fix checkLockAcquired: support re-entrant locking (#9272)

* fix checkLockAcquired: support re-entrant locking

* add test; address comments

* add comment

packages/flutter_tools/lib/src/cache.dart

@@ -4,6 +4,8 @@
 
 import 'dart:async';
 
+import 'package:meta/meta.dart';
+
 import 'base/context.dart';
 import 'base/file_system.dart';
 import 'base/logger.dart';
@@ -33,10 +35,19 @@ class Cache {
   ///
   /// This is used by the tests since they run simultaneously and all in one
   /// process and so it would be a mess if they had to use the lock.
+  @visibleForTesting
   static void disableLocking() {
     _lockEnabled = false;
   }
 
+  /// Turn on the [lock]/[releaseLockEarly] mechanism.
+  ///
+  /// This is used by the tests.
+  @visibleForTesting
+  static void enableLocking() {
+    _lockEnabled = true;
+  }
+
   /// Lock the cache directory.
   ///
   /// This happens automatically on startup (see [FlutterCommandRunner.runCommand]).
@@ -48,7 +59,7 @@ class Cache {
     if (!_lockEnabled)
       return null;
     assert(_lock == null);
-    _lock = fs.file(fs.path.join(flutterRoot, 'bin', 'cache', 'lockfile')).openSync(mode: FileMode.WRITE);
+    _lock = await fs.file(fs.path.join(flutterRoot, 'bin', 'cache', 'lockfile')).open(mode: FileMode.WRITE);
     bool locked = false;
     bool printed = false;
     while (!locked) {
@@ -77,7 +88,7 @@ class Cache {
   /// Checks if the current process owns the lock for the cache directory at
   /// this very moment; throws a [StateError] if it doesn't.
   static void checkLockAcquired() {
-    if (_lockEnabled && _lock == null) {
+    if (_lockEnabled && _lock == null && platform.environment['FLUTTER_ALREADY_LOCKED'] != 'true') {
       throw new StateError(
         'The current process does not own the lock for the cache directory. This is a bug in Flutter CLI tools.',
       );

packages/flutter_tools/test/src/cache_test.dart

@@ -0,0 +1,67 @@
+// Copyright 2016 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+import 'dart:async';
+
+import 'package:file/file.dart';
+import 'package:file/memory.dart';
+import 'package:mockito/mockito.dart';
+import 'package:test/test.dart';
+import 'package:platform/platform.dart';
+
+import 'package:flutter_tools/src/cache.dart';
+
+import 'context.dart';
+
+void main() {
+  group('$Cache.checkLockAcquired', () {
+    setUp(() {
+      Cache.enableLocking();
+    });
+
+    tearDown(() {
+      // Restore locking to prevent potential side-effects in
+      // tests outside this group (this option is globally shared).
+      Cache.enableLocking();
+    });
+
+    test('should throw when locking is not acquired', () {
+      expect(() => Cache.checkLockAcquired(), throwsStateError);
+    });
+
+    test('should not throw when locking is disabled', () {
+      Cache.disableLocking();
+      Cache.checkLockAcquired();
+    });
+
+    testUsingContext('should not throw when lock is acquired', () async {
+      await Cache.lock();
+      Cache.checkLockAcquired();
+    }, overrides: <Type, Generator>{
+      FileSystem: () => new MockFileSystem(),
+    });
+
+    testUsingContext('should not throw when FLUTTER_ALREADY_LOCKED is set', () async {
+      Cache.checkLockAcquired();
+    }, overrides: <Type, Generator>{
+      Platform: () => new FakePlatform()..environment = <String, String>{'FLUTTER_ALREADY_LOCKED': 'true'},
+    });
+  });
+}
+
+class MockFileSystem extends MemoryFileSystem {
+  @override
+  File file(dynamic path) {
+    return new MockFile();
+  }
+}
+
+class MockFile extends Mock implements File {
+  @override
+  Future<RandomAccessFile> open({FileMode mode: FileMode.READ}) async {
+    return new MockRandomAccessFile();
+  }
+}
+
+class MockRandomAccessFile extends Mock implements RandomAccessFile {}