have the ssh communicator return fatal errors

This will let the retry loop abort when there are errors which aren't
going to ever be corrected.

builtin/provisioners/remote-exec/resource_provisioner.go

@@ -169,9 +169,9 @@ func runScripts(
 
 	// Wait and retry until we establish the connection
 	err := communicator.Retry(ctx, func() error {
-		err := comm.Connect(o)
-		return err
+		return comm.Connect(o)
 	})
+
 	if err != nil {
 		return err
 	}

communicator/ssh/communicator.go

@@ -63,6 +63,14 @@ type sshConfig struct {
 	sshAgent *sshAgent
 }
 
+type fatalError struct {
+	error
+}
+
+func (e fatalError) FatalError() error {
+	return e.error
+}
+
 // New creates a new communicator implementation over SSH.
 func New(s *terraform.InstanceState) (*Communicator, error) {
 	connInfo, err := parseConnectionInfo(s)
@@ -159,8 +167,8 @@ func (c *Communicator) Connect(o terraform.UIOutput) (err error) {
 	host := fmt.Sprintf("%s:%d", c.connInfo.Host, c.connInfo.Port)
 	sshConn, sshChan, req, err := ssh.NewClientConn(c.conn, host, c.config.config)
 	if err != nil {
-		log.Printf("handshake error: %s", err)
-		return err
+		log.Printf("fatal handshake error: %s", err)
+		return fatalError{err}
 	}
 
 	c.client = ssh.NewClient(sshConn, sshChan, req)
@@ -168,7 +176,7 @@ func (c *Communicator) Connect(o terraform.UIOutput) (err error) {
 	if c.config.sshAgent != nil {
 		log.Printf("[DEBUG] Telling SSH config to forward to agent")
 		if err := c.config.sshAgent.ForwardToAgent(c.client); err != nil {
-			return err
+			return fatalError{err}
 		}
 
 		log.Printf("[DEBUG] Setting up a session to request agent forwarding")