core: create flag to prevent fatal error on bad status code

cli/cli-flags.js

@@ -204,12 +204,16 @@ function getYargsParser(manualArgv) {
         type: 'boolean',
         describe: 'Disables collection of the full page screenshot, which can be quite large',
       },
+      'ignore-status-code': {
+        type: 'boolean',
+        describe: 'Disables failing on all error status codes, and instead issues a warning.',
+      },
     })
     .group([
       'save-assets', 'list-all-audits', 'list-locales', 'list-trace-categories', 'additional-trace-categories',
       'config-path', 'preset', 'chrome-flags', 'port', 'hostname', 'form-factor', 'screenEmulation', 'emulatedUserAgent',
       'max-wait-for-load', 'enable-error-reporting', 'gather-mode', 'audit-mode',
-      'only-audits', 'only-categories', 'skip-audits', 'budget-path', 'disable-full-page-screenshot',
+      'only-audits', 'only-categories', 'skip-audits', 'budget-path', 'disable-full-page-screenshot', 'ignore-status-code',
     ], 'Configuration:')
 
     // Output

core/config/constants.js

@@ -113,6 +113,7 @@ const defaultSettings = {
   disableFullPageScreenshot: false,
   skipAboutBlank: false,
   blankPage: 'about:blank',
+  ignoreStatusCode: false,
 
   // the following settings have no defaults but we still want ensure that `key in settings`
   // in config will work in a typechecked way

core/gather/navigation-runner.js

@@ -175,6 +175,7 @@ async function _computeNavigationResult(
     ? getPageLoadError(navigationError, {
       url: mainDocumentUrl,
       loadFailureMode: navigationContext.navigation.loadFailureMode,
+      ignoreStatusCode: navigationContext.resolvedConfig.settings.ignoreStatusCode,
       networkRecords: debugData.records,
       warnings,
     })

core/lib/navigation-error.js

@@ -16,6 +16,13 @@ const UIStrings = {
    */
   warningXhtml:
     'The page MIME type is XHTML: Lighthouse does not explicitly support this document type',
+  /**
+   * Warning shown in report when the page under test returns an error code, which Lighthouse is not able to reliably load
+   * so we display a warning.
+   */
+  warningStatusCode: 'Lighthouse was unable to reliably load the page you requested. Make sure' +
+    ' you are testing the correct URL and that the server is properly responding' +
+    ' to all requests.',
 };
 
 const str_ = i18n.createIcuMessageFn(import.meta.url, UIStrings);
@@ -27,9 +34,10 @@ const XHTML_MIME_TYPE = 'application/xhtml+xml';
 /**
  * Returns an error if the original network request failed or wasn't found.
  * @param {LH.Artifacts.NetworkRequest|undefined} mainRecord
+ * @param {{warnings: Array<string | LH.IcuMessage>, ignoreStatusCode?: LH.Config.Settings['ignoreStatusCode']}} context
  * @return {LH.LighthouseError|undefined}
  */
-function getNetworkError(mainRecord) {
+function getNetworkError(mainRecord, context) {
   if (!mainRecord) {
     return new LighthouseError(LighthouseError.errors.NO_DOCUMENT_REQUEST);
   } else if (mainRecord.failed) {
@@ -47,9 +55,13 @@ function getNetworkError(mainRecord) {
         LighthouseError.errors.FAILED_DOCUMENT_REQUEST, {errorDetails: netErr});
     }
   } else if (mainRecord.hasErrorStatusCode()) {
-    return new LighthouseError(LighthouseError.errors.ERRORED_DOCUMENT_REQUEST, {
-      statusCode: `${mainRecord.statusCode}`,
-    });
+    if ( context.ignoreStatusCode) {
+      context.warnings.push(str_(UIStrings.warningStatusCode));
+    } else {
+      return new LighthouseError(LighthouseError.errors.ERRORED_DOCUMENT_REQUEST, {
+        statusCode: `${mainRecord.statusCode}`,
+      });
+    }
   }
 }
 
@@ -113,7 +125,7 @@ function getNonHtmlError(finalRecord) {
  * Returns an error if the page load should be considered failed, e.g. from a
  * main document request failure, a security issue, etc.
  * @param {LH.LighthouseError|undefined} navigationError
- * @param {{url: string, loadFailureMode: LH.Config.SharedPassNavigationJson['loadFailureMode'], networkRecords: Array<LH.Artifacts.NetworkRequest>, warnings: Array<string | LH.IcuMessage>}} context
+ * @param {{url: string, loadFailureMode: LH.Config.SharedPassNavigationJson['loadFailureMode'], ignoreStatusCode?: LH.Config.Settings['ignoreStatusCode'], networkRecords: Array<LH.Artifacts.NetworkRequest>, warnings: Array<string | LH.IcuMessage>}} context
  * @return {LH.LighthouseError|undefined}
  */
 function getPageLoadError(navigationError, context) {
@@ -144,7 +156,7 @@ function getPageLoadError(navigationError, context) {
     context.warnings.push(str_(UIStrings.warningXhtml));
   }
 
-  const networkError = getNetworkError(mainRecord);
+  const networkError = getNetworkError(mainRecord, context);
   const interstitialError = getInterstitialError(mainRecord, networkRecords);
   const nonHtmlError = getNonHtmlError(finalRecord);
 

core/scripts/i18n/collect-strings.js

@@ -720,6 +720,8 @@ function checkKnownFixedCollisions(strings) {
       'Document has a valid $MARKDOWN_SNIPPET_0$',
       'Failing Elements',
       'Failing Elements',
+      'Lighthouse was unable to reliably load the page you requested. Make sure you are testing the correct URL and that the server is properly responding to all requests.',
+      'Lighthouse was unable to reliably load the page you requested. Make sure you are testing the correct URL and that the server is properly responding to all requests.',
       'Name',
       'Name',
       'Pages that use portals are not currently eligible for back/forward cache.',