Fix threading issues in aw form database

android_webview/browser/aw_form_database_service.cc

 // Copyright (c) 2013 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "android_webview/browser/aw_form_database_service.h"
 #include "base/logging.h"
 #include "components/autofill/core/browser/webdata/autofill_table.h"
 #include "components/webdata/common/webdata_constants.h"
 #include "content/public/browser/browser_thread.h"
 #include "ui/base/l10n/l10n_util_android.h"
@@ skipping 48 matching lines @@
     pending_query_handle_ = 0;
   }
 }
 
 scoped_refptr<autofill::AutofillWebDataService>
 AwFormDatabaseService::get_autofill_webdata_service() {
   return autofill_data_;
 }
 
 void AwFormDatabaseService::ClearFormData() {
+  base::AutoLock lock(lock_);

[joth, 2013/09/05 00:59:10]

I don't understand what problem this lock is solving

+  // TODO(sgurun) investigate if it is better to do on DB thread
+  BrowserThread::PostTask(
+      BrowserThread::UI,
+      FROM_HERE,
+      base::Bind(&AwFormDatabaseService::ClearFormDataLocked,
+                 base::Unretained(this)));
+}
+
+void AwFormDatabaseService::ClearFormDataLocked() {
   base::Time begin;
   base::Time end = base::Time::Max();
   autofill_data_->RemoveFormElementsAddedBetween(begin, end);
   autofill_data_->RemoveAutofillDataModifiedBetween(begin, end);
 }
 
 bool AwFormDatabaseService::HasFormData() {
-  BrowserThread::PostTask(BrowserThread::DB, FROM_HERE,
-      base::Bind(&AwFormDatabaseService::HasFormDataImpl,
+  base::AutoLock lock(lock_);
+  // TODO(sgurun) investigate if it is better to do on DB thread

[joth, 2013/09/05 00:59:10]

I can answer this: it's *definitely* better done on DB thread. 
1/ in general, bouncing a synchronous call to UI thread risks a deadlock if the
caller is (naughtily) blocking UI thread waiting for a result from the
background thread using this API
2/ as written, the code here will 100% deadlock if called from UI thread.

+  BrowserThread::PostTask(
+      BrowserThread::UI,
+      FROM_HERE,
+      base::Bind(&AwFormDatabaseService::HasFormDataLocked,
                  base::Unretained(this)));
+  // TODO(sgurun) Could be unnecessarily blocking UI thread
   completion_.Wait();
   return has_form_data_;
 }
 
-void AwFormDatabaseService::HasFormDataImpl() {
-  DCHECK(BrowserThread::CurrentlyOn(BrowserThread::DB));
+void AwFormDatabaseService::HasFormDataLocked() {
   pending_query_handle_ = autofill_data_->HasFormElements(this);
 }
 
 
 void AwFormDatabaseService::OnWebDataServiceRequestDone(
     WebDataServiceBase::Handle h,
     const WDTypedResult* result) {
 
   DCHECK(BrowserThread::CurrentlyOn(BrowserThread::DB));
   DCHECK_EQ(pending_query_handle_, h);
   pending_query_handle_ = 0;
   has_form_data_ = false;
 
   if (result) {
     DCHECK_EQ(AUTOFILL_VALUE_RESULT, result->GetType());
     const WDResult<bool>* autofill_result =
         static_cast<const WDResult<bool>*>(result);
     has_form_data_ = autofill_result->GetValue();
   }
   completion_.Signal();
 }
 
 }  // namespace android_webview