DatabaseBackend should not post a task to an already terminated database thread

Source/modules/webdatabase/DatabaseContext.cpp

 /*
  * Copyright (C) 2008 Apple Inc. All Rights Reserved.
  * Copyright (C) 2011 Google, Inc. All Rights Reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions
  * are met:
  * 1. Redistributions of source code must retain the above copyright
  *    notice, this list of conditions and the following disclaimer.
  * 2. Redistributions in binary form must reproduce the above copyright
@@ skipping 147 matching lines @@
 
         // Create the database thread on first request - but not if at least one database was already opened,
         // because in that case we already had a database thread and terminated it and should not create another.
         m_databaseThread = DatabaseThread::create();
         m_databaseThread->start();
     }
 
     return m_databaseThread.get();
 }
 
+bool DatabaseContext::databaseThreadAvailable()
+{
+    return databaseThread() && !m_hasRequestedTermination;
+}
+
 void DatabaseContext::stopDatabases()
 {
     // Though we initiate termination of the DatabaseThread here in
     // stopDatabases(), we can't clear the m_databaseThread ref till we get to
     // the destructor. This is because the Databases that are managed by
     // DatabaseThread still rely on this ref between the context and the thread
     // to execute the task for closing the database. By the time we get to the
     // destructor, we're guaranteed that the databases are destructed (which is
     // why our ref count is 0 then and we're destructing). Then, the
     // m_databaseThread RefPtr destructor will deref and delete the
     // DatabaseThread.
 
-    if (m_databaseThread && !m_hasRequestedTermination) {
+    if (databaseThreadAvailable()) {
         TaskSynchronizer sync;
         m_databaseThread->requestTermination(&sync);
         m_hasRequestedTermination = true;
         sync.waitForTaskCompletion();
     }
 }
 
 bool DatabaseContext::allowDatabaseAccess() const
 {
     return toDocument(executionContext())->isActive();
 }
 
 SecurityOrigin* DatabaseContext::securityOrigin() const
 {
     return executionContext()->securityOrigin();
 }
 
 bool DatabaseContext::isContextThread() const
 {
     return executionContext()->isContextThread();
 }
 
 } // namespace blink