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, so that's the position
+      // a cursor should be reset to if the prefetch is invalidated.
+      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");
   cursor_.swap(saved_cursor_);
   saved_cursor_.reset();
 
   if (closed_)
     return;
   if (cursor_) {
-    for (int i = 0; i < used_prefetches; ++i) {
+    // First prefetched result is always used.
+    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