all: Send Close BEP msg on intentional disconnect (#5440)

This avoids waiting until next ping and timeout until the connection is actually
closed both by notifying the peer of the disconnect and by immediately closing
the local end of the connection after that. As a nice side effect, info level
logging about dropped connections now have the actual reason in it, not a generic
timeout error which looks like a real problem with the connection.

lib/connections/structs.go

@@ -9,7 +9,6 @@ package connections
 import (
 	"crypto/tls"
 	"fmt"
-	"io"
 	"net"
 	"net/url"
 	"time"
@@ -23,7 +22,6 @@ import (
 // that can be closed and has some metadata.
 type Connection interface {
 	protocol.Connection
-	io.Closer
 	Type() string
 	Transport() string
 	RemoteAddr() net.Addr
@@ -39,6 +37,11 @@ type completeConn struct {
 	protocol.Connection
 }
 
+func (c completeConn) Close(err error) {
+	c.Connection.Close(err)
+	c.internalConn.Close()
+}
+
 // internalConn is the raw TLS connection plus some metadata on where it
 // came from (type, priority).
 type internalConn struct {
@@ -82,6 +85,14 @@ func (t connType) Transport() string {
 	}
 }
 
+func (c internalConn) Close() {
+	// *tls.Conn.Close() does more than it says on the tin. Specifically, it
+	// sends a TLS alert message, which might block forever if the
+	// connection is dead and we don't have a deadline set.
+	c.SetWriteDeadline(time.Now().Add(250 * time.Millisecond))
+	c.Conn.Close()
+}
+
 func (c internalConn) Type() string {
 	return c.connType.String()
 }

lib/model/model.go

@@ -8,11 +8,9 @@ package model
 
 import (
 	"bytes"
-	"crypto/tls"
 	"encoding/json"
 	"errors"
 	"fmt"
-	"io"
 	"net"
 	"path/filepath"
 	"reflect"
@@ -128,6 +126,9 @@ var (
 	errFolderNotRunning  = errors.New("folder is not running")
 	errFolderMissing     = errors.New("no such folder")
 	errNetworkNotAllowed = errors.New("network not allowed")
+	// errors about why a connection is closed
+	errIgnoredFolderRemoved = errors.New("folder no longer ignored")
+	errReplacingConnection  = errors.New("replacing connection")
 )
 
 // NewModel creates and starts a new model. The model starts in read-only mode,
@@ -226,7 +227,7 @@ func (m *Model) startFolderLocked(folder string) config.FolderType {
 
 	// Close connections to affected devices
 	for _, id := range cfg.DeviceIDs() {
-		m.closeLocked(id)
+		m.closeLocked(id, fmt.Errorf("started folder %v", cfg.Description()))
 	}
 
 	v, ok := fs.Sequence(protocol.LocalDeviceID), true
@@ -339,7 +340,7 @@ func (m *Model) RemoveFolder(cfg config.FolderConfiguration) {
 	// Delete syncthing specific files
 	cfg.Filesystem().RemoveAll(config.DefaultMarkerName)
 
-	m.tearDownFolderLocked(cfg)
+	m.tearDownFolderLocked(cfg, fmt.Errorf("removing folder %v", cfg.Description()))
 	// Remove it from the database
 	db.DropFolder(m.db, cfg.ID)
 
@@ -347,12 +348,12 @@ func (m *Model) RemoveFolder(cfg config.FolderConfiguration) {
 	m.fmut.Unlock()
 }
 
-func (m *Model) tearDownFolderLocked(cfg config.FolderConfiguration) {
+func (m *Model) tearDownFolderLocked(cfg config.FolderConfiguration, err error) {
 	// Close connections to affected devices
 	// Must happen before stopping the folder service to abort ongoing
 	// transmissions and thus allow timely service termination.
 	for _, dev := range cfg.Devices {
-		m.closeLocked(dev.DeviceID)
+		m.closeLocked(dev.DeviceID, err)
 	}
 
 	// Stop the services running for this folder and wait for them to finish
@@ -398,14 +399,26 @@ func (m *Model) RestartFolder(from, to config.FolderConfiguration) {
 	defer m.fmut.Unlock()
 	defer m.pmut.Unlock()
 
-	m.tearDownFolderLocked(from)
-	if to.Paused {
-		l.Infoln("Paused folder", to.Description())
-	} else {
-		m.addFolderLocked(to)
-		folderType := m.startFolderLocked(to.ID)
-		l.Infoln("Restarted folder", to.Description(), fmt.Sprintf("(%s)", folderType))
+	var infoMsg string
+	var errMsg string
+	switch {
+	case to.Paused:
+		infoMsg = "Paused"
+		errMsg = "pausing"
+	case from.Paused:
+		infoMsg = "Unpaused"
+		errMsg = "unpausing"
+	default:
+		infoMsg = "Restarted"
+		errMsg = "restarting"
 	}
+
+	m.tearDownFolderLocked(from, fmt.Errorf("%v folder %v", errMsg, to.Description()))
+	if !to.Paused {
+		m.addFolderLocked(to)
+		m.startFolderLocked(to.ID)
+	}
+	l.Infof("%v folder %v (%v)", infoMsg, to.Description(), to.Type)
 }
 
 func (m *Model) UsageReportingStats(version int, preview bool) map[string]interface{} {
@@ -1342,21 +1355,21 @@ func (m *Model) Closed(conn protocol.Connection, err error) {
 }
 
 // close will close the underlying connection for a given device
-func (m *Model) close(device protocol.DeviceID) {
+func (m *Model) close(device protocol.DeviceID, err error) {
 	m.pmut.Lock()
-	m.closeLocked(device)
+	m.closeLocked(device, err)
 	m.pmut.Unlock()
 }
 
 // closeLocked will close the underlying connection for a given device
-func (m *Model) closeLocked(device protocol.DeviceID) {
+func (m *Model) closeLocked(device protocol.DeviceID, err error) {
 	conn, ok := m.conn[device]
 	if !ok {
 		// There is no connection to close
 		return
 	}
 
-	closeRawConn(conn)
+	conn.Close(err)
 }
 
 // Implements protocol.RequestResponse
@@ -1719,7 +1732,7 @@ func (m *Model) AddConnection(conn connections.Connection, hello protocol.HelloR
 		// back into Closed() for the cleanup.
 		closed := m.closed[deviceID]
 		m.pmut.Unlock()
-		closeRawConn(oldConn)
+		oldConn.Close(errReplacingConnection)
 		<-closed
 		m.pmut.Lock()
 	}
@@ -2582,6 +2595,10 @@ func (m *Model) CommitConfiguration(from, to config.Configuration) bool {
 			continue
 		}
 
+		if fromCfg.Paused && toCfg.Paused {
+			continue
+		}
+
 		// This folder exists on both sides. Settings might have changed.
 		// Check if anything differs that requires a restart.
 		if !reflect.DeepEqual(fromCfg.RequiresRestartOnly(), toCfg.RequiresRestartOnly()) {
@@ -2616,12 +2633,12 @@ func (m *Model) CommitConfiguration(from, to config.Configuration) bool {
 
 		// Ignored folder was removed, reconnect to retrigger the prompt.
 		if len(fromCfg.IgnoredFolders) > len(toCfg.IgnoredFolders) {
-			m.close(deviceID)
+			m.close(deviceID, errIgnoredFolderRemoved)
 		}
 
 		if toCfg.Paused {
 			l.Infoln("Pausing", deviceID)
-			m.close(deviceID)
+			m.close(deviceID, errDevicePaused)
 			events.Default.Log(events.DevicePaused, map[string]string{"device": deviceID.String()})
 		} else {
 			events.Default.Log(events.DeviceResumed, map[string]string{"device": deviceID.String()})
@@ -2717,17 +2734,6 @@ func getChunk(data []string, skip, get int) ([]string, int, int) {
 	return data[skip : skip+get], 0, 0
 }
 
-func closeRawConn(conn io.Closer) error {
-	if conn, ok := conn.(*tls.Conn); ok {
-		// If the underlying connection is a *tls.Conn, Close() does more
-		// than it says on the tin. Specifically, it sends a TLS alert
-		// message, which might block forever if the connection is dead
-		// and we don't have a deadline set.
-		conn.SetWriteDeadline(time.Now().Add(250 * time.Millisecond))
-	}
-	return conn.Close()
-}
-
 func stringSliceWithout(ss []string, s string) []string {
 	for i := range ss {
 		if ss[i] == s {

lib/model/model_test.go

@@ -316,11 +316,10 @@ type fakeConnection struct {
 	mut                      sync.Mutex
 }
 
-func (f *fakeConnection) Close() error {
+func (f *fakeConnection) Close(_ error) {
 	f.mut.Lock()
 	defer f.mut.Unlock()
 	f.closed = true
-	return nil
 }
 
 func (f *fakeConnection) Start() {

lib/protocol/protocol.go

@@ -143,6 +143,7 @@ type RequestResponse interface {
 
 type Connection interface {
 	Start()
+	Close(err error)
 	ID() DeviceID
 	Name() string
 	Index(folder string, files []FileInfo) error
@@ -171,6 +172,7 @@ type rawConnection struct {
 	nextIDMut sync.Mutex
 
 	outbox      chan asyncMessage
+	sendClose   chan asyncMessage
 	closed      chan struct{}
 	once        sync.Once
 	compression Compression
@@ -214,6 +216,7 @@ func NewConnection(deviceID DeviceID, reader io.Reader, writer io.Writer, receiv
 		cw:          cw,
 		awaiting:    make(map[int32]chan asyncResult),
 		outbox:      make(chan asyncMessage),
+		sendClose:   make(chan asyncMessage),
 		closed:      make(chan struct{}),
 		compression: compress,
 	}
@@ -334,7 +337,7 @@ func (c *rawConnection) ping() bool {
 
 func (c *rawConnection) readerLoop() (err error) {
 	defer func() {
-		c.close(err)
+		c.internalClose(err)
 	}()
 
 	fourByteBuf := make([]byte, 4)
@@ -636,10 +639,15 @@ func (c *rawConnection) writerLoop() {
 				close(hm.done)
 			}
 			if err != nil {
-				c.close(err)
+				c.internalClose(err)
 				return
 			}
 
+		case m := <-c.sendClose:
+			c.writeMessage(m)
+			close(m.done)
+			return // No message must be sent after the Close message.
+
 		case <-c.closed:
 			return
 		}
@@ -801,24 +809,47 @@ func (c *rawConnection) shouldCompressMessage(msg message) bool {
 	}
 }
 
-func (c *rawConnection) close(err error) {
+// Close is called when the connection is regularely closed and thus the Close
+// BEP message is sent before terminating the actual connection. The error
+// argument specifies the reason for closing the connection.
+func (c *rawConnection) Close(err error) {
 	c.once.Do(func() {
-		l.Debugln("close due to", err)
-		close(c.closed)
+		done := make(chan struct{})
+		c.sendClose <- asyncMessage{&Close{err.Error()}, done}
+		<-done
 
-		c.awaitingMut.Lock()
-		for i, ch := range c.awaiting {
-			if ch != nil {
-				close(ch)
-				delete(c.awaiting, i)
-			}
-		}
-		c.awaitingMut.Unlock()
-
-		c.receiver.Closed(c, err)
+		// No more sends are necessary, therefore closing the underlying
+		// connection can happen at the same time as the internal cleanup.
+		// And this prevents a potential deadlock due to calling c.receiver.Closed
+		go c.commonClose(err)
 	})
 }
 
+// internalClose is called if there is an unexpected error during normal operation.
+func (c *rawConnection) internalClose(err error) {
+	c.once.Do(func() {
+		c.commonClose(err)
+	})
+}
+
+// commonClose is a utility function that must only be called from within
+// rawConnection.once.Do (i.e. in Close and close).
+func (c *rawConnection) commonClose(err error) {
+	l.Debugln("close due to", err)
+	close(c.closed)
+
+	c.awaitingMut.Lock()
+	for i, ch := range c.awaiting {
+		if ch != nil {
+			close(ch)
+			delete(c.awaiting, i)
+		}
+	}
+	c.awaitingMut.Unlock()
+
+	c.receiver.Closed(c, err)
+}
+
 // The pingSender makes sure that we've sent a message within the last
 // PingSendInterval. If we already have something sent in the last
 // PingSendInterval/2, we do nothing. Otherwise we send a ping message. This
@@ -859,7 +890,7 @@ func (c *rawConnection) pingReceiver() {
 			d := time.Since(c.cr.Last())
 			if d > ReceiveTimeout {
 				l.Debugln(c.id, "ping timeout", d)
-				c.close(ErrTimeout)
+				c.internalClose(ErrTimeout)
 			}
 
 			l.Debugln(c.id, "last read within", d)

lib/protocol/protocol_test.go

@@ -56,7 +56,7 @@ func TestClose(t *testing.T) {
 	c0.ClusterConfig(ClusterConfig{})
 	c1.ClusterConfig(ClusterConfig{})
 
-	c0.close(errors.New("manual close"))
+	c0.internalClose(errors.New("manual close"))
 
 	<-c0.closed
 	if err := m0.closedError(); err == nil || !strings.Contains(err.Error(), "manual close") {