add ssh keepalive messages to communicator

Long running remote-exec commands with no output may be cutoff during
execution. Enable ssh keepalives for all ssh connections.

communicator/ssh/communicator.go

@@ -3,6 +3,7 @@ package ssh
 import (
 	"bufio"
 	"bytes"
+	"context"
 	"errors"
 	"fmt"
 	"io"
@@ -26,6 +27,9 @@ import (
 const (
 	// DefaultShebang is added at the top of a SSH script file
 	DefaultShebang = "#!/bin/sh\n"
+
+	// enable ssh keeplive probes by default
+	keepAliveInterval = 2 * time.Second
 )
 
 // randShared is a global random generator object that is shared.
@@ -37,11 +41,12 @@ var randShared *rand.Rand
 
 // Communicator represents the SSH communicator
 type Communicator struct {
-	connInfo *connectionInfo
-	client   *ssh.Client
-	config   *sshConfig
-	conn     net.Conn
-	address  string
+	connInfo        *connectionInfo
+	client          *ssh.Client
+	config          *sshConfig
+	conn            net.Conn
+	address         string
+	cancelKeepAlive context.CancelFunc
 
 	lock sync.Mutex
 }
@@ -203,11 +208,39 @@ func (c *Communicator) Connect(o terraform.UIOutput) (err error) {
 		}
 	}
 
+	if err != nil {
+		return err
+	}
+
 	if o != nil {
 		o.Output("Connected!")
 	}
 
-	return err
+	ctx, cancelKeepAlive := context.WithCancel(context.TODO())
+	c.cancelKeepAlive = cancelKeepAlive
+
+	// Start a keepalive goroutine to help maintain the connection for
+	// long-running commands.
+	log.Printf("[DEBUG] starting ssh KeepAlives")
+	go func() {
+		t := time.NewTicker(keepAliveInterval)
+		defer t.Stop()
+		for {
+			select {
+			case <-t.C:
+				// there's no useful response to these, just abort when there's
+				// an error.
+				_, _, err := c.client.SendRequest("keepalive@terraform.io", true, nil)
+				if err != nil {
+					return
+				}
+			case <-ctx.Done():
+				return
+			}
+		}
+	}()
+
+	return nil
 }
 
 // Disconnect implementation of communicator.Communicator interface
@@ -215,6 +248,10 @@ func (c *Communicator) Disconnect() error {
 	c.lock.Lock()
 	defer c.lock.Unlock()
 
+	if c.cancelKeepAlive != nil {
+		c.cancelKeepAlive()
+	}
+
 	if c.config.sshAgent != nil {
 		if err := c.config.sshAgent.Close(); err != nil {
 			return err

communicator/ssh/communicator_test.go

@@ -179,6 +179,48 @@ func TestStart(t *testing.T) {
 	}
 }
 
+// TestKeepAlives verifies that the keepalive messages don't interfere with
+// normal operation of the client.
+func TestKeepAlives(t *testing.T) {
+	address := newMockLineServer(t, nil)
+	parts := strings.Split(address, ":")
+
+	r := &terraform.InstanceState{
+		Ephemeral: terraform.EphemeralState{
+			ConnInfo: map[string]string{
+				"type":     "ssh",
+				"user":     "user",
+				"password": "pass",
+				"host":     parts[0],
+				"port":     parts[1],
+				"timeout":  "30s",
+			},
+		},
+	}
+
+	c, err := New(r)
+	if err != nil {
+		t.Fatalf("error creating communicator: %s", err)
+	}
+
+	if err := c.Connect(nil); err != nil {
+		t.Fatal(err)
+	}
+
+	var cmd remote.Cmd
+	stdout := new(bytes.Buffer)
+	cmd.Command = "echo foo"
+	cmd.Stdout = stdout
+
+	// wait a bit before executing the command, so that at least 1 keepalive is sent
+	time.Sleep(3 * time.Second)
+
+	err = c.Start(&cmd)
+	if err != nil {
+		t.Fatalf("error executing remote command: %s", err)
+	}
+}
+
 func TestLostConnection(t *testing.T) {
 	address := newMockLineServer(t, nil)
 	parts := strings.Split(address, ":")