sftpd: properly handle listener accept errors

continue on temporary errors and exit from the serve loop for the
other ones

ftpd/ftpd.go

@@ -141,7 +141,7 @@ func (c *Configuration) Initialize(configDir string) error {
 		PassivePortRange: c.PassivePortRange,
 	}
 
-	exitChannel := make(chan error)
+	exitChannel := make(chan error, 1)
 
 	for idx, binding := range c.Bindings {
 		if !binding.IsValid() {

sftpd/internal_test.go

@@ -1866,3 +1866,71 @@ func TestRecoverer(t *testing.T) {
 	assert.EqualError(t, err, common.ErrGenericFailure.Error())
 	assert.Len(t, common.Connections.GetStats(), 0)
 }
+
+func TestListernerAcceptErrors(t *testing.T) {
+	errFake := errors.New("a fake error")
+	listener := newFakeListener(errFake)
+	c := Configuration{}
+	err := c.serve(listener, nil)
+	require.EqualError(t, err, errFake.Error())
+	err = listener.Close()
+	require.NoError(t, err)
+
+	errNetFake := &fakeNetError{error: errFake}
+	listener = newFakeListener(errNetFake)
+	err = c.serve(listener, nil)
+	require.EqualError(t, err, errFake.Error())
+	err = listener.Close()
+	require.NoError(t, err)
+}
+
+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,
+	}
+}

sftpd/server.go

@@ -15,7 +15,6 @@ import (
 	"strings"
 	"time"
 
-	"github.com/pires/go-proxyproto"
 	"github.com/pkg/sftp"
 	"golang.org/x/crypto/ssh"
 
@@ -224,7 +223,7 @@ func (c *Configuration) Initialize(configDir string) error {
 	c.configureLoginBanner(serverConfig, configDir)
 	c.checkSSHCommands()
 
-	exitChannel := make(chan error)
+	exitChannel := make(chan error, 1)
 	serviceStatus.Bindings = nil
 
 	for _, binding := range c.Bindings {
@@ -234,7 +233,27 @@ func (c *Configuration) Initialize(configDir string) error {
 		serviceStatus.Bindings = append(serviceStatus.Bindings, binding)
 
 		go func(binding Binding) {
-			exitChannel <- c.listenAndServe(binding, serverConfig)
+			addr := binding.GetAddress()
+			listener, err := net.Listen("tcp", addr)
+			if err != nil {
+				logger.Warn(logSender, "", "error starting listener on address %v: %v", addr, err)
+				exitChannel <- err
+				return
+			}
+
+			if binding.ApplyProxyConfig {
+				proxyListener, err := common.Config.GetProxyListener(listener)
+				if err != nil {
+					logger.Warn(logSender, "", "error enabling proxy listener: %v", err)
+					exitChannel <- err
+					return
+				}
+				if proxyListener != nil {
+					listener = proxyListener
+				}
+			}
+
+			exitChannel <- c.serve(listener, serverConfig)
 		}(binding)
 	}
 
@@ -244,34 +263,32 @@ func (c *Configuration) Initialize(configDir string) error {
 	return <-exitChannel
 }
 
-func (c *Configuration) listenAndServe(binding Binding, serverConfig *ssh.ServerConfig) error {
+func (c *Configuration) serve(listener net.Listener, serverConfig *ssh.ServerConfig) error {
-	addr := binding.GetAddress()
-	listener, err := net.Listen("tcp", addr)
-	if err != nil {
-		logger.Warn(logSender, "", "error starting listener on address %v: %v", addr, err)
-		return err
-	}
-	var proxyListener *proxyproto.Listener
-
-	if binding.ApplyProxyConfig {
-		proxyListener, err = common.Config.GetProxyListener(listener)
-		if err != nil {
-			logger.Warn(logSender, "", "error enabling proxy listener: %v", err)
-			return err
-		}
-	}
 	logger.Info(logSender, "", "server listener registered, address: %v", listener.Addr().String())
+	var tempDelay time.Duration // how long to sleep on accept failure
+
 	for {
-		var conn net.Conn
+		conn, err := listener.Accept()
-		if proxyListener != nil {
+		if err != nil {
-			conn, err = proxyListener.Accept()
+			if ne, ok := err.(net.Error); ok && ne.Temporary() {
+				if tempDelay == 0 {
+					tempDelay = 5 * time.Millisecond
 				} else {
-			conn, err = listener.Accept()
+					tempDelay *= 2
 				}
-		if conn != nil && err == nil {
+				if max := 1 * time.Second; tempDelay > max {
+					tempDelay = max
+				}
+				logger.Warn(logSender, "", "accept error: %v; retrying in %v", err, tempDelay)
+				time.Sleep(tempDelay)
+				continue
+			}
+			logger.Warn(logSender, "", "unrecoverable accept error: %v", err)
+			return err
+		}
+
 		go c.AcceptInboundConnection(conn, serverConfig)
 	}
-	}
 }
 
 func (c *Configuration) configureSecurityOptions(serverConfig *ssh.ServerConfig) {

sftpd/sftpd_test.go

@@ -8420,7 +8420,7 @@ func sftpDownloadFile(remoteSourcePath string, localDestPath string, expectedSiz
 }
 
 func sftpUploadNonBlocking(localSourcePath string, remoteDestPath string, expectedSize int64, client *sftp.Client) <-chan error {
-	c := make(chan error)
+	c := make(chan error, 1)
 	go func() {
 		c <- sftpUploadFile(localSourcePath, remoteDestPath, expectedSize, client)
 	}()
@@ -8428,7 +8428,7 @@ func sftpUploadNonBlocking(localSourcePath string, remoteDestPath string, expect
 }
 
 func sftpDownloadNonBlocking(remoteSourcePath string, localDestPath string, expectedSize int64, client *sftp.Client) <-chan error {
-	c := make(chan error)
+	c := make(chan error, 1)
 	go func() {
 		c <- sftpDownloadFile(remoteSourcePath, localDestPath, expectedSize, client)
 	}()

webdavd/webdavd.go

@@ -140,7 +140,7 @@ func (c *Configuration) Initialize(configDir string) error {
 
 	server.status.Bindings = nil
 
-	exitChannel := make(chan error)
+	exitChannel := make(chan error, 1)
 
 	for _, binding := range c.Bindings {
 		if !binding.IsValid() {