Fix errors from original http.Client response being double-wrapped wh…

…en using the RoundTripper (#95)

This causes *url.Errors coming from the standard http.Client to be passed through instead of double wrapped.

roundtripper.go

@@ -1,7 +1,9 @@
 package retryablehttp
 
 import (
+	"errors"
 	"net/http"
+	"net/url"
 	"sync"
 )
 
@@ -39,5 +41,12 @@ func (rt *RoundTripper) RoundTrip(req *http.Request) (*http.Response, error) {
 	}
 
 	// Execute the request.
-	return rt.Client.Do(retryableReq)
+	resp, err := rt.Client.Do(retryableReq)
+	// If we got an error returned by standard library's `Do` method, unwrap it
+	// otherwise we will wind up erroneously re-nesting the error.
+	if _, ok := err.(*url.Error); ok {
+		return resp, errors.Unwrap(err)
+	}
+
+	return resp, err
 }

roundtripper_test.go

@@ -2,9 +2,13 @@ package retryablehttp
 
 import (
 	"context"
+	"errors"
 	"io/ioutil"
+	"net"
 	"net/http"
 	"net/http/httptest"
+	"net/url"
+	"reflect"
 	"sync/atomic"
 	"testing"
 )
@@ -87,3 +91,51 @@ func TestRoundTripper_RoundTrip(t *testing.T) {
 		t.Fatalf("expected %q, got %q", "success!", v)
 	}
 }
+
+func TestRoundTripper_TransportFailureErrorHandling(t *testing.T) {
+	// Make a client with some custom settings to verify they are used.
+	retryClient := NewClient()
+	retryClient.CheckRetry = func(_ context.Context, resp *http.Response, err error) (bool, error) {
+		if err != nil {
+			return true, err
+		}
+
+		return false, nil
+	}
+
+	retryClient.ErrorHandler = PassthroughErrorHandler
+
+	expectedError := &url.Error{
+		Op:  "Get",
+		URL: "http://this-url-does-not-exist-ed2fb.com/",
+		Err: &net.OpError{
+			Op:  "dial",
+			Net: "tcp",
+			Err: &net.DNSError{
+				Name:       "this-url-does-not-exist-ed2fb.com",
+				Err:        "no such host",
+				IsNotFound: true,
+			},
+		},
+	}
+
+	// Get the standard client and execute the request.
+	client := retryClient.StandardClient()
+	_, err := client.Get("http://this-url-does-not-exist-ed2fb.com/")
+
+	// assert expectations
+	if !reflect.DeepEqual(normalizeError(err), expectedError) {
+		t.Fatalf("expected %q, got %q", expectedError, err)
+	}
+}
+
+func normalizeError(err error) error {
+	var dnsError *net.DNSError
+
+	if errors.As(err, &dnsError) {
+		// this field is populated with the DNS server on on CI, but not locally
+		dnsError.Server = ""
+	}
+
+	return err
+}