feat(gateway): add DisableHTMLErrors option

CHANGELOG.md

@@ -29,6 +29,9 @@ The following emojis are used to highlight certain changes:
 ### Added
 
 * ✨ The `routing/http` implements Delegated Peer Routing introduced in [IPIP-417](https://github.com/ipfs/specs/pull/417).
+* An option `DisableHTMLErrors` has been added to `gateway.Config`. When this option
+  is `true`, pretty HTML error pages for web browsers are disabled. Instead, a
+  `text/plain` page with the raw error message as the body is returned.
 
 ### Changed
 

gateway/errors.go

@@ -159,7 +159,7 @@ func webError(w http.ResponseWriter, r *http.Request, c *Config, err error, defa
 		code = gwErr.StatusCode
 	}
 
-	acceptsHTML := strings.Contains(r.Header.Get("Accept"), "text/html")
+	acceptsHTML := !c.DisableHTMLErrors && strings.Contains(r.Header.Get("Accept"), "text/html")
 	if acceptsHTML {
 		w.Header().Set("Content-Type", "text/html")
 		w.WriteHeader(code)

gateway/errors_test.go

@@ -87,4 +87,38 @@ func TestWebError(t *testing.T) {
 		webError(w, r, config, err, http.StatusInternalServerError)
 		require.Equal(t, http.StatusTeapot, w.Result().StatusCode)
 	})
+
+	t.Run("Error is sent as HTML when 'Accept' header contains 'text/html'", func(t *testing.T) {
+		t.Parallel()
+
+		w := httptest.NewRecorder()
+		r := httptest.NewRequest(http.MethodGet, "/blah", nil)
+		r.Header.Set("Accept", "something/else, text/html")
+		webError(w, r, config, NewErrorStatusCodeFromStatus(http.StatusTeapot), http.StatusInternalServerError)
+		require.Equal(t, http.StatusTeapot, w.Result().StatusCode)
+		require.Contains(t, w.Result().Header.Get("Content-Type"), "text/html")
+	})
+
+	t.Run("Error is sent as plain text when 'Accept' header does not contain 'text/html'", func(t *testing.T) {
+		t.Parallel()
+
+		w := httptest.NewRecorder()
+		r := httptest.NewRequest(http.MethodGet, "/blah", nil)
+		r.Header.Set("Accept", "application/json")
+		webError(w, r, config, NewErrorStatusCodeFromStatus(http.StatusTeapot), http.StatusInternalServerError)
+		require.Equal(t, http.StatusTeapot, w.Result().StatusCode)
+		require.Contains(t, w.Result().Header.Get("Content-Type"), "text/plain")
+	})
+
+	t.Run("Error is sent as plain text when 'Accept' header contains 'text/html' and config.DisableHTMLErrors is true", func(t *testing.T) {
+		t.Parallel()
+
+		config := &Config{Headers: map[string][]string{}, DisableHTMLErrors: true}
+		w := httptest.NewRecorder()
+		r := httptest.NewRequest(http.MethodGet, "/blah", nil)
+		r.Header.Set("Accept", "something/else, text/html")
+		webError(w, r, config, NewErrorStatusCodeFromStatus(http.StatusTeapot), http.StatusInternalServerError)
+		require.Equal(t, http.StatusTeapot, w.Result().StatusCode)
+		require.Contains(t, w.Result().Header.Get("Content-Type"), "text/plain")
+	})
 }

gateway/gateway.go

@@ -40,6 +40,11 @@ type Config struct {
 	// overridden per FQDN in PublicGateways. To be used with WithHostname.
 	NoDNSLink bool
 
+	// DisableHTMLErrors disables pretty HTML pages when an error occurs. Instead, a `text/plain`
+	// page will be sent with the raw error message. This can be useful if this gateway
+	// is being proxied by other service, which wants to use the error message.
+	DisableHTMLErrors bool
+
 	// PublicGateways configures the behavior of known public gateways. Each key is
 	// a fully qualified domain name (FQDN). To be used with WithHostname.
 	PublicGateways map[string]*PublicGateway