Clairambault <florent@clairambault.fr>
---
 server.go      | 22 ++++++++++++++++
 server_test.go | 71 ++++++++++++++++++++++++++++++++++++++++++++++++++
 2 files changed, 93 insertions(+)

diff --git a/server.go b/server.go
index 5eb44059..d8a5ed93 100644
--- a/server.go
+++ b/server.go
@@ -6,6 +6,7 @@ import (
 	"errors"
 	"fmt"
 	"net"
+	"time"
 
 	"github.com/fclairamb/ftpserverlib/log"
 )
@@ -177,6 +178,8 @@ func (server *FtpServer) Listen() error {
 
 // Serve accepts and processes any new incoming client
 func (server *FtpServer) Serve() error {
+	var tempDelay time.Duration // how long to sleep on accept failure
+
 	for {
 		connection, err := server.listener.Accept()
 
@@ -190,6 +193,25 @@ func (server *FtpServer) Serve() error {
 				}
 			}
 
+			if ne, ok := err.(net.Error); ok && ne.Temporary() {
+				if tempDelay == 0 {
+					tempDelay = 5 * time.Millisecond
+				} else {
+					tempDelay *= 2
+				}
+
+				if max := 1 * time.Second; tempDelay > max {
+					tempDelay = max
+				}
+
+				server.Logger.Warn(
+					"accept error", err,
+					"retry delay", tempDelay)
+				time.Sleep(tempDelay)
+
+				continue
+			}
+
 			server.Logger.Error("Listener accept error", "err", err)
 
 			return err
diff --git a/server_test.go b/server_test.go
index a1eccd9e..5ca5eb03 100644
--- a/server_test.go
+++ b/server_test.go
@@ -1,11 +1,82 @@
 package ftpserver
 
 import (
+	"errors"
+	"net"
 	"testing"
 
 	"github.com/stretchr/testify/require"
+
+	"github.com/fclairamb/ftpserverlib/log"
 )
 
+var errListenerAccept = errors.New("error accepting a connection")
+
+type fakeNetError struct {
+	error
+	count int
+}
+
+func (e *fakeNetError) Timeout() bool {
+	return false
+}
+
+func (e *fakeNetError) Temporary() bool {
+	e.count++
+
+	return e.count < 10
+}
+
+func (e *fakeNetError) Error() string {
+	return e.error.Error()
+}
+
+type fakeListener struct {
+	server net.Conn
+	client net.Conn
+	err    error
+}
+
+func (l *fakeListener) Accept() (net.Conn, error) {
+	return l.client, l.err
+}
+
+func (l *fakeListener) Close() error {
+	errClient := l.client.Close()
+	errServer := l.server.Close()
+
+	if errServer != nil {
+		return errServer
+	}
+
+	return errClient
+}
+
+func (l *fakeListener) Addr() net.Addr {
+	return l.server.LocalAddr()
+}
+
+func newFakeListener(err error) net.Listener {
+	server, client := net.Pipe()
+
+	return &fakeListener{
+		server: server,
+		client: client,
+		err:    err,
+	}
+}
+
+func TestListernerAcceptErrors(t *testing.T) {
+	errNetFake := &fakeNetError{error: errListenerAccept}
+
+	server := FtpServer{
+		listener: newFakeListener(errNetFake),
+		Logger:   log.Nothing(),
+	}
+	err := server.Serve()
+	require.EqualError(t, err, errListenerAccept.Error())
+}
+
 func TestPortCommandFormatOK(t *testing.T) {
 	net, err := parsePORTAddr("127,0,0,1,239,163")
 	require.NoError(t, err, "Problem parsing")