IndexedDB: Fix cursor prefetching edge cases

content/browser/indexed_db/indexed_db_cursor.cc

Index: content/browser/indexed_db/indexed_db_cursor.cc
diff --git a/content/browser/indexed_db/indexed_db_cursor.cc b/content/browser/indexed_db/indexed_db_cursor.cc
index 6d7bc1c2d5914df345f3d0b1f652f12e57f0b036..fe4d32e9ffa5ea3fd9063129fbad4a1689fe8f82 100644
--- a/content/browser/indexed_db/indexed_db_cursor.cc
+++ b/content/browser/indexed_db/indexed_db_cursor.cc
@@ -108,8 +108,7 @@ void IndexedDBCursor::CursorPrefetchIterationOperation(
   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;
 
@@ -119,6 +118,11 @@ void IndexedDBCursor::CursorPrefetchIterationOperation(
       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());
 
@@ -152,15 +156,29 @@ void IndexedDBCursor::CursorPrefetchIterationOperation(
       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);
     }