TaskScheduler: Add SequenceToken and ScopedSetSequenceTokenForCurrentThread

base/sequence_checker_unittest.cc

Index: base/sequence_checker_unittest.cc
diff --git a/base/sequence_checker_unittest.cc b/base/sequence_checker_unittest.cc
index 196bb1cc7972309ddba5c85ba42d074ddb3138c3..c23335bac9b0dcb4d91ffc2c31b72b02c2b2d189 100644
--- a/base/sequence_checker_unittest.cc
+++ b/base/sequence_checker_unittest.cc
@@ -2,334 +2,277 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-#include "base/sequence_checker.h"
-
 #include <stddef.h>
 
 #include <memory>
-#include <utility>
+#include <string>
 
 #include "base/bind.h"
 #include "base/bind_helpers.h"
-#include "base/location.h"
-#include "base/logging.h"
+#include "base/callback_forward.h"
 #include "base/macros.h"
-#include "base/memory/ref_counted.h"
+#include "base/message_loop/message_loop.h"
+#include "base/sequence_checker_impl.h"
+#include "base/sequence_token.h"
 #include "base/single_thread_task_runner.h"
 #include "base/test/sequenced_worker_pool_owner.h"
-#include "base/threading/thread.h"
+#include "base/threading/simple_thread.h"
 #include "testing/gtest/include/gtest/gtest.h"
 
-// Duplicated from base/sequence_checker.h so that we can be good citizens
-// there and undef the macro.
-#if (!defined(NDEBUG) || defined(DCHECK_ALWAYS_ON))
-#define ENABLE_SEQUENCE_CHECKER 1
-#else
-#define ENABLE_SEQUENCE_CHECKER 0
-#endif
-
 namespace base {
 
 namespace {
 
-const size_t kNumWorkerThreads = 3;
+constexpr size_t kNumWorkerThreads = 3;
 
-// Simple class to exercise the basics of SequenceChecker.
-// DoStuff should verify that it's called on a valid sequenced thread.
-// SequenceCheckedObject can be destroyed on any thread (like WeakPtr).
-class SequenceCheckedObject {
+// Runs a callback on another thread.
+class RunCallbackThread : public SimpleThread {
  public:
-  SequenceCheckedObject() {}
-  ~SequenceCheckedObject() {}
-
-  // Verifies that it was called on the same thread as the constructor.
-  void DoStuff() {
-    DCHECK(sequence_checker_.CalledOnValidSequencedThread());
-  }
-
-  void DetachFromSequence() {
-    sequence_checker_.DetachFromSequence();
+  explicit RunCallbackThread(const Closure& callback)
+      : SimpleThread("RunCallbackThread"), callback_(callback) {
+    Start();
+    Join();
   }
 
  private:
-  SequenceChecker sequence_checker_;
+  // SimpleThread:
+  void Run() override { callback_.Run(); }
+
+  const Closure callback_;
 
-  DISALLOW_COPY_AND_ASSIGN(SequenceCheckedObject);
+  DISALLOW_COPY_AND_ASSIGN(RunCallbackThread);
 };
 
 class SequenceCheckerTest : public testing::Test {
- public:
-  SequenceCheckerTest() : other_thread_("sequence_checker_test_other_thread") {}
-
-  void SetUp() override {
-    other_thread_.Start();
-    ResetPool();
-  }
-
-  void TearDown() override {
-    other_thread_.Stop();
-  }
-
  protected:
-  base::Thread* other_thread() { return &other_thread_; }
+  SequenceCheckerTest() : pool_owner_(kNumWorkerThreads, "test") {}
 
-  const scoped_refptr<SequencedWorkerPool>& pool() {
-    return pool_owner_->pool();
+  void PostToSequencedWorkerPool(const Closure& callback,
+                                 const std::string& token_name) {
+    pool_owner_.pool()->PostNamedSequencedWorkerTask(token_name, FROM_HERE,
+                                                     callback);
   }
 
-  void PostDoStuffToWorkerPool(SequenceCheckedObject* sequence_checked_object,
-                               const std::string& token_name) {
-    pool()->PostNamedSequencedWorkerTask(
-        token_name,
-        FROM_HERE,
-        base::Bind(&SequenceCheckedObject::DoStuff,
-                   base::Unretained(sequence_checked_object)));
+  void FlushSequencedWorkerPoolForTesting() {
+    pool_owner_.pool()->FlushForTesting();
   }
 
-  void PostDoStuffToOtherThread(
-      SequenceCheckedObject* sequence_checked_object) {
-    other_thread()->task_runner()->PostTask(
-        FROM_HERE, base::Bind(&SequenceCheckedObject::DoStuff,
-                              base::Unretained(sequence_checked_object)));
-  }
-
-  void PostDeleteToOtherThread(
-      std::unique_ptr<SequenceCheckedObject> sequence_checked_object) {
-    other_thread()->message_loop()->task_runner()->DeleteSoon(
-        FROM_HERE, sequence_checked_object.release());
-  }
-
-  // Destroys the SequencedWorkerPool instance, blocking until it is fully shut
-  // down, and creates a new instance.
-  void ResetPool() {
-    pool_owner_.reset(new SequencedWorkerPoolOwner(kNumWorkerThreads, "test"));
-  }
-
-  void MethodOnDifferentThreadDeathTest();
-  void DetachThenCallFromDifferentThreadDeathTest();
-  void DifferentSequenceTokensDeathTest();
-  void WorkerPoolAndSimpleThreadDeathTest();
-  void TwoDifferentWorkerPoolsDeathTest();
-
  private:
   MessageLoop message_loop_;  // Needed by SequencedWorkerPool to function.
-  base::Thread other_thread_;
-  std::unique_ptr<SequencedWorkerPoolOwner> pool_owner_;
+  SequencedWorkerPoolOwner pool_owner_;
+
+  DISALLOW_COPY_AND_ASSIGN(SequenceCheckerTest);
 };
 
-TEST_F(SequenceCheckerTest, CallsAllowedOnSameThread) {
-  std::unique_ptr<SequenceCheckedObject> sequence_checked_object(
-      new SequenceCheckedObject);
+void ExpectCalledOnValidSequencedThread(SequenceCheckerImpl* sequence_checker) {
+  ASSERT_TRUE(sequence_checker);
 
-  // Verify that DoStuff doesn't assert.
-  sequence_checked_object->DoStuff();
+  // This should bind |sequence_checker| to the current sequence if it wasn't
+  // already bound to a sequence.
+  EXPECT_TRUE(sequence_checker->CalledOnValidSequencedThread());
 
-  // Verify that the destructor doesn't assert.
-  sequence_checked_object.reset();
+  // Since |sequence_checker| is now bound to the current sequence, another call
+  // to CalledOnValidSequencedThread() should return true.
+  EXPECT_TRUE(sequence_checker->CalledOnValidSequencedThread());
 }
 
-TEST_F(SequenceCheckerTest, DestructorAllowedOnDifferentThread) {
-  std::unique_ptr<SequenceCheckedObject> sequence_checked_object(
-      new SequenceCheckedObject);
-
-  // Verify the destructor doesn't assert when called on a different thread.
-  PostDeleteToOtherThread(std::move(sequence_checked_object));
-  other_thread()->Stop();
+void ExpectCalledOnValidSequencedThreadWithSequenceToken(
+    SequenceCheckerImpl* sequence_checker,
+    internal::SequenceToken sequence_token) {
+  internal::ScopedSetCurrentSequenceToken scoped_set_current_sequence_token(
+      sequence_token);
+  ExpectCalledOnValidSequencedThread(sequence_checker);
 }
 
-TEST_F(SequenceCheckerTest, DetachFromSequence) {
-  std::unique_ptr<SequenceCheckedObject> sequence_checked_object(
-      new SequenceCheckedObject);
-
-  // Verify that DoStuff doesn't assert when called on a different thread after
-  // a call to DetachFromSequence.
-  sequence_checked_object->DetachFromSequence();
-
-  PostDoStuffToOtherThread(sequence_checked_object.get());
-  other_thread()->Stop();
+void ExpectNotCalledOnValidSequencedThread(
+    SequenceCheckerImpl* sequence_checker) {
+  ASSERT_TRUE(sequence_checker);
+  EXPECT_FALSE(sequence_checker->CalledOnValidSequencedThread());
 }
 
-TEST_F(SequenceCheckerTest, SameSequenceTokenValid) {
-  std::unique_ptr<SequenceCheckedObject> sequence_checked_object(
-      new SequenceCheckedObject);
+}  // namespace
 
-  sequence_checked_object->DetachFromSequence();
-  PostDoStuffToWorkerPool(sequence_checked_object.get(), "A");
-  PostDoStuffToWorkerPool(sequence_checked_object.get(), "A");
-  PostDoStuffToWorkerPool(sequence_checked_object.get(), "A");
-  PostDoStuffToWorkerPool(sequence_checked_object.get(), "A");
-  pool()->FlushForTesting();
+TEST_F(SequenceCheckerTest, CallsAllowedOnSameThreadNoSequenceToken) {
+  SequenceCheckerImpl sequence_checker;
+  EXPECT_TRUE(sequence_checker.CalledOnValidSequencedThread());
+}
 
-  PostDeleteToOtherThread(std::move(sequence_checked_object));
-  other_thread()->Stop();
+TEST_F(SequenceCheckerTest, CallsAllowedOnSameThreadSameSequenceToken) {
+  internal::ScopedSetCurrentSequenceToken scoped_set_current_sequence_token(
+      internal::SequenceToken::Create());

[gab, 2016/07/21 21:14:24]

internal::SequenceToken::Create() will become invalid after this call and the
checks below are done on freed state, need to use a variable.

(below as well)

[fdoray, 2016/07/25 13:24:28]

ScopedSetCurrentSequenceToken copies the argument it receives, so this shouldn't
be an issue.

[gab, 2016/07/25 14:09:31]

Ah, good point :-).

+  SequenceCheckerImpl sequence_checker;
+  EXPECT_TRUE(sequence_checker.CalledOnValidSequencedThread());
 }
 
-TEST_F(SequenceCheckerTest, DetachSequenceTokenValid) {
-  std::unique_ptr<SequenceCheckedObject> sequence_checked_object(
-      new SequenceCheckedObject);
+TEST_F(SequenceCheckerTest, CallsDisallowedOnDifferentThreadsNoSequenceToken) {
+  SequenceCheckerImpl sequence_checker;
+  RunCallbackThread thread(Bind(&ExpectNotCalledOnValidSequencedThread,
+                                Unretained(&sequence_checker)));
+}
 
-  sequence_checked_object->DetachFromSequence();
-  PostDoStuffToWorkerPool(sequence_checked_object.get(), "A");
-  PostDoStuffToWorkerPool(sequence_checked_object.get(), "A");
-  pool()->FlushForTesting();
+TEST_F(SequenceCheckerTest, CallsAllowedOnDifferentThreadsSameSequenceToken) {
+  const internal::SequenceToken sequence_token(
+      internal::SequenceToken::Create());
 
-  sequence_checked_object->DetachFromSequence();
-  PostDoStuffToWorkerPool(sequence_checked_object.get(), "B");
-  PostDoStuffToWorkerPool(sequence_checked_object.get(), "B");
-  pool()->FlushForTesting();
+  internal::ScopedSetCurrentSequenceToken scoped_set_current_sequence_token(
+      sequence_token);
+  SequenceCheckerImpl sequence_checker;
 

[gab, 2016/07/21 21:14:24]

Also EXPECT_TRUE(sequence_checker.CalledOnValidSequencedThread()); here to
really drive the essence of this test.

[fdoray, 2016/07/25 13:24:28]

Done.

-  PostDeleteToOtherThread(std::move(sequence_checked_object));
-  other_thread()->Stop();
+  RunCallbackThread thread(
+      Bind(&ExpectCalledOnValidSequencedThreadWithSequenceToken,
+           Unretained(&sequence_checker), sequence_token));
 }
 
-#if GTEST_HAS_DEATH_TEST || !ENABLE_SEQUENCE_CHECKER
+TEST_F(SequenceCheckerTest, CallsDisallowedOnSameThreadDifferentSequenceToken) {
+  std::unique_ptr<SequenceCheckerImpl> sequence_checker;
 
-void SequenceCheckerTest::MethodOnDifferentThreadDeathTest() {
-  std::unique_ptr<SequenceCheckedObject> sequence_checked_object(
-      new SequenceCheckedObject);
+  {
+    internal::ScopedSetCurrentSequenceToken scoped_set_current_sequence_token(
+        internal::SequenceToken::Create());
+    sequence_checker.reset(new SequenceCheckerImpl);
+  }
 
-  // DoStuff should assert in debug builds only when called on a
-  // different thread.
-  PostDoStuffToOtherThread(sequence_checked_object.get());
-  other_thread()->Stop();
-}
+  {
+    // Different SequenceToken.
+    internal::ScopedSetCurrentSequenceToken scoped_set_current_sequence_token(
+        internal::SequenceToken::Create());
+    EXPECT_FALSE(sequence_checker->CalledOnValidSequencedThread());
+  }
 
-#if ENABLE_SEQUENCE_CHECKER
-TEST_F(SequenceCheckerTest, MethodNotAllowedOnDifferentThreadDeathTestInDebug) {
-  // The default style "fast" does not support multi-threaded tests.
-  ::testing::FLAGS_gtest_death_test_style = "threadsafe";
-  ASSERT_DEATH({
-    MethodOnDifferentThreadDeathTest();
-  }, "");
+  // No SequenceToken.
+  EXPECT_FALSE(sequence_checker->CalledOnValidSequencedThread());
 }
-#else
-TEST_F(SequenceCheckerTest, MethodAllowedOnDifferentThreadDeathTestInRelease) {
-  MethodOnDifferentThreadDeathTest();
-}
-#endif  // ENABLE_SEQUENCE_CHECKER
 
-void SequenceCheckerTest::DetachThenCallFromDifferentThreadDeathTest() {
-  std::unique_ptr<SequenceCheckedObject> sequence_checked_object(
-      new SequenceCheckedObject);
+TEST_F(SequenceCheckerTest, DetachFromSequence) {
+  std::unique_ptr<SequenceCheckerImpl> sequence_checker;
 
-  // DoStuff doesn't assert when called on a different thread
-  // after a call to DetachFromSequence.
-  sequence_checked_object->DetachFromSequence();
-  PostDoStuffToOtherThread(sequence_checked_object.get());
-  other_thread()->Stop();
+  {
+    internal::ScopedSetCurrentSequenceToken scoped_set_current_sequence_token(
+        internal::SequenceToken::Create());
+    sequence_checker.reset(new SequenceCheckerImpl);
+  }
 
-  // DoStuff should assert in debug builds only after moving to
-  // another thread.
-  sequence_checked_object->DoStuff();
-}
+  sequence_checker->DetachFromSequence();
 
-#if ENABLE_SEQUENCE_CHECKER
-TEST_F(SequenceCheckerTest, DetachFromSequenceDeathTestInDebug) {
-  // The default style "fast" does not support multi-threaded tests.
-  ::testing::FLAGS_gtest_death_test_style = "threadsafe";
-  ASSERT_DEATH({
-    DetachThenCallFromDifferentThreadDeathTest();
-  }, "");
-}
-#else
-TEST_F(SequenceCheckerTest, DetachFromThreadDeathTestInRelease) {
-  DetachThenCallFromDifferentThreadDeathTest();
+  {
+    // Verify that CalledOnValidSequencedThread() returns true when called with
+    // a different sequence token after a call to DetachFromSequence().
+    internal::ScopedSetCurrentSequenceToken scoped_set_current_sequence_token(
+        internal::SequenceToken::Create());
+    EXPECT_TRUE(sequence_checker->CalledOnValidSequencedThread());
+  }
 }
-#endif  // ENABLE_SEQUENCE_CHECKER
 
-void SequenceCheckerTest::DifferentSequenceTokensDeathTest() {
-  std::unique_ptr<SequenceCheckedObject> sequence_checked_object(
-      new SequenceCheckedObject);
+TEST_F(SequenceCheckerTest, DetachFromSequenceNoSequenceToken) {
+  SequenceCheckerImpl sequence_checker;
+  sequence_checker.DetachFromSequence();
 
-  sequence_checked_object->DetachFromSequence();
-  PostDoStuffToWorkerPool(sequence_checked_object.get(), "A");
-  PostDoStuffToWorkerPool(sequence_checked_object.get(), "A");
-  PostDoStuffToWorkerPool(sequence_checked_object.get(), "B");
-  PostDoStuffToWorkerPool(sequence_checked_object.get(), "B");
-  pool()->FlushForTesting();
+  // Verify that CalledOnValidSequencedThread() returns true when called on a
+  // different thread after a call to DetachFromSequence().
+  RunCallbackThread thread(
+      Bind(&ExpectCalledOnValidSequencedThread, Unretained(&sequence_checker)));
 
-  PostDeleteToOtherThread(std::move(sequence_checked_object));
-  other_thread()->Stop();
+  EXPECT_FALSE(sequence_checker.CalledOnValidSequencedThread());
 }
 
-#if ENABLE_SEQUENCE_CHECKER
-TEST_F(SequenceCheckerTest, DifferentSequenceTokensDeathTestInDebug) {
-  // The default style "fast" does not support multi-threaded tests.
-  ::testing::FLAGS_gtest_death_test_style = "threadsafe";
-  ASSERT_DEATH({
-    DifferentSequenceTokensDeathTest();
-  }, "");
+TEST_F(SequenceCheckerTest, SequencedWorkerPool_SameSequenceTokenValid) {
+  SequenceCheckerImpl sequence_checker;
+  sequence_checker.DetachFromSequence();
+
+  PostToSequencedWorkerPool(
+      Bind(&ExpectCalledOnValidSequencedThread, Unretained(&sequence_checker)),
+      "A");
+  PostToSequencedWorkerPool(
+      Bind(&ExpectCalledOnValidSequencedThread, Unretained(&sequence_checker)),
+      "A");
+  PostToSequencedWorkerPool(
+      Bind(&ExpectCalledOnValidSequencedThread, Unretained(&sequence_checker)),
+      "A");
+  PostToSequencedWorkerPool(
+      Bind(&ExpectCalledOnValidSequencedThread, Unretained(&sequence_checker)),
+      "A");
+  FlushSequencedWorkerPoolForTesting();
 }
-#else
-TEST_F(SequenceCheckerTest, DifferentSequenceTokensDeathTestInRelease) {
-  DifferentSequenceTokensDeathTest();
+
+TEST_F(SequenceCheckerTest, SequencedWorkerPool_DetachSequenceTokenValid) {
+  SequenceCheckerImpl sequence_checker;
+  sequence_checker.DetachFromSequence();
+
+  PostToSequencedWorkerPool(
+      Bind(&ExpectCalledOnValidSequencedThread, Unretained(&sequence_checker)),
+      "A");
+  PostToSequencedWorkerPool(
+      Bind(&ExpectCalledOnValidSequencedThread, Unretained(&sequence_checker)),
+      "A");
+  FlushSequencedWorkerPoolForTesting();
+
+  sequence_checker.DetachFromSequence();
+
+  PostToSequencedWorkerPool(
+      Bind(&ExpectCalledOnValidSequencedThread, Unretained(&sequence_checker)),
+      "B");
+  PostToSequencedWorkerPool(
+      Bind(&ExpectCalledOnValidSequencedThread, Unretained(&sequence_checker)),
+      "B");
+  FlushSequencedWorkerPoolForTesting();
 }
-#endif  // ENABLE_SEQUENCE_CHECKER
 
-void SequenceCheckerTest::WorkerPoolAndSimpleThreadDeathTest() {
-  std::unique_ptr<SequenceCheckedObject> sequence_checked_object(
-      new SequenceCheckedObject);
+TEST_F(SequenceCheckerTest,
+       SequencedWorkerPool_DifferentSequenceTokensInvalid) {
+  SequenceCheckerImpl sequence_checker;
+  sequence_checker.DetachFromSequence();
+
+  PostToSequencedWorkerPool(
+      Bind(&ExpectCalledOnValidSequencedThread, Unretained(&sequence_checker)),
+      "A");
+  PostToSequencedWorkerPool(
+      Bind(&ExpectCalledOnValidSequencedThread, Unretained(&sequence_checker)),
+      "A");
+  FlushSequencedWorkerPoolForTesting();
+
+  PostToSequencedWorkerPool(Bind(&ExpectNotCalledOnValidSequencedThread,
+                                 Unretained(&sequence_checker)),
+                            "B");
+  PostToSequencedWorkerPool(Bind(&ExpectNotCalledOnValidSequencedThread,
+                                 Unretained(&sequence_checker)),
+                            "B");
+  FlushSequencedWorkerPoolForTesting();
+}
 
-  sequence_checked_object->DetachFromSequence();
-  PostDoStuffToWorkerPool(sequence_checked_object.get(), "A");
-  PostDoStuffToWorkerPool(sequence_checked_object.get(), "A");
-  pool()->FlushForTesting();
+TEST_F(SequenceCheckerTest,
+       SequencedWorkerPool_WorkerPoolAndSimpleThreadInvalid) {
+  SequenceCheckerImpl sequence_checker;
+  sequence_checker.DetachFromSequence();
 
-  PostDoStuffToOtherThread(sequence_checked_object.get());
-  other_thread()->Stop();
-}
+  PostToSequencedWorkerPool(
+      Bind(&ExpectCalledOnValidSequencedThread, Unretained(&sequence_checker)),
+      "A");
+  PostToSequencedWorkerPool(
+      Bind(&ExpectCalledOnValidSequencedThread, Unretained(&sequence_checker)),
+      "A");
+  FlushSequencedWorkerPoolForTesting();
 
-#if ENABLE_SEQUENCE_CHECKER
-TEST_F(SequenceCheckerTest, WorkerPoolAndSimpleThreadDeathTestInDebug) {
-  // The default style "fast" does not support multi-threaded tests.
-  ::testing::FLAGS_gtest_death_test_style = "threadsafe";
-  ASSERT_DEATH({
-    WorkerPoolAndSimpleThreadDeathTest();
-  }, "");
+  EXPECT_FALSE(sequence_checker.CalledOnValidSequencedThread());
 }
-#else
-TEST_F(SequenceCheckerTest, WorkerPoolAndSimpleThreadDeathTestInRelease) {
-  WorkerPoolAndSimpleThreadDeathTest();
-}
-#endif  // ENABLE_SEQUENCE_CHECKER
 
-void SequenceCheckerTest::TwoDifferentWorkerPoolsDeathTest() {
-  std::unique_ptr<SequenceCheckedObject> sequence_checked_object(
-      new SequenceCheckedObject);
+TEST_F(SequenceCheckerTest,
+       SequencedWorkerPool_TwoDifferentWorkerPoolsInvalid) {
+  SequenceCheckerImpl sequence_checker;
+  sequence_checker.DetachFromSequence();
 
-  sequence_checked_object->DetachFromSequence();
-  PostDoStuffToWorkerPool(sequence_checked_object.get(), "A");
-  PostDoStuffToWorkerPool(sequence_checked_object.get(), "A");
-  pool()->FlushForTesting();
+  PostToSequencedWorkerPool(
+      Bind(&ExpectCalledOnValidSequencedThread, Unretained(&sequence_checker)),
+      "A");
+  PostToSequencedWorkerPool(
+      Bind(&ExpectCalledOnValidSequencedThread, Unretained(&sequence_checker)),
+      "A");
+  FlushSequencedWorkerPoolForTesting();
 
   SequencedWorkerPoolOwner second_pool_owner(kNumWorkerThreads, "test2");
   second_pool_owner.pool()->PostNamedSequencedWorkerTask(
-      "A",
-      FROM_HERE,
-      base::Bind(&SequenceCheckedObject::DoStuff,
-                 base::Unretained(sequence_checked_object.get())));
+      "A", FROM_HERE, base::Bind(&ExpectNotCalledOnValidSequencedThread,
+                                 base::Unretained(&sequence_checker)));
   second_pool_owner.pool()->FlushForTesting();
 }
 
-#if ENABLE_SEQUENCE_CHECKER
-TEST_F(SequenceCheckerTest, TwoDifferentWorkerPoolsDeathTestInDebug) {
-  // The default style "fast" does not support multi-threaded tests.
-  ::testing::FLAGS_gtest_death_test_style = "threadsafe";
-  ASSERT_DEATH({
-    TwoDifferentWorkerPoolsDeathTest();
-  }, "");
-}
-#else
-TEST_F(SequenceCheckerTest, TwoDifferentWorkerPoolsDeathTestInRelease) {
-  TwoDifferentWorkerPoolsDeathTest();
-}
-#endif  // ENABLE_SEQUENCE_CHECKER
-
-#endif  // GTEST_HAS_DEATH_TEST || !ENABLE_SEQUENCE_CHECKER
-
-}  // namespace
-
 }  // namespace base
-
-// Just in case we ever get lumped together with other compilation units.
-#undef ENABLE_SEQUENCE_CHECKER