Oilpan: Prepare to move AbstractSQLTransactionBackend, SQLTransactionBackend, and SQLTransactionCoo…

Source/modules/webdatabase/SQLTransactionBackend.h

Index: Source/modules/webdatabase/SQLTransactionBackend.h
diff --git a/Source/modules/webdatabase/SQLTransactionBackend.h b/Source/modules/webdatabase/SQLTransactionBackend.h
index 33c7d3b0aba1738b5194d4d26397d555df78d7b0..54ba940d569dafe95b0a47cf144912fb4192238c 100644
--- a/Source/modules/webdatabase/SQLTransactionBackend.h
+++ b/Source/modules/webdatabase/SQLTransactionBackend.h
@@ -57,12 +57,13 @@ public:
     virtual void handleCommitFailedAfterPostflight(SQLTransactionBackend*) = 0;
 };
 
-class SQLTransactionBackend FINAL : public SQLTransactionStateMachine<SQLTransactionBackend>, public AbstractSQLTransactionBackend {
+class SQLTransactionBackend FINAL : public AbstractSQLTransactionBackend, public SQLTransactionStateMachine<SQLTransactionBackend> {
 public:
-    static PassRefPtr<SQLTransactionBackend> create(DatabaseBackend*,
+    static PassRefPtrWillBeRawPtr<SQLTransactionBackend> create(DatabaseBackend*,
         PassRefPtrWillBeRawPtr<AbstractSQLTransaction>, PassRefPtr<SQLTransactionWrapper>, bool readOnly);
 
     virtual ~SQLTransactionBackend();
+    virtual void trace(Visitor*) OVERRIDE;
 
     void lockAcquired();
     void performNextStep();
@@ -108,10 +109,10 @@ private:
 
     void getNextStatement();
 
-    RefPtrWillBeCrossThreadPersistent<AbstractSQLTransaction> m_frontend; // Has a reference cycle, and will break in doCleanup().
+    RefPtrWillBeMember<AbstractSQLTransaction> m_frontend; // Has a reference cycle, and will break in doCleanup().

[haraken, 2014/03/19 04:59:20]

Who guarantees that the thread that created the SQLTransaction is alive while
the database thread is holding the SQLTransaction?

I think it's safe to store "an on-heap object created from one thread" into "a
member of another thread" only if it's guaranteed that the creation thread
outlives the thread that holds the member. Is there any way to assert the
guarantee?

[tkent, 2014/03/19 05:15:13]

It's guaranteed.  A main/worker thread creates a dedicated database thread, and
it waits for  the database thread termination at [1].

[1]
https://code.google.com/p/chromium/codesearch#chromium/src/third_party/WebKit...

     RefPtr<SQLStatementBackend> m_currentStatementBackend;
 
-    RefPtrWillBeCrossThreadPersistent<DatabaseBackend> m_database;
+    RefPtrWillBeMember<DatabaseBackend> m_database;
     RefPtr<SQLTransactionWrapper> m_wrapper;
     RefPtr<SQLError> m_transactionError;