Default onLog falls back to print. (#4049)

Author: davidmorgan

build_test/CHANGELOG.md

@@ -5,6 +5,8 @@
   read and resolved.
 - Add `loadIsolateSources` to `ReaderWriterTesting`. It loads all real
   sources visible to the test into memory.
+- `testBuilder` default `onLog` now works outside of tests: it falls
+  back to `print` instead of crashing.
 
 ## 3.0.0
 

build_test/lib/src/test_builder.dart

@@ -230,12 +230,7 @@ Future<TestBuilderResult> testBuilders(
   TestReaderWriter? readerWriter,
   bool enableLowResourceMode = false,
 }) async {
-  onLog ??=
-      (log) => printOnFailure(
-        '$log'
-        '${log.error == null ? '' : '  ${log.error}'}'
-        '${log.stackTrace == null ? '' : '  ${log.stackTrace}'}',
-      );
+  onLog ??= _printOnFailureOrPrint;
 
   var inputIds = {
     for (var descriptor in sourceAssets.keys) makeAssetId(descriptor),
@@ -370,3 +365,17 @@ class TestBuilderResult {
 
   TestBuilderResult({required this.buildResult, required this.readerWriter});
 }
+
+void _printOnFailureOrPrint(LogRecord record) {
+  final message =
+      '$record'
+      '${record.error == null ? '' : '  ${record.error}'}'
+      '${record.stackTrace == null ? '' : '  ${record.stackTrace}'}';
+  try {
+    // This throws if a test is not currently running.
+    printOnFailure(message);
+  } catch (_) {
+    // Print instead.
+    print(message);
+  }
+}

build_test/test/test_builder_test.dart

@@ -10,7 +10,23 @@ import 'package:glob/glob.dart';
 import 'package:package_config/package_config.dart';
 import 'package:test/test.dart';
 
-void main() {
+Future<void> main() async {
+  // Default logging uses `printOnFailure` which crashes outside tests; check
+  // that it falls back to `print` outside tests.
+  final printed = <String>[];
+  await runZonedGuarded(
+    () async {
+      await testBuilder(TestBuilder(), {'a|lib/a.dart': ''}, rootPackage: 'a');
+    },
+    (_, _) {},
+    zoneSpecification: ZoneSpecification(
+      print: (_, _, _, string) {
+        printed.add(string);
+      },
+    ),
+  );
+  if (printed.isEmpty) throw StateError('Expected some prints.');
+
   test('can glob files in the root package', () async {
     var assets = {
       'a|lib/a.globPlaceholder': '',