aws/config: Update Request Send to always ensure HTTPResponse is not nil

Updates SDK Request.Send to always ensure HTTPResponse member is not
nil, and its Header and Body fields are not nil as well.

Fixes aws#4211

CHANGELOG_PENDING.md

@@ -3,3 +3,5 @@
 ### SDK Enhancements
 
 ### SDK Bugs
+* `aws/request`: Update Request Send to always ensure Request.HTTPResponse is populated.
+    * Fixes [#4211](https://github.com/aws/aws-sdk-go/issues/4211)

aws/request/request.go

@@ -4,6 +4,7 @@ import (
 	"bytes"
 	"fmt"
 	"io"
+	"io/ioutil"
 	"net/http"
 	"net/url"
 	"reflect"
@@ -525,6 +526,14 @@ func (r *Request) GetBody() io.ReadSeeker {
 // Send will not close the request.Request's body.
 func (r *Request) Send() error {
 	defer func() {
+		// Ensure a non-nil HTTPResponse parameter is set to ensure handlers
+		// checking for HTTPResponse values, don't fail.
+		if r.HTTPResponse == nil {
+			r.HTTPResponse = &http.Response{
+				Header: http.Header{},
+				Body:   ioutil.NopCloser(&bytes.Buffer{}),
+			}
+		}
 		// Regardless of success or failure of the request trigger the Complete
 		// request handlers.
 		r.Handlers.Complete.Run(r)

aws/request/request_test.go

@@ -1257,6 +1257,37 @@ func TestRequestMarshaledEndpointWithNonDefaultPort(t *testing.T) {
 	}
 }
 
+func TestRequestCompleteWithoutHTTPResponse(t *testing.T) {
+	s := awstesting.NewClient(aws.NewConfig().WithRegion("mock-region"))
+	r := s.NewRequest(&request.Operation{
+		Name:       "FooBar",
+		HTTPMethod: "GET",
+		HTTPPath:   "/",
+	}, nil, nil)
+	r.Handlers.Sign.Clear()
+	r.Handlers.Sign.PushFront(func(r *request.Request) {
+		r.Error = fmt.Errorf("failed")
+	})
+	r.Handlers.Complete.PushBack(func(r *request.Request) {
+		if r.HTTPResponse == nil {
+			t.Fatalf("expect HTTPResponse not to be nil")
+		}
+		if r.HTTPResponse.Header == nil {
+			t.Fatalf("expect HTTPResponse.Header not to be nil")
+		}
+		if r.HTTPResponse.Body == nil {
+			t.Fatalf("expect HTTPResponse.Body not to be nil")
+		}
+	})
+	err := r.Send()
+	if err == nil {
+		t.Fatalf("expect error, got none")
+	}
+	if e, a := "failed", err.Error(); !strings.Contains(a, e) {
+		t.Errorf("expect %v error in %v", e, a)
+	}
+}
+
 type stubSeekFail struct {
 	Err error
 }