Fix threading issues in aw form database

android_webview/browser/aw_form_database_service.cc

Index: android_webview/browser/aw_form_database_service.cc
diff --git a/android_webview/browser/aw_form_database_service.cc b/android_webview/browser/aw_form_database_service.cc
index 2b168deba9ebd9aa0fd9dbc7d9d3b679b7cc33df..c498820a28b753a932cb74eb5aa281153b9fad75 100644
--- a/android_webview/browser/aw_form_database_service.cc
+++ b/android_webview/browser/aw_form_database_service.cc
@@ -66,6 +66,16 @@ AwFormDatabaseService::get_autofill_webdata_service() {
 }
 
 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);
@@ -73,15 +83,19 @@ void AwFormDatabaseService::ClearFormData() {
 }
 
 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);
 }