Fix a race condition in meta.load-css() (#1376)

We weren't properly awaiting a call to CssStylesheet.accept(), which
meant that it could try to continue doing work asynchronously in the
wrong context.

Closes #1318

CHANGELOG.md

@@ -16,6 +16,9 @@
 * Fix an edge case where `@extend` wouldn't affect a selector within a
   pseudo-selector such as `:is()` that itself extended other selectors.
 
+* Fix a race condition where `meta.load-css()` could trigger an internal error
+  when running in asynchronous mode.
+
 ## 1.35.1
 
 * Fix a bug where the quiet dependency flag didn't silence warnings in some

lib/src/extend/extension_store.dart

@@ -262,8 +262,7 @@ class ExtensionStore {
         _extensionsByExtender.putIfAbsent(simple, () => []).add(extension);
         // Only source specificity for the original selector is relevant.
         // Selectors generated by `@extend` don't get new specificity.
-        _sourceSpecificity.putIfAbsent(
-          simple, () => complex.maxSpecificity);
+        _sourceSpecificity.putIfAbsent(simple, () => complex.maxSpecificity);
       }
 
       if (selectors != null || existingExtensions != null) {

lib/src/visitor/async_evaluate.dart

@@ -592,7 +592,7 @@ class _EvaluateVisitor
   /// The [stackFrame] and [nodeWithSpan] are used for the name and location of
   /// the stack frame for the duration of the [callback].
   Future<void> _loadModule(Uri url, String stackFrame, AstNode nodeWithSpan,
-      void callback(Module module),
+      FutureOr<void> callback(Module module),
       {Uri? baseUrl,
       Configuration? configuration,
       bool namesInErrors = false}) async {
@@ -606,7 +606,7 @@ class _EvaluateVisitor
             configuration.nodeWithSpan.span);
       }
 
-      _addExceptionSpan(nodeWithSpan, () => callback(builtInModule));
+      await _addExceptionSpanAsync(nodeWithSpan, () => callback(builtInModule));
       return;
     }
 
@@ -643,7 +643,7 @@ class _EvaluateVisitor
       }
 
       try {
-        callback(module);
+        await callback(module);
       } on SassRuntimeException {
         rethrow;
       } on MultiSpanSassException catch (error) {
@@ -3212,7 +3212,7 @@ class _EvaluateVisitor
 
   /// Like [_addExceptionSpan], but for an asynchronous [callback].
   Future<T> _addExceptionSpanAsync<T>(
-      AstNode nodeWithSpan, Future<T> callback()) async {
+      AstNode nodeWithSpan, FutureOr<T> callback()) async {
     try {
       return await callback();
     } on MultiSpanSassScriptException catch (error) {

lib/src/visitor/evaluate.dart

@@ -5,7 +5,7 @@
 // DO NOT EDIT. This file was generated from async_evaluate.dart.
 // See tool/grind/synchronize.dart for details.
 //
-// Checksum: 1249b1184a46950409776772cb1c6003b80ebb31
+// Checksum: b36c6c618b222a025ed67c1df8afb66e037a132b
 //
 // ignore_for_file: unused_import
 

test/dart_api_test.dart

@@ -225,4 +225,16 @@ a {
       });
     });
   });
+
+  // Regression test for #1318
+  test("meta.load-module() doesn't have a race condition", () async {
+    await d.file("other.scss", '/**//**/').create();
+    expect(compileStringAsync("""
+      @use 'sass:meta';
+      @include meta.load-css("other.scss");
+    """, loadPaths: [d.sandbox]), completion(equals("/**/\n/**/")));
+
+    // Give the race condition time to appear.
+    await pumpEventQueue();
+  });
 }