Fix EC2Metadata Client overriding http.DefaultClient incorrectly

The EC2Metadata Client was incorrectly overriding the http.DefaultClient
when users had modified the default client's parameters. The metadata
client will now only set its alternate dial timeout if no http client
was provided, or if the provided client was never modified from the
original http.Client{}.

Also updates the logic so DefaultTransport is reused instead of
hardcoded.

Fix #504

aws/ec2metadata/service.go

@@ -6,6 +6,7 @@ import (
 	"io/ioutil"
 	"net"
 	"net/http"
+	"reflect"
 	"time"
 
 	"github.com/aws/aws-sdk-go/aws"
@@ -26,6 +27,10 @@ type EC2Metadata struct {
 // New creates a new instance of the EC2Metadata client with a session.
 // This client is safe to use across multiple goroutines.
 //
+// If an unmodified HTTP client is provided from the stdlib default, or no client
+// it is safe to override the dial's timeout and keep alive for shorter connections.
+// If any client is provided which is not equal to the original default.
+//
 // Example:
 //     // Create a EC2Metadata client from just a session.
 //     svc := ec2metadata.New(mySession)
@@ -41,22 +46,22 @@ func New(p client.ConfigProvider, cfgs ...*aws.Config) *EC2Metadata {
 // a client when not using a session. Generally using just New with a session
 // is preferred.
 func NewClient(cfg aws.Config, handlers request.Handlers, endpoint, signingRegion string, opts ...func(*client.Client)) *EC2Metadata {
-	// If the default http client is provided, replace it with a custom
-	// client using default timeouts.
-	if cfg.HTTPClient == http.DefaultClient {
-		cfg.HTTPClient = &http.Client{
-			Transport: &http.Transport{
-				Proxy: http.ProxyFromEnvironment,
-				Dial: (&net.Dialer{
-					// use a shorter timeout than default because the metadata
-					// service is local if it is running, and to fail faster
-					// if not running on an ec2 instance.
-					Timeout:   5 * time.Second,
-					KeepAlive: 30 * time.Second,
-				}).Dial,
-				TLSHandshakeTimeout: 10 * time.Second,
-			},
+	if cfg.HTTPClient == nil || reflect.DeepEqual(*cfg.HTTPClient, http.Client{}) {
+		// If a unmodified default http client is provided it is safe to add
+		// custom timeouts.
+		httpClient := *http.DefaultClient
+		if t, ok := http.DefaultTransport.(*http.Transport); ok {
+			transport := *t
+			transport.Dial = (&net.Dialer{
+				// use a shorter timeout than default because the metadata
+				// service is local if it is running, and to fail faster
+				// if not running on an ec2 instance.
+				Timeout:   5 * time.Second,
+				KeepAlive: 30 * time.Second,
+			}).Dial
+			httpClient.Transport = &transport
 		}
+		cfg.HTTPClient = &httpClient
 	}
 
 	svc := &EC2Metadata{

aws/ec2metadata/service_test.go

@@ -0,0 +1,40 @@
+package ec2metadata_test
+
+import (
+	"net/http"
+	"testing"
+
+	"github.com/aws/aws-sdk-go/aws/ec2metadata"
+	"github.com/aws/aws-sdk-go/aws/session"
+	"github.com/stretchr/testify/assert"
+)
+
+func TestClientOverrideDefaultHTTPClientDialTimeout(t *testing.T) {
+	svc := ec2metadata.New(session.New())
+
+	assert.NotEqual(t, http.DefaultClient, svc.Config.HTTPClient)
+
+	tr, ok := svc.Config.HTTPClient.Transport.(*http.Transport)
+	assert.True(t, ok)
+	assert.NotNil(t, tr)
+
+	assert.NotNil(t, tr.Dial)
+}
+
+func TestClientNotOverrideDefaultHTTPClientDialTimeout(t *testing.T) {
+	origClient := *http.DefaultClient
+	http.DefaultClient.Transport = &http.Transport{}
+	defer func() {
+		http.DefaultClient = &origClient
+	}()
+
+	svc := ec2metadata.New(session.New())
+
+	assert.Equal(t, http.DefaultClient, svc.Config.HTTPClient)
+
+	tr, ok := svc.Config.HTTPClient.Transport.(*http.Transport)
+	assert.True(t, ok)
+	assert.NotNil(t, tr)
+
+	assert.Nil(t, tr.Dial)
+}