ebs: parallelize fast-launch enablement

Enabling fast-launch for an AMI is an operation that takes time. Prior
to the patch that mandates running the operation on the source AMI,
parallelising would have had no effect, as the operation was only run
once per build.

After the patch though, we need to run this operation on each AMI that
was copied, in which case, running in parallel may result in a
significant reduction in build times.

Example on the copy + fast-launch acceptance tests:

- Before this patch: 2436s, i.e. 40 minutes
- After this patch: 2062s, i.e. 34 minutes

builder/ebs/step_enable_fast_launch.go

@@ -5,8 +5,10 @@ package ebs
 
 import (
 	"context"
+	"errors"
 	"fmt"
 	"log"
+	"sync"
 	"time"
 
 	"github.com/aws/aws-sdk-go/aws"
@@ -47,76 +49,108 @@ func (s *stepEnableFastLaunch) Run(ctx context.Context, state multistep.StateBag
 		return multistep.ActionContinue
 	}
 
+	wg := &sync.WaitGroup{}
+	wg.Add(len(amis))
+
+	errWg := &sync.WaitGroup{}
+	errWg.Add(1)
+
+	errCh := make(chan (error), 1)
+	var errs []error
+	go func() {
+		defer errWg.Done()
+		for err := range errCh {
+			errs = append(errs, err)
+		}
+	}()
+
 	for region, ami := range amis {
 		ec2connif, err := common.GetRegionConn(s.AccessConfig, region)
 		if err != nil {
 			state.Put("error", fmt.Errorf("Failed to get connection to region %q: %s", region, err))
 			return multistep.ActionHalt
 		}
 
-		// Casting is somewhat unsafe, but since the retryer below only
-		// accepts this type, and not ec2iface.EC2API, we can safely
-		// do this here, unless the `GetRegionConn` function evolves
-		// later, in which case this will fail.
-		ec2conn := ec2connif.(*ec2.EC2)
-
-		ui.Say(fmt.Sprintf("Enabling fast boot for AMI %s in region %s", ami, region))
-
-		fastLaunchInput := s.prepareFastLaunchRequest(ami, region, state)
-
-		// Create a timeout for the EnableFastLaunch call.
-		timeoutCtx, cancel := context.WithTimeout(ctx, time.Minute)
-		defer cancel()
-
-		err = retry.Config{
-			Tries: 5,
-			ShouldRetry: func(err error) bool {
-				log.Printf("Enabling fast launch failed: %s", err)
-				return true
-			},
-			RetryDelay: (&retry.Backoff{InitialBackoff: 500 * time.Millisecond, MaxBackoff: 30 * time.Second, Multiplier: 2}).Linear,
-		}.Run(timeoutCtx, func(ctx context.Context) error {
-			var err error
-			_, err = ec2conn.EnableFastLaunch(fastLaunchInput)
-			return err
-		})
-		if err != nil {
-			err := fmt.Errorf("Error enabling fast boot for AMI in region %s: %s", region, err)
-			state.Put("error", err)
-			ui.Error(err.Error())
-			return multistep.ActionHalt
-		}
+		go func(region, ami string) {
+			defer wg.Done()
+
+			// Casting is somewhat unsafe, but since the retryer below only
+			// accepts this type, and not ec2iface.EC2API, we can safely
+			// do this here, unless the `GetRegionConn` function evolves
+			// later, in which case this will fail.
+			ec2conn := ec2connif.(*ec2.EC2)
+
+			ui.Say(fmt.Sprintf("Enabling fast boot for AMI %s in region %s", ami, region))
+
+			fastLaunchInput := s.prepareFastLaunchRequest(ami, region, state)
+
+			// Create a timeout for the EnableFastLaunch call.
+			timeoutCtx, cancel := context.WithTimeout(ctx, time.Minute)
+			defer cancel()
+
+			err = retry.Config{
+				Tries: 5,
+				ShouldRetry: func(err error) bool {
+					log.Printf("Enabling fast launch failed: %s", err)
+					return true
+				},
+				RetryDelay: (&retry.Backoff{InitialBackoff: 500 * time.Millisecond, MaxBackoff: 30 * time.Second, Multiplier: 2}).Linear,
+			}.Run(timeoutCtx, func(ctx context.Context) error {
+				var err error
+				_, err = ec2conn.EnableFastLaunch(fastLaunchInput)
+				return err
+			})
+			if err != nil {
+				errCh <- fmt.Errorf("Error enabling fast boot for AMI in region %s: %s", region, err)
+				return
+			}
 
-		// Wait for the image to become ready
-		ui.Say("Waiting for fast launch to become ready...")
-		waitErr := s.PollingConfig.WaitUntilFastLaunchEnabled(ctx, ec2conn, ami)
-		if waitErr != nil {
-			err := fmt.Errorf("Failed to enable fast launch: %s", err)
-			state.Put("error", err)
-			ui.Error(err.Error())
-			return multistep.ActionHalt
-		}
+			// Wait for the image to become ready
+			ui.Say(fmt.Sprintf("Waiting for fast launch to become ready on AMI %q in region %s...", ami, region))
+			waitErr := s.PollingConfig.WaitUntilFastLaunchEnabled(ctx, ec2conn, ami)
+			if waitErr != nil {
+				errCh <- fmt.Errorf("Failed to enable fast launch: %s", err)
+				return
+			}
 
-		flStatus, err := ec2conn.DescribeFastLaunchImages(&ec2.DescribeFastLaunchImagesInput{
-			ImageIds: []*string{
-				&ami,
-			},
-		})
-		if err != nil {
-			err := fmt.Errorf("Failed to get fast-launch status for AMI %q: %s", ami, err)
-			state.Put("error", err)
-			ui.Error(err.Error())
-			return multistep.ActionHalt
-		}
+			flStatus, err := ec2conn.DescribeFastLaunchImages(&ec2.DescribeFastLaunchImagesInput{
+				ImageIds: []*string{
+					&ami,
+				},
+			})
+			if err != nil {
+				errCh <- fmt.Errorf("Failed to get fast-launch status for AMI %q: %s", ami, err)
+				return
+			}
 
-		for _, img := range flStatus.FastLaunchImages {
-			if *img.State != "enabled" {
-				err := fmt.Errorf("Failed to enable fast-launch for AMI %q: %s", ami, *img.StateTransitionReason)
-				state.Put("error", err)
-				ui.Error(err.Error())
-				return multistep.ActionHalt
+			for _, img := range flStatus.FastLaunchImages {
+				if *img.State != "enabled" {
+					errCh <- fmt.Errorf("Failed to enable fast-launch for AMI %q: %s", ami, *img.StateTransitionReason)
+					return
+				}
 			}
-		}
+		}(region, ami)
+	}
+
+	wg.Wait()
+	close(errCh)
+	// Wait until we finished queueing the errors before continuing
+	//
+	// Otherwise we may end-up queueing an error and leaving the routine
+	// that processes with an error, and the error check below may execute
+	// before the error gets queued up, which may result in a build
+	// succeeding even if there was an error with the fast-launch on one AMI.
+	errWg.Wait()
+
+	for _, err := range errs {
+		ui.Error(err.Error())
+	}
+
+	if errs != nil {
+		err := errors.New("Failed to enable fast-launch because of errors")
+		ui.Error(err.Error())
+		state.Put("error", err)
+		return multistep.ActionHalt
 	}
 
 	return multistep.ActionContinue