From 335a0af676b942903fbabcc1cee75775998a0e4e Mon Sep 17 00:00:00 2001
From: Jeff Gilbert <jgilbert@mozilla.com>
Date: Thu, 2 Jul 2020 01:45:36 +0000
Subject: [PATCH] Bug 1647491 - Hold ref to previous webgl frontbuffer while
 in-flight. r=lsalzman

Differential Revision: https://phabricator.services.mozilla.com/D81921
---
 gfx/gl/GLScreenBuffer.cpp | 15 ++++++++++-----
 gfx/gl/GLScreenBuffer.h   |  9 +++++----
 2 files changed, 15 insertions(+), 9 deletions(-)

diff --git a/gfx/gl/GLScreenBuffer.cpp b/gfx/gl/GLScreenBuffer.cpp
index dc468baa7a44..0398dd7dc6a2 100644
--- a/gfx/gl/GLScreenBuffer.cpp
+++ b/gfx/gl/GLScreenBuffer.cpp
@@ -48,11 +48,15 @@ UniquePtr<SwapChainPresenter> SwapChain::Acquire(const gfx::IntSize& size) {
   }
 
   auto ret = MakeUnique<SwapChainPresenter>(*this);
-  ret->SwapBackBuffer(std::move(surf));
+  const auto old = ret->SwapBackBuffer(surf);
+  MOZ_ALWAYS_TRUE(!old);
   return ret;
 }
 
-void SwapChain::ClearPool() { mPool = {}; }
+void SwapChain::ClearPool() {
+  mPool = {};
+  mPrevFrontBuffer = nullptr;
+}
 
 // -
 
@@ -69,7 +73,8 @@ SwapChainPresenter::~SwapChainPresenter() {
 
   auto newFront = SwapBackBuffer(nullptr);
   if (newFront) {
-    mSwapChain->mFrontBuffer = std::move(newFront);
+    mSwapChain->mPrevFrontBuffer = mSwapChain->mFrontBuffer;
+    mSwapChain->mFrontBuffer = newFront;
   }
 }
 
@@ -80,8 +85,8 @@ std::shared_ptr<SharedSurface> SwapChainPresenter::SwapBackBuffer(
     mBackBuffer->ProducerRelease();
     mBackBuffer->Commit();
   }
-  auto old = std::move(mBackBuffer);
-  mBackBuffer = std::move(back);
+  auto old = mBackBuffer;
+  mBackBuffer = back;
   if (mBackBuffer) {
     mBackBuffer->WaitForBufferOwnership();
     mBackBuffer->ProducerAcquire();
diff --git a/gfx/gl/GLScreenBuffer.h b/gfx/gl/GLScreenBuffer.h
index ea67518ab66e..916e1898e70a 100644
--- a/gfx/gl/GLScreenBuffer.h
+++ b/gfx/gl/GLScreenBuffer.h
@@ -56,12 +56,13 @@ class SwapChain final {
  public:
   UniquePtr<SurfaceFactory> mFactory;
 
- private:
-  std::shared_ptr<SharedSurface> mFrontBuffer;
-  SwapChainPresenter* mPresenter = nullptr;
-
  private:
   std::queue<std::shared_ptr<SharedSurface>> mPool;
+  std::shared_ptr<SharedSurface> mFrontBuffer;
+ public:
+  std::shared_ptr<SharedSurface> mPrevFrontBuffer; // Hold this ref while it's in-flight.
+ private:
+  SwapChainPresenter* mPresenter = nullptr;
 
  public:
   SwapChain();