sftp: fix timeout on hashing large files by sending keepalives

Before this fix the SFTP sessions could timeout when doing hashes if
they took longer than the --timeout parameter.

This patch sends keepalive packets every minute while a shell command
is running to keep the connection open.

See: https://forum.rclone.org/t/rclone-check-over-sftp-failure-to-calculate-md5-hash-for-large-files/27487

backend/sftp/sftp.go

@@ -42,7 +42,8 @@ const (
 	hashCommandNotSupported = "none"
 	minSleep                = 100 * time.Millisecond
 	maxSleep                = 2 * time.Second
-	decayConstant           = 2 // bigger for slower decay, exponential
+	decayConstant           = 2           // bigger for slower decay, exponential
+	keepAliveInterval       = time.Minute // send keepalives every this long while running commands
 )
 
 var (
@@ -339,6 +340,32 @@ func (c *conn) wait() {
 	c.err <- c.sshClient.Conn.Wait()
 }
 
+// Send a keepalive over the ssh connection
+func (c *conn) sendKeepAlive() {
+	_, _, err := c.sshClient.SendRequest("keepalive@openssh.com", true, nil)
+	if err != nil {
+		fs.Debugf(nil, "Failed to send keep alive: %v", err)
+	}
+}
+
+// Send keepalives every interval over the ssh connection until done is closed
+func (c *conn) sendKeepAlives(interval time.Duration) (done chan struct{}) {
+	done = make(chan struct{})
+	go func() {
+		t := time.NewTicker(interval)
+		defer t.Stop()
+		for {
+			select {
+			case <-t.C:
+				c.sendKeepAlive()
+			case <-done:
+				return
+			}
+		}
+	}()
+	return done
+}
+
 // Closes the connection
 func (c *conn) close() error {
 	sftpErr := c.sftpClient.Close()
@@ -1098,6 +1125,9 @@ func (f *Fs) run(ctx context.Context, cmd string) ([]byte, error) {
 	}
 	defer f.putSftpConnection(&c, err)
 
+	// Send keepalives while the connection is open
+	defer close(c.sendKeepAlives(keepAliveInterval))
+
 	session, err := c.sshClient.NewSession()
 	if err != nil {
 		return nil, fmt.Errorf("run: get SFTP session: %w", err)

docs/content/sftp.md

@@ -620,7 +620,7 @@ issue](https://github.com/pkg/sftp/issues/156) is fixed.
 Note that since SFTP isn't HTTP based the following flags don't work
 with it: `--dump-headers`, `--dump-bodies`, `--dump-auth`
 
-Note that `--timeout` isn't supported (but `--contimeout` is).
+Note that `--timeout` and `--contimeout` are both supported.
 
 
 ## C14 {#c14}