feat: diff the configs and output them

app_dart/lib/src/request_handlers/scheduler/batch_backfiller.dart

@@ -65,7 +65,7 @@ final class BatchBackfiller extends RequestHandler {
   Future<void> _backfillExperimentalBranch(RepositorySlug slug) async {
     final fsGrid = await _firestore.queryRecentCommitsAndTasks(
       slug,
-      commitLimit: config.backfillerCommitLimit,
+      commitLimit: config.flags.backfillerCommitLimit,
       branch: 'ios-experimental',
     );
     await _doBackfillFrom(slug, fsGrid, forceLowPriority: true);

app_dart/lib/src/service/config.dart

@@ -569,15 +569,15 @@ class DynamicConfigUpdater {
 
   void stopUpdateLoop() {
     if (_status != UpdaterStatus.running) return;
-    log.info('Stopping config update loop...');
+    log.info('ConfigUpdater: Stopping config update loop...');
     _status = UpdaterStatus.stopping;
   }
 
   void startUpdateLoop(Config config) async {
     if (_status != UpdaterStatus.stopped) return;
     _status = UpdaterStatus.running;
 
-    log.info('Starting config update loop...');
+    log.info('ConfigUpdater: Starting config update loop...');
 
     // What we've decided:
     //   1. Each instance will **start** with a valid DynamicConfig
@@ -589,18 +589,51 @@ class DynamicConfigUpdater {
         _delay + Duration(milliseconds: _random.nextInt(1000)),
       );
       if (_status != UpdaterStatus.running) {
-        log.info('Stopped config update loop');
+        log.info('ConfigUpdater: Stopped config update loop');
         _status = UpdaterStatus.stopped;
         return;
       }
       try {
         final dynamicConfig = await fetchDynamicConfig();
-        config._dynamicConfig = dynamicConfig;
+        final diffs = diffConfigChanges(
+          config._dynamicConfig.toJson(),
+          dynamicConfig.toJson(),
+        );
+        if (diffs.isNotEmpty) {
+          log.info('ConfigUpdater: ${diffs.join(',')}');
+          config._dynamicConfig = dynamicConfig;
+        }
       } catch (e, s) {
-        log.error('Unable to fetch DynamicConfig!', e, s);
+        log.error('ConfigUpdater: Unable to fetch DynamicConfig!', e, s);
       }
     }
   }
+
+  /// Produce a simple diff of the changing flags.
+  List<String> diffConfigChanges(
+    Map<String, Object?> oldFlags,
+    Map<String, Object?> newFlags, {
+    List<String>? diffs,
+    String chain = 'flags',
+  }) {
+    diffs ??= <String>[];
+
+    for (final MapEntry(:key, :value) in oldFlags.entries) {
+      if (value is Map) {
+        diffConfigChanges(
+          value as Map<String, Object?>,
+          newFlags[key] as Map<String, Object?>,
+          diffs: diffs,
+          chain: '$chain.$key',
+        );
+        continue;
+      }
+      if (value != newFlags[key]) {
+        diffs.add('$chain.$key $value -> ${newFlags[key]}');
+      }
+    }
+    return diffs;
+  }
 }
 
 enum UpdaterStatus { stopped, running, stopping }

app_dart/test/service/dynamic_config_updater_test.dart

@@ -75,7 +75,7 @@ void main() {
       bufferedLoggerOf(
         containsOnce(
           logThat(
-            message: equals('Starting config update loop...'),
+            message: equals('ConfigUpdater: Starting config update loop...'),
             severity: atMostInfo,
           ),
         ),
@@ -86,7 +86,7 @@ void main() {
       bufferedLoggerOf(
         containsOnce(
           logThat(
-            message: equals('Stopping config update loop...'),
+            message: equals('ConfigUpdater: Stopping config update loop...'),
             severity: atMostInfo,
           ),
         ),
@@ -97,7 +97,7 @@ void main() {
       bufferedLoggerOf(
         containsOnce(
           logThat(
-            message: equals('Stopped config update loop'),
+            message: equals('ConfigUpdater: Stopped config update loop'),
             severity: atMostInfo,
           ),
         ),
@@ -131,6 +131,29 @@ void main() {
     );
     expect(config.flags.backfillerCommitLimit, 100);
   });
+
+  test('handles updating values', () async {
+    updater.startUpdateLoop(config);
+    await Future<void>.delayed(const Duration(milliseconds: 50));
+    mockHttpFile = updatedConfigYaml;
+    await Future<void>.delayed(const Duration(milliseconds: 50));
+    updater.stopUpdateLoop();
+
+    expect(config.flags.backfillerCommitLimit, 200);
+    expect(
+      log,
+      bufferedLoggerOf(
+        containsOnce(
+          logThat(
+            message: equals(
+              'ConfigUpdater: flags.backfillerCommitLimit 100 -> 200',
+            ),
+            severity: atMostInfo,
+          ),
+        ),
+      ),
+    );
+  });
 }
 
 final class _FakeRandom extends Fake implements Random {
@@ -149,3 +172,12 @@ const goodConfigYaml = '''
 
 backfillerCommitLimit: 100
 ''';
+
+const updatedConfigYaml = '''
+# Defines the config options for Flutter CI (Cocoon)
+#
+# The schema for this file is defined in DynamicConfig of
+# app_dart/lib/src/service/config.dart
+
+backfillerCommitLimit: 200
+''';