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..0853b810f285e20279f88a58764a037a4db1d903 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 +118,12 @@ void IndexedDBCursor::CursorPrefetchIterationOperation(
       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());
 
@@ -152,7 +157,8 @@ 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");
   cursor_.swap(saved_cursor_);
   saved_cursor_.reset();
@@ -160,7 +166,9 @@ void IndexedDBCursor::PrefetchReset(int used_prefetches, int) {
   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);
     }