Improve documentation and tests for base::OnTaskRunnerDeleter.

base/sequenced_task_runner_unittest.cc

Index: base/sequenced_task_runner_unittest.cc
diff --git a/base/sequenced_task_runner_unittest.cc b/base/sequenced_task_runner_unittest.cc
index 93ac20f7190a15e991f581c4effd67f0fe5b4bf7..7451e1d7a421fbdab915429fc15ede77461990eb 100644
--- a/base/sequenced_task_runner_unittest.cc
+++ b/base/sequenced_task_runner_unittest.cc
@@ -5,6 +5,7 @@
 #include "base/sequenced_task_runner.h"
 
 #include "base/bind.h"
+#include "base/gtest_prod_util.h"
 #include "base/message_loop/message_loop.h"
 #include "base/run_loop.h"
 #include "base/threading/thread.h"
@@ -13,60 +14,89 @@
 namespace base {
 namespace {
 
-struct DeleteCounter {
-  DeleteCounter(int* counter, scoped_refptr<SequencedTaskRunner> task_runner)
-      : counter_(counter),
-        task_runner_(std::move(task_runner)) {
-  }
-  ~DeleteCounter() {
-    ++*counter_;
-    EXPECT_TRUE(!task_runner_ || task_runner_->RunsTasksOnCurrentThread());
+class FlagOnDelete {
+ public:
+  FlagOnDelete(bool* deleted,
+               scoped_refptr<SequencedTaskRunner> expected_deletion_sequence)
+      : deleted_(deleted),
+        expected_deletion_sequence_(std::move(expected_deletion_sequence)) {}
+
+ private:
+  friend class DeleteHelper<FlagOnDelete>;
+  FRIEND_TEST_ALL_PREFIXES(SequencedTaskRunnerTest,
+                           OnTaskRunnerDeleterTargetStoppedEarly);
+
+  ~FlagOnDelete() {
+    EXPECT_FALSE(*deleted_);
+    *deleted_ = true;
+    if (expected_deletion_sequence_)
+      EXPECT_TRUE(expected_deletion_sequence_->RunsTasksInCurrentSequence());
   }
 
-  int* counter_;
-  scoped_refptr<SequencedTaskRunner> task_runner_;
+  bool* deleted_;
+  const scoped_refptr<SequencedTaskRunner> expected_deletion_sequence_;
+
+  DISALLOW_COPY_AND_ASSIGN(FlagOnDelete);
 };
 
-}  // namespace
+class SequencedTaskRunnerTest : public testing::Test {
+ protected:
+  SequencedTaskRunnerTest() : foreign_thread_("foreign") {}
+
+  void SetUp() override {
+    main_runner_ = message_loop_.task_runner();
 
-TEST(SequencedTaskRunnerTest, OnTaskRunnerDeleter) {
-  base::MessageLoop message_loop;
-  base::Thread thread("Foreign");
-  thread.Start();
+    foreign_thread_.Start();
+    foreign_runner_ = foreign_thread_.task_runner();
+  }
 
-  scoped_refptr<SequencedTaskRunner> current_thread =
-      message_loop.task_runner();
-  scoped_refptr<SequencedTaskRunner> foreign_thread =
-      thread.task_runner();
+  scoped_refptr<SequencedTaskRunner> main_runner_;
+  scoped_refptr<SequencedTaskRunner> foreign_runner_;
 
-  using SequenceBoundUniquePtr =
-      std::unique_ptr<DeleteCounter, OnTaskRunnerDeleter>;
+  Thread foreign_thread_;
 
-  int counter = 0;
-  SequenceBoundUniquePtr ptr(new DeleteCounter(&counter, current_thread),
-                             OnTaskRunnerDeleter(current_thread));
-  EXPECT_EQ(0, counter);
-  foreign_thread->PostTask(
+ private:
+  MessageLoop message_loop_;
+
+  DISALLOW_COPY_AND_ASSIGN(SequencedTaskRunnerTest);
+};
+
+using SequenceBoundUniquePtr =
+    std::unique_ptr<FlagOnDelete, OnTaskRunnerDeleter>;
+
+TEST_F(SequencedTaskRunnerTest, OnTaskRunnerDeleterOnMainThread) {
+  bool deleted_on_main_thread = false;
+  SequenceBoundUniquePtr ptr(
+      new FlagOnDelete(&deleted_on_main_thread, main_runner_),
+      OnTaskRunnerDeleter(main_runner_));
+  EXPECT_FALSE(deleted_on_main_thread);
+  foreign_runner_->PostTask(
       FROM_HERE, BindOnce([](SequenceBoundUniquePtr) {}, Passed(&ptr)));
 
   {
     RunLoop run_loop;
-    foreign_thread->PostTaskAndReply(FROM_HERE, BindOnce([] {}),
-                                     run_loop.QuitClosure());
+    foreign_runner_->PostTaskAndReply(FROM_HERE, BindOnce([] {}),
+                                      run_loop.QuitClosure());
     run_loop.Run();
   }
-  EXPECT_EQ(1, counter);
+  EXPECT_TRUE(deleted_on_main_thread);
+}
 
-  DeleteCounter* raw = new DeleteCounter(&counter, nullptr);
-  SequenceBoundUniquePtr ptr2(raw, OnTaskRunnerDeleter(foreign_thread));
-  EXPECT_EQ(1, counter);
+TEST_F(SequencedTaskRunnerTest, OnTaskRunnerDeleterTargetStoppedEarly) {
+  bool deleted_on_main_thread = false;
+  FlagOnDelete* raw = new FlagOnDelete(&deleted_on_main_thread, main_runner_);
+  SequenceBoundUniquePtr ptr(raw, OnTaskRunnerDeleter(foreign_runner_));
+  EXPECT_FALSE(deleted_on_main_thread);
 
-  thread.Stop();
-  ptr2 = nullptr;
-  ASSERT_EQ(1, counter);
+  // Stopping the target ahead of deleting |ptr| should make its
+  // OnTaskRunnerDeleter no-op.
+  foreign_thread_.Stop();
+  ptr = nullptr;
+  EXPECT_FALSE(deleted_on_main_thread);
 
   delete raw;
-  EXPECT_EQ(2, counter);
+  EXPECT_TRUE(deleted_on_main_thread);
 }
 
+}  // namespace
 }  // namespace base