fix behavior when not using IAP, try to use more sophisticated streaming than buffer.String()

builder/googlecompute/config.go

@@ -336,12 +336,14 @@ func (c *Config) Prepare(raws ...interface{}) ([]string, error) {
 	}
 
 	// Configure IAP: Update SSH config to use localhost proxy instead
-	if c.Comm.Type == "ssh" {
-		c.Comm.SSHHost = "localhost"
-	} else {
-		err := fmt.Errorf("Error: IAP tunnel currently only implemnted for" +
-			" SSH communicator")
-		errs = packer.MultiErrorAppend(errs, err)
+	if c.IAPConfig.IAP {
+		if c.Comm.Type == "ssh" {
+			c.Comm.SSHHost = "localhost"
+		} else {
+			err := fmt.Errorf("Error: IAP tunnel currently only implemnted for" +
+				" SSH communicator")
+			errs = packer.MultiErrorAppend(errs, err)
+		}
 	}
 
 	// Process required parameters.

builder/googlecompute/step_start_tunnel.go

@@ -205,5 +205,9 @@ func (s *StepStartTunnel) Run(ctx context.Context, state multistep.StateBag) mul
 
 // Cleanup stops the IAP tunnel and cleans up processes.
 func (s *StepStartTunnel) Cleanup(state multistep.StateBag) {
+	if !s.IAPConf.IAP {
+		log.Printf("Skipping cleanup of IAP tunnel; \"iap\" is false.")
+		return
+	}
 	s.tunnelDriver.StopTunnel()
 }

builder/googlecompute/tunnel_driver.go

@@ -3,6 +3,7 @@
 package googlecompute
 
 import (
+	"bufio"
 	"bytes"
 	"context"
 	"fmt"
@@ -38,23 +39,24 @@ func (t *TunnelDriverLinux) StartTunnel(cancelCtx context.Context, tempScriptFil
 			err)
 		return err
 	}
-	// Wait for tunnel to launch and gather response. TODO: do this without
-	// a sleep.
-	time.Sleep(30 * time.Second)
-
-	// Track stdout.
-	sout := stdout.String()
-	if sout != "" {
-		log.Printf("[start-iap-tunnel] stdout is:")
-	}
 
+	time.Sleep(10 * time.Second)
+	// read stdout
+	scanner := bufio.NewScanner(&stderr)
 	log.Printf("[start-iap-tunnel] stderr is:")
-	serr := stderr.String()
-	log.Println(serr)
-	if strings.Contains(serr, "ERROR") {
-		errIdx := strings.Index(serr, "ERROR:")
-		return fmt.Errorf("ERROR: %s", serr[errIdx+7:])
+	for scanner.Scan() {
+		line := scanner.Text()
+		log.Println(line)
+
+		if strings.Contains(line, "ERROR") {
+			errIdx := strings.Index(line, "ERROR:")
+			return fmt.Errorf("ERROR: %s", line[errIdx+7:])
+		}
 	}
+	if err := scanner.Err(); err != nil {
+		log.Printf("Error scanning tunnel stderr: %s", err)
+	}
+
 	// Store successful command on step so we can access it to cancel it
 	// later.
 	t.cmd = cmd