Fix Bug in RingBuffer

gpu/command_buffer/client/ring_buffer.cc

 // Copyright (c) 2010 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.
 
 // This file contains the implementation of the RingBuffer class.
 
 #include "../client/ring_buffer.h"
 #include <algorithm>
 #include "../client/cmd_buffer_helper.h"
 
@@ skipping 22 matching lines @@
   helper_->WaitForToken(block.token);
   in_use_offset_ += block.size;
   if (in_use_offset_ == size_) {
     in_use_offset_ = 0;
   }
   // If they match then the entire buffer is free.
   if (in_use_offset_ == free_offset_) {
     in_use_offset_ = 0;
     free_offset_ = 0;
   }
-  blocks_.pop_back();
+  blocks_.pop_front();
 }
 
 RingBuffer::Offset RingBuffer::Alloc(unsigned int size) {
   DCHECK_LE(size, size_) << "attempt to allocate more than maximum memory";
+  DCHECK(blocks_.empty() || blocks_.back().valid)
+      << "Attempt to alloc another block before freeing the previous.";
   // Similarly to malloc, an allocation of 0 allocates at least 1 byte, to
   // return different pointers every time.
   if (size == 0) size = 1;
 
   // Wait until there is enough room.
   while (size > GetLargestFreeSizeNoWaiting()) {
     FreeOldestBlock();
   }
 
   Offset offset = free_offset_;
@@ skipping 17 matching lines @@
       DCHECK(!block.valid) << "block that corresponds to offset already freed";
       block.token = token;
       block.valid = true;
       return;
     }
   }
   NOTREACHED() << "attempt to free non-existant block";
 }
 
 unsigned int RingBuffer::GetLargestFreeSizeNoWaiting() {
+  // TODO(gman): Should check what the current token is and free up to that
+  //    point.
   if (free_offset_ == in_use_offset_) {
     if (blocks_.empty()) {
       // The entire buffer is free.
       DCHECK_EQ(free_offset_, 0u);
       return size_;
     } else {
       // The entire buffer is in use.
       return 0;
     }
   } else if (free_offset_ > in_use_offset_) {
     // It's free from free_offset_ to size_
     return size_ - free_offset_;
   } else {
     // It's free from free_offset_ -> in_use_offset_;
     return in_use_offset_ - free_offset_;
   }
 }
 
 }  // namespace gpu