IndexedDB: Fix cursor prefetching edge cases

content/browser/indexed_db/indexed_db_cursor.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 "content/browser/indexed_db/indexed_db_cursor.h"
 
 #include "base/bind.h"
 #include "base/logging.h"
 #include "content/browser/indexed_db/indexed_db_callbacks.h"
 #include "content/browser/indexed_db/indexed_db_database_error.h"
@@ skipping 90 matching lines @@
 void IndexedDBCursor::CursorPrefetchIterationOperation(
     int number_to_fetch,
     scoped_refptr<IndexedDBCallbacks> callbacks,
     IndexedDBTransaction* /*transaction*/) {
   IDB_TRACE("IndexedDBCursor::CursorPrefetchIterationOperation");
 
   std::vector<IndexedDBKey> found_keys;
   std::vector<IndexedDBKey> found_primary_keys;
   std::vector<std::string> found_values;
 
-  if (cursor_)
-    saved_cursor_.reset(cursor_->Clone());
+  saved_cursor_.reset();
   const size_t max_size_estimate = 10 * 1024 * 1024;
   size_t size_estimate = 0;
 
   for (int i = 0; i < number_to_fetch; ++i) {
     if (!cursor_ || !cursor_->Continue()) {
       cursor_.reset();
       break;
     }
 
+    if (i == 0) {
+      // First prefetched result is always used and can't be reset.
+      saved_cursor_.reset(cursor_->Clone());
+    }
+
     found_keys.push_back(cursor_->key());
     found_primary_keys.push_back(cursor_->primary_key());
 
     switch (cursor_type_) {
       case indexed_db::CURSOR_KEY_ONLY:
         found_values.push_back(std::string());
         break;
       case indexed_db::CURSOR_KEY_AND_VALUE: {
         std::string value;
         value.swap(*cursor_->value());
@@ skipping 13 matching lines @@
 
   if (!found_keys.size()) {
     callbacks->OnSuccess(static_cast<std::string*>(NULL));
     return;
   }
 
   callbacks->OnSuccessWithPrefetch(
       found_keys, found_primary_keys, found_values);
 }
 
-void IndexedDBCursor::PrefetchReset(int used_prefetches, int) {
+void IndexedDBCursor::PrefetchReset(int used_prefetches,
+                                    int /* unused_prefetches */) {
   IDB_TRACE("IndexedDBCursor::PrefetchReset");
+  transaction_->ScheduleTask(

[jsbell, 2014/01/03 23:05:11]

I haven't convinced myself one way or another whether resets should be tasks or
not.

[jsbell, 2014/01/06 17:51:55]

After further reflection, this shouldn't be necessary. The prefetch mechanism is
predicated on there not being any other requests outstanding while the prefetch
cache is being used. So resetting the cache synchronously is fine.

+      task_type_,
+      base::Bind(&IndexedDBCursor::CursorPrefetchResetOperation,
+                 this,
+                 used_prefetches));
+}
+
+void IndexedDBCursor::CursorPrefetchResetOperation(
+    int used_prefetches,
+    IndexedDBTransaction* /*transaction*/) {
+  IDB_TRACE("IndexedDBCursor::CursorPrefetchIterationOperation");
+
   cursor_.swap(saved_cursor_);
   saved_cursor_.reset();
 
   if (closed_)
     return;
   if (cursor_) {
-    for (int i = 0; i < used_prefetches; ++i) {
+    DCHECK_GT(used_prefetches, 0);
+    for (int i = 0; i < used_prefetches - 1; ++i) {
       bool ok = cursor_->Continue();
       DCHECK(ok);
     }
   }
 }
 
 void IndexedDBCursor::Close() {
   IDB_TRACE("IndexedDBCursor::Close");
   closed_ = true;
   cursor_.reset();
   saved_cursor_.reset();
 }
 
 }  // namespace content