fix(devtool): handle trailing slash in URL and improve error message (#155)

A trailing slash in the URL caused double-slash paths like "//json/version",
which failed with the misleading error "could not resolve IP for 127.0.0.1".

Now we strip trailing slashes and show the actual endpoints tried on failure.

Fixes #152

Author: mafredri

devtool/devtool.go

@@ -37,7 +37,7 @@ type DevTools struct {
 
 // New returns a DevTools instance that uses URL.
 func New(url string, opts ...DevToolsOption) *DevTools {
-	devtools := &DevTools{url: url}
+	devtools := &DevTools{url: strings.TrimRight(url, "/")}
 	for _, o := range opts {
 		o(devtools)
 	}
@@ -297,6 +297,7 @@ func (d *DevTools) resolveHost(ctx context.Context) error {
 		return err
 	}
 
+	var tried []string
 	newURL := ""
 	for _, a := range addrs {
 		host[0] = a
@@ -305,7 +306,9 @@ func (d *DevTools) resolveHost(ctx context.Context) error {
 
 		// The selection of "/json/version" here is arbitrary,
 		// it just needs to exist and not have side-effects.
-		req, err := http.NewRequest(http.MethodGet, try+"/json/version", nil)
+		endpoint := try + "/json/version"
+		tried = append(tried, endpoint)
+		req, err := http.NewRequest(http.MethodGet, endpoint, nil)
 		if err != nil {
 			return err
 		}
@@ -322,7 +325,7 @@ func (d *DevTools) resolveHost(ctx context.Context) error {
 		}
 	}
 	if newURL == "" {
-		return errors.New("could not resolve IP for " + origHost)
+		return fmt.Errorf("could not find a working endpoint for %q, tried: %v", origHost, tried)
 	}
 	d.url = newURL
 

devtool/devtool_test.go

@@ -253,6 +253,28 @@ func TestDevTools_InvalidURL(t *testing.T) {
 	}
 }
 
+func TestDevTools_TrailingSlash(t *testing.T) {
+	th := newTestHandler(t)
+	srv := httptest.NewServer(th)
+	defer srv.Close()
+
+	// Create DevTools with trailing slash in URL.
+	devt := New(srv.URL + "/")
+	th.hostnameLookup = true
+
+	th.status = http.StatusOK
+	th.body = read(t, filepath.Join("testdata", "version.json"))
+
+	ctx, cancel := context.WithCancel(context.Background())
+	defer cancel()
+
+	// This should succeed without double-slash issues.
+	_, err := devt.Version(ctx)
+	if err != nil {
+		t.Errorf("Version failed with trailing slash URL: %v", err)
+	}
+}
+
 func showDiff(t testing.TB, got, want []byte) {
 	gr := bufio.NewReader(bytes.NewReader(got))
 	wr := bufio.NewReader(bytes.NewReader(want))