Read AWS Region from EC2 Metadata

Tackles kubernetes/autoscaler#1208. EC2 Metadata helpfully supplies AWS
Region info, so rather than _requiring_ an environment variable, this
patch enables `aws_manager.go` to retrieve that information from EC2
itself.

Example YAMLs no longer reference the vestigial `AWS_REGION` environment
variable, but supplying it to `cluster-autoscaler` still relays it to
the AWS SDK like before.

cluster-autoscaler/cloudprovider/aws/aws_manager.go

@@ -22,11 +22,13 @@ import (
 	"fmt"
 	"io"
 	"math/rand"
+	"os"
 	"regexp"
 	"strings"
 	"time"
 
 	"github.com/aws/aws-sdk-go/aws"
+	"github.com/aws/aws-sdk-go/aws/ec2metadata"
 	"github.com/aws/aws-sdk-go/aws/session"
 	"github.com/aws/aws-sdk-go/service/autoscaling"
 	"github.com/aws/aws-sdk-go/service/ec2"
@@ -64,6 +66,18 @@ type asgTemplate struct {
 	Tags         []*autoscaling.TagDescription
 }
 
+// getRegion deduces the current AWS Region.
+func getRegion(cfg ...*aws.Config) string {
+	region, present := os.LookupEnv("AWS_REGION")
+	if !present {
+		svc := ec2metadata.New(session.New(), cfg...)
+		if r, err := svc.Region(); err == nil {
+			region = r
+		}
+	}
+	return region
+}
+
 // createAwsManagerInternal allows for a customer autoScalingWrapper to be passed in by tests
 func createAWSManagerInternal(
 	configReader io.Reader,
@@ -80,7 +94,7 @@ func createAWSManagerInternal(
 	}
 
 	if autoScalingService == nil || ec2Service == nil {
-		sess := session.New()
+		sess := session.New(aws.NewConfig().WithRegion(getRegion()))
 
 		if autoScalingService == nil {
 			autoScalingService = &autoScalingWrapper{autoscaling.New(sess)}

cluster-autoscaler/cloudprovider/aws/aws_manager_test.go

@@ -18,6 +18,9 @@ package aws
 
 import (
 	"fmt"
+	"net/http"
+	"net/http/httptest"
+	"os"
 	"testing"
 
 	"github.com/aws/aws-sdk-go/aws"
@@ -30,6 +33,32 @@ import (
 	kubeletapis "k8s.io/kubernetes/pkg/kubelet/apis"
 )
 
+// TestGetRegion ensures correct source supplies AWS Region.
+func TestGetRegion(t *testing.T) {
+	key := "AWS_REGION"
+	originalRegion, originalPresent := os.LookupEnv(key)
+	defer func(region string, present bool) {
+		os.Unsetenv(key)
+		if present {
+			os.Setenv(key, region)
+		}
+	}(originalRegion, originalPresent)
+	// Ensure environment variable retains precedence.
+	expected1 := "the-shire-1"
+	os.Setenv(key, expected1)
+	assert.Equal(t, expected1, getRegion())
+	// Ensure without environment variable, EC2 Metadata used... and it merely
+	// chops the last character off the Availability Zone.
+	expected2 := "mordor-2"
+	expected2a := expected2 + "a"
+	os.Unsetenv(key)
+	server := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		w.Write([]byte(expected2a))
+	}))
+	cfg := aws.NewConfig().WithEndpoint(server.URL)
+	assert.Equal(t, expected2, getRegion(cfg))
+}
+
 func TestBuildGenericLabels(t *testing.T) {
 	labels := buildGenericLabels(&asgTemplate{
 		InstanceType: &instanceType{

cluster-autoscaler/cloudprovider/aws/examples/cluster-autoscaler-autodiscover.yaml

@@ -138,9 +138,6 @@ spec:
             - --skip-nodes-with-local-storage=false
             - --expander=least-waste
             - --node-group-auto-discovery=asg:tag=k8s.io/cluster-autoscaler/enabled,k8s.io/cluster-autoscaler/<YOUR CLUSTER NAME>
-          env:
-            - name: AWS_REGION
-              value: us-east-1
           volumeMounts:
             - name: ssl-certs
               mountPath: /etc/ssl/certs/ca-certificates.crt

cluster-autoscaler/cloudprovider/aws/examples/cluster-autoscaler-multi-asg.yaml

@@ -139,9 +139,6 @@ spec:
             - --expander=least-waste
             - --nodes=1:10:k8s-worker-asg-1
             - --nodes=1:3:k8s-worker-asg-2
-          env:
-            - name: AWS_REGION
-              value: us-east-1
           volumeMounts:
             - name: ssl-certs
               mountPath: /etc/ssl/certs/ca-certificates.crt

cluster-autoscaler/cloudprovider/aws/examples/cluster-autoscaler-one-asg.yaml

@@ -137,9 +137,6 @@ spec:
             - --cloud-provider=aws
             - --skip-nodes-with-local-storage=false
             - --nodes=1:10:k8s-worker-asg-1
-          env:
-            - name: AWS_REGION
-              value: us-east-1
           volumeMounts:
             - name: ssl-certs
               mountPath: /etc/ssl/certs/ca-certificates.crt

cluster-autoscaler/cloudprovider/aws/examples/cluster-autoscaler-run-on-master.yaml

@@ -144,9 +144,6 @@ spec:
             - --cloud-provider=aws
             - --skip-nodes-with-local-storage=false
             - --nodes={{ node_asg_min }}:{{ node_asg_max }}:{{ name }}
-          env:
-            - name: AWS_REGION
-              value: {{ region }}
           volumeMounts:
             - name: ssl-certs
               mountPath: /etc/ssl/certs/ca-certificates.crt