[native_assets_builder] Fix hook kernel compilation caching (#1406)

pkgs/native_assets_builder/lib/src/build_runner/build_runner.dart

@@ -610,9 +610,20 @@ ${e.message}
     }
   }
 
-  /// Compiles the hook to dill and caches the dill.
+  /// Compiles the hook to kernel and caches the kernel.
   ///
-  /// It does not reuse the cached dill for different [config]s, due to
+  /// If any of the Dart source files, or the package config changed after
+  /// the last time the kernel file is compiled, the kernel file is
+  /// recompiled. Otherwise a cached version is used.
+  ///
+  /// Due to some OSes only providing last-modified timestamps with second
+  /// precision. The kernel compilation cache might be considered stale if
+  /// the last modification and the kernel compilation happened within one
+  /// second of each other. We error on the side of caution, rather recompile
+  /// one time too many, then not recompiling when recompilation should have
+  /// happened.
+  ///
+  /// It does not reuse the cached kernel for different [config]s, due to
   /// reentrancy requirements. For more info see:
   /// https://github.com/dart-lang/native/issues/1319
   Future<(bool success, File kernelFile, DateTime lastSourceChange)>
@@ -639,7 +650,7 @@ ${e.message}
       final dartSourceFiles = depFileContents
           .trim()
           .split(' ')
-          .skip(1) // '<dill>:'
+          .skip(1) // '<kernel file>:'
           .map((u) => Uri.file(u).fileSystemEntity)
           .toList();
       final dartFilesLastChange = await dartSourceFiles.lastModified();
@@ -648,8 +659,9 @@ ${e.message}
       sourceLastChange = packageConfigLastChange.isAfter(dartFilesLastChange)
           ? packageConfigLastChange
           : dartFilesLastChange;
-      final dillLastChange = await kernelFile.lastModified();
-      mustCompile = sourceLastChange.isAfter(dillLastChange);
+      final kernelLastChange = await kernelFile.lastModified();
+      mustCompile = sourceLastChange == kernelLastChange ||
+          sourceLastChange.isAfter(kernelLastChange);
     }
     final bool success;
     if (!mustCompile) {

pkgs/native_assets_builder/test/build_runner/build_runner_caching_test.dart

@@ -110,36 +110,55 @@ void main() async {
   test(
     'add C file, modify hook',
     timeout: longTimeout,
-    onPlatform: {
-      'linux': const Skip('https://github.com/dart-lang/native/issues/1391.'),
-    },
     () async {
       await inTempDir((tempUri) async {
         await copyTestProjects(targetUri: tempUri);
         final packageUri = tempUri.resolve('native_add/');
 
+        final logMessages = <String>[];
+        final logger = createCapturingLogger(logMessages);
+
         await runPubGet(workingDirectory: packageUri, logger: logger);
+        logMessages.clear();
         // Make sure the first compile is at least one second after the
         // package_config.json is written, otherwise dill compilation isn't
         // cached.
         await Future<void>.delayed(const Duration(seconds: 1));
 
+        final result = await build(packageUri, logger, dartExecutable);
         {
-          final result = await build(packageUri, logger, dartExecutable);
-          await expectSymbols(
-              asset: result.assets.single as NativeCodeAssetImpl,
-              symbols: ['add']);
+          final compiledHook = logMessages
+              .where((m) =>
+                  m.contains('dart compile kernel') ||
+                  m.contains('dart.exe compile kernel'))
+              .isNotEmpty;
+          expect(compiledHook, isTrue);
         }
+        logMessages.clear();
+        await expectSymbols(
+          asset: result.assets.single as NativeCodeAssetImpl,
+          symbols: ['add'],
+        );
 
         await copyTestProjects(
             sourceUri: testDataUri.resolve('native_add_add_source/'),
             targetUri: packageUri);
 
         {
           final result = await build(packageUri, logger, dartExecutable);
+          {
+            final compiledHook = logMessages
+                .where((m) =>
+                    m.contains('dart compile kernel') ||
+                    m.contains('dart.exe compile kernel'))
+                .isNotEmpty;
+            expect(compiledHook, isTrue);
+          }
+          logMessages.clear();
           await expectSymbols(
-              asset: result.assets.single as NativeCodeAssetImpl,
-              symbols: ['add', 'multiply']);
+            asset: result.assets.single as NativeCodeAssetImpl,
+            symbols: ['add', 'multiply'],
+          );
         }
       });
     },

pkgs/native_assets_builder/test/build_runner/build_runner_test.dart

@@ -24,6 +24,11 @@ void main() async {
         logger: logger,
       );
 
+      // Make sure the first compile is at least one second after the
+      // package_config.json is written, otherwise dill compilation isn't
+      // cached.
+      await Future<void>.delayed(const Duration(seconds: 1));
+
       // Trigger a build, should invoke build for libraries with native assets.
       {
         final logMessages = <String>[];