Catch FormatException from bad simulator log output (#90966)

packages/flutter_tools/lib/src/ios/simulators.dart

@@ -858,9 +858,14 @@ class _IOSSimulatorLogReader extends DeviceLogReader {
     // The log command predicate handles filtering, so every log eventMessage should be decoded and added.
     final Match eventMessageMatch = _unifiedLoggingEventMessageRegex.firstMatch(line);
     if (eventMessageMatch != null) {
-      final dynamic decodedJson = jsonDecode(eventMessageMatch.group(1));
-      if (decodedJson is String) {
-        _linesController.add(decodedJson);
+      final String message = eventMessageMatch.group(1);
+      try {
+        final dynamic decodedJson = jsonDecode(message);
+        if (decodedJson is String) {
+          _linesController.add(decodedJson);
+        }
+      } on FormatException {
+        globals.printError('Logger returned non-JSON response: $message');
       }
     }
   }

packages/flutter_tools/test/general.shard/ios/simulators_test.dart

@@ -622,6 +622,12 @@ Dec 20 17:04:32 md32-11-vm1 Another App[88374]: Ignore this text'''
     });
 
     group('unified logging', () {
+      BufferLogger logger;
+
+      setUp(() {
+        logger = BufferLogger.test();
+      });
+
       testUsingContext('log reader handles escaped multiline messages', () async {
         const String logPredicate = 'eventType = logEvent AND processImagePath ENDSWITH "My Super Awesome App" '
           'AND (senderImagePath ENDSWITH "/Flutter" OR senderImagePath ENDSWITH "/libswiftCore.dylib" '
@@ -676,6 +682,47 @@ Dec 20 17:04:32 md32-11-vm1 Another App[88374]: Ignore this text'''
         ProcessManager: () => fakeProcessManager,
         FileSystem: () => fileSystem,
       });
+
+      testUsingContext('log reader handles bad output', () async {
+        const String logPredicate = 'eventType = logEvent AND processImagePath ENDSWITH "My Super Awesome App" '
+            'AND (senderImagePath ENDSWITH "/Flutter" OR senderImagePath ENDSWITH "/libswiftCore.dylib" '
+            'OR processImageUUID == senderImageUUID) AND NOT(eventMessage CONTAINS ": could not find icon '
+            'for representation -> com.apple.") AND NOT(eventMessage BEGINSWITH "assertion failed: ") '
+            'AND NOT(eventMessage CONTAINS " libxpc.dylib ")';
+        fakeProcessManager.addCommand(const FakeCommand(
+            command:  <String>[
+              'xcrun',
+              'simctl',
+              'spawn',
+              '123456',
+              'log',
+              'stream',
+              '--style',
+              'json',
+              '--predicate',
+              logPredicate,
+            ],
+            stdout: '"eventMessage" : "message with incorrect escaping""',
+        ));
+
+        final IOSSimulator device = IOSSimulator(
+          '123456',
+          simulatorCategory: 'iOS 11.0',
+          simControl: simControl,
+        );
+        final DeviceLogReader logReader = device.getLogReader(
+          app: await BuildableIOSApp.fromProject(mockIosProject, null),
+        );
+
+        final List<String> lines = await logReader.logLines.toList();
+        expect(lines, isEmpty);
+        expect(logger.errorText, contains('Logger returned non-JSON response'));
+        expect(fakeProcessManager.hasRemainingExpectations, isFalse);
+      }, overrides: <Type, Generator>{
+        ProcessManager: () => fakeProcessManager,
+        FileSystem: () => fileSystem,
+        Logger: () => logger,
+      });
     });
   });