Example of inappropriate cross-thread weak-ptr use.

sql/connection_unittest.cc

Index: sql/connection_unittest.cc
diff --git a/sql/connection_unittest.cc b/sql/connection_unittest.cc
index 1c08881227d5304fc2a74a4e13ec788289809983..ef044ac8d18148022f389029e4bbeb908d33b7bb 100644
--- a/sql/connection_unittest.cc
+++ b/sql/connection_unittest.cc
@@ -12,7 +12,10 @@
 #include "base/logging.h"
 #include "base/macros.h"
 #include "base/metrics/statistics_recorder.h"
+#include "base/single_thread_task_runner.h"
 #include "base/test/histogram_tester.h"
+#include "base/thread_task_runner_handle.h"
+#include "base/threading/thread.h"
 #include "base/trace_event/process_memory_dump.h"
 #include "sql/connection.h"
 #include "sql/connection_memory_dump_provider.h"
@@ -1488,4 +1491,71 @@ TEST_F(SQLConnectionTest, GetAppropriateMmapSize) {
 }
 #endif
 
+namespace {
+
+class Demo {
+ public:
+  explicit Demo(scoped_refptr<base::SingleThreadTaskRunner> task_runner)
+      : background_task_runner_(task_runner),
+        weak_ptr_factory_(this) {
+  }
+  ~Demo() {
+  }
+
+  void Ping(
+      scoped_refptr<base::SingleThreadTaskRunner> callback_runner,
+      const base::Closure& callback) {
+    LOG(ERROR) << "Received ping, sleeping";
+    base::PlatformThread::Sleep(base::TimeDelta::FromSeconds(1));
+    LOG(ERROR) << "Sending pong";
+    callback_runner->PostTask(FROM_HERE, callback);
+  }
+
+  void SendPing() {
+    // This use of weak pointer is valid, because it will be
+    // dereferenced when the task is posted back on the current
+    // thread.
+    base::Closure callback(base::Bind(&Demo::Pong,
+                                      weak_ptr_factory_.GetWeakPtr()));
+
+    // This use of weak pointer is not valid, because it will be
+    // dereferenced when the task is posted on the background thread.
+    // It fails the sequence_checker_.CalledOnValidSequencedThread()
+    // DCHECK in WeakReference::Flag::IsValid().
+    background_task_runner_->PostTask(
+        FROM_HERE,
+        base::Bind(&Demo::Ping, weak_ptr_factory_.GetWeakPtr(),
+                   base::ThreadTaskRunnerHandle::Get(), callback));
+  }
+
+  void Pong() {
+    LOG(ERROR) << "Received pong, sleeping";
+    base::PlatformThread::Sleep(base::TimeDelta::FromSeconds(1));
+    LOG(ERROR) << "Sending ping";
+    SendPing();
+  }
+
+  void Start() {
+    LOG(ERROR) << "Sending initial ping";
+    SendPing();
+  }
+
+ private:
+  scoped_refptr<base::SingleThreadTaskRunner> background_task_runner_;
+
+  base::WeakPtrFactory<Demo> weak_ptr_factory_;
+};
+
+}  // namespace
+
+TEST_F(SQLConnectionTest, WeakDemo) {
+  base::MessageLoop message_loop;
+
+  base::Thread background_thread("demo");
+  ASSERT_TRUE(background_thread.Start());
+
+  Demo d(background_thread.task_runner());
+  d.Start();
+}
+
 }  // namespace sql