Add code for acquiring region from env; Refactor code to reduce EC2 calls

Author: Shreya Anil Naik

pkg/aws/cloud.go

@@ -6,6 +6,7 @@ import (
 	"github.com/aws/aws-sdk-go/aws/session"
 	"github.com/pkg/errors"
 	"github.com/prometheus/client_golang/prometheus"
+	"os"
 	"sigs.k8s.io/aws-load-balancer-controller/pkg/aws/metrics"
 	"sigs.k8s.io/aws-load-balancer-controller/pkg/aws/services"
 	"sigs.k8s.io/aws-load-balancer-controller/pkg/aws/throttle"
@@ -42,24 +43,25 @@ type Cloud interface {
 
 // NewCloud constructs new Cloud implementation.
 func NewCloud(cfg CloudConfig, metricsRegisterer prometheus.Registerer) (Cloud, error) {
-	metadataSess := session.Must(session.NewSession(aws.NewConfig()))
-	metadata := services.NewEC2Metadata(metadataSess)
-	if len(cfg.Region) == 0 {
-		region, err := metadata.Region()
-		if err != nil {
-			return nil, errors.Wrap(err, "failed to introspect region from EC2Metadata, specify --aws-region instead if EC2Metadata is unavailable")
-		}
-		cfg.Region = region
-	}
 
 	if len(cfg.VpcID) == 0 {
+		metadataSess := session.Must(session.NewSession(aws.NewConfig()))
+		metadata := services.NewEC2Metadata(metadataSess)
 		vpcId, err := metadata.VpcID()
 		if err != nil {
 			return nil, errors.Wrap(err, "failed to introspect vpcID from EC2Metadata, specify --aws-vpc-id instead if EC2Metadata is unavailable")
 		}
 		cfg.VpcID = vpcId
 	}
 
+	if len(cfg.Region) == 0 {
+		region := os.Getenv("AWS_DEFAULT_REGION")
+		if region == "" {
+			region = os.Getenv("AWS_REGION")
+		}
+		cfg.Region = region
+	}
+
 	awsCFG := aws.NewConfig().WithRegion(cfg.Region).WithSTSRegionalEndpoint(endpoints.RegionalSTSEndpoint).WithMaxRetries(cfg.MaxRetries)
 	sess := session.Must(session.NewSession(awsCFG))
 	injectUserAgent(&sess.Handlers)