Verify that importer 'contents' are a string-type (#167)

Implements verification added on sass/dart-sass#1816

See sass/sass-spec#1838

Author: Goodwine

lib/src/importer-registry.ts

@@ -110,6 +110,21 @@ export class ImporterRegistry<sync extends 'sync' | 'async'> {
         return thenOr(importer.load(new URL(request.getUrl())), result => {
           const proto = new InboundMessage.ImportResponse();
           if (result) {
+            if (typeof result.contents !== 'string') {
+              throw Error(
+                `Invalid argument (contents): must be a string but was: ${
+                  (result.contents as {}).constructor.name
+                }`
+              );
+            }
+
+            if (result.sourceMapUrl && !result.sourceMapUrl.protocol) {
+              throw Error(
+                'Invalid argument (sourceMapUrl): must be absolute but was: ' +
+                  result.sourceMapUrl
+              );
+            }
+
             const success = new InboundMessage.ImportResponse.ImportSuccess();
             success.setContents(result.contents);
             success.setSyntax(utils.protofySyntax(result.syntax));

lib/src/legacy/importer.ts

@@ -260,9 +260,22 @@ export class LegacyImporterWrapper<sync extends 'sync' | 'async'>
       thenOr(
         this.invokeCallback(this.callbacks[n], self, url, prev),
         result => {
-          if (result !== null) return result;
-          if (n === this.callbacks.length - 1) return null;
-          return invokeNthCallback(n + 1);
+          if (result === null) {
+            if (n === this.callbacks.length - 1) return null;
+            return invokeNthCallback(n + 1);
+          }
+          if (
+            'contents' in result &&
+            result.contents &&
+            typeof result.contents !== 'string'
+          ) {
+            throw new Error(
+              `Invalid argument (contents): must be a string but was: ${
+                (result.contents as {}).constructor.name
+              }`
+            );
+          }
+          return result;
         }
       );