builder/amazon/common: remove duplicates from ami_region

/cc @jmassara

builder/amazon/common/ami_config.go

@@ -63,11 +63,28 @@ func (c *AMIConfig) Prepare(t *packer.ConfigTemplate) []error {
 	}
 
 	if len(c.AMIRegions) > 0 {
+		regionSet := make(map[string]struct{})
+		regions := make([]string, 0, len(c.AMIRegions))
+
 		for _, region := range c.AMIRegions {
+			// If we already saw the region, then don't look again
+			if _, ok := regionSet[region]; ok {
+				continue
+			}
+
+			// Mark that we saw the region
+			regionSet[region] = struct{}{}
+
+			// Verify the region is real
 			if _, ok := aws.Regions[region]; !ok {
 				errs = append(errs, fmt.Errorf("Unknown region: %s", region))
+				continue
 			}
+
+			regions = append(regions, region)
 		}
+
+		c.AMIRegions = regions
 	}
 
 	if len(errs) > 0 {

builder/amazon/common/ami_config_test.go

 package common
 
 import (
+	"reflect"
 	"testing"
 )
 
@@ -10,7 +11,7 @@ func testAMIConfig() *AMIConfig {
 	}
 }
 
-func TestAMIConfigPrepare_Region(t *testing.T) {
+func TestAMIConfigPrepare_name(t *testing.T) {
 	c := testAMIConfig()
 	if err := c.Prepare(nil); err != nil {
 		t.Fatalf("shouldn't have err: %s", err)
@@ -21,3 +22,26 @@ func TestAMIConfigPrepare_Region(t *testing.T) {
 		t.Fatal("should have error")
 	}
 }
+
+func TestAMIConfigPrepare_regions(t *testing.T) {
+	c := testAMIConfig()
+	c.AMIRegions = nil
+	if err := c.Prepare(nil); err != nil {
+		t.Fatalf("shouldn't have err: %s", err)
+	}
+
+	c.AMIRegions = []string{"foo"}
+	if err := c.Prepare(nil); err == nil {
+		t.Fatal("should have error")
+	}
+
+	c.AMIRegions = []string{"us-east-1", "us-west-1", "us-east-1"}
+	if err := c.Prepare(nil); err != nil {
+		t.Fatalf("bad: %s", err)
+	}
+
+	expected := []string{"us-east-1", "us-west-1"}
+	if !reflect.DeepEqual(c.AMIRegions, expected) {
+		t.Fatalf("bad: %#v", c.AMIRegions)
+	}
+}