builder/aws: Add pre validate step, to validate things before building.

builder/amazon/common/step_pre_validate.go

+package common
+
+import (
+	"fmt"
+
+	"github.com/aws/aws-sdk-go/aws"
+	"github.com/aws/aws-sdk-go/service/ec2"
+	"github.com/mitchellh/multistep"
+	"github.com/mitchellh/packer/packer"
+)
+
+// StepPreValidate provides an opportunity to pre-validate any configuration for
+// the build before actually doing any time consuming work
+//
+type StepPreValidate struct {
+	DestAmiName string
+}
+
+func (s *StepPreValidate) Run(state multistep.StateBag) multistep.StepAction {
+	ec2conn := state.Get("ec2").(*ec2.EC2)
+	ui := state.Get("ui").(packer.Ui)
+
+	ui.Say("Prevalidating AMI Name...")
+	resp, err := ec2conn.DescribeImages(&ec2.DescribeImagesInput{
+		Filters: []*ec2.Filter{&ec2.Filter{
+			Name:   aws.String("name"),
+			Values: []*string{aws.String(s.DestAmiName)},
+		}}})
+
+	if err != nil {
+		err := fmt.Errorf("Error querying AMI: %s", err)
+		state.Put("error", err)
+		ui.Error(err.Error())
+		return multistep.ActionHalt
+	}
+
+	if len(resp.Images) > 0 {
+		err := fmt.Errorf("Error: an AMI with that name already exists")
+		state.Put("error", err)
+		ui.Error(err.Error())
+		return multistep.ActionHalt
+	}
+
+	return multistep.ActionContinue
+}
+
+func (s *StepPreValidate) Cleanup(multistep.StateBag) {}

builder/amazon/ebs/builder.go

@@ -78,6 +78,9 @@ func (b *Builder) Run(ui packer.Ui, hook packer.Hook, cache packer.Cache) (packe
 
 	// Build the steps
 	steps := []multistep.Step{
+		&awscommon.StepPreValidate{
+			DestAmiName: b.config.AMIName,
+		},
 		&awscommon.StepSourceAMIInfo{
 			SourceAmi:          b.config.SourceAmi,
 			EnhancedNetworking: b.config.AMIEnhancedNetworking,