Transport: Fix memory leak when retrying 5xx responses

To prevent a memory leak when a request with a retryable status code is executed (502, etc),
drain and close the body before the request is retried.

Fixes #159

(cherry picked from commit c50a178)

Author: karmi

estransport/estransport.go

@@ -226,8 +226,9 @@ func (c *Client) Perform(req *http.Request) (*http.Response, error) {
 
 	for i := 1; i <= c.maxRetries; i++ {
 		var (
-			conn        *Connection
-			shouldRetry bool
+			conn            *Connection
+			shouldRetry     bool
+			shouldCloseBody bool
 		)
 
 		// Get connection from the pool
@@ -308,6 +309,7 @@ func (c *Client) Perform(req *http.Request) (*http.Response, error) {
 			for _, code := range c.retryOnStatus {
 				if res.StatusCode == code {
 					shouldRetry = true
+					shouldCloseBody = true
 				}
 			}
 		}
@@ -317,6 +319,14 @@ func (c *Client) Perform(req *http.Request) (*http.Response, error) {
 			break
 		}
 
+		// Drain and close body when retrying after response
+		if shouldCloseBody && i < c.maxRetries {
+			if res.Body != nil {
+				io.Copy(ioutil.Discard, res.Body)
+				res.Body.Close()
+			}
+		}
+
 		// Delay the retry if a backoff function is configured
 		if c.retryBackoff != nil {
 			time.Sleep(c.retryBackoff(i))

estransport/estransport_internal_test.go

@@ -501,6 +501,50 @@ func TestTransportPerformRetries(t *testing.T) {
 		}
 	})
 
+	t.Run("Close response body for a 5xx response", func(t *testing.T) {
+		var (
+			i       int
+			numReqs = 5
+		)
+
+		u, _ := url.Parse("http://foo.bar")
+		tp, _ := New(Config{
+			URLs:       []*url.URL{u, u, u},
+			MaxRetries: numReqs,
+			Transport: &mockTransp{
+				RoundTripFunc: func(req *http.Request) (*http.Response, error) {
+					i++
+					fmt.Printf("Request #%d", i)
+					fmt.Print(": 502\n")
+					body := ioutil.NopCloser(strings.NewReader(`MOCK`))
+					return &http.Response{StatusCode: 502, Body: body}, nil
+				},
+			}})
+
+		req, _ := http.NewRequest("GET", "/", nil)
+
+		res, err := tp.Perform(req)
+
+		if err != nil {
+			t.Fatalf("Unexpected error: %s", err)
+		}
+
+		if i != numReqs {
+			t.Errorf("Unexpected number of requests, want=%d, got=%d", numReqs, i)
+		}
+
+		if res.StatusCode != 502 {
+			t.Errorf("Unexpected response: %+v", res)
+		}
+
+		resBody, _ := ioutil.ReadAll(res.Body)
+		res.Body.Close()
+
+		if string(resBody) != "MOCK" {
+			t.Errorf("Unexpected body, want=MOCK, got=%s", resBody)
+		}
+	})
+
 	t.Run("Retry request and return error when max retries exhausted", func(t *testing.T) {
 		var (
 			i       int