Framebuffer clear() needs to consider the situation some draw buffers are disabled.

gpu/command_buffer/service/gles2_cmd_decoder.cc

 // Copyright (c) 2012 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 "gpu/command_buffer/service/gles2_cmd_decoder.h"
 
 #include <stdio.h>
 
 #include <algorithm>
 #include <list>
@@ skipping 3055 matching lines @@
   if (!framebuffer) {
     if (backbuffer_needs_clear_bits_) {
       glClearColor(0, 0, 0, (GLES2Util::GetChannelsForFormat(
           offscreen_target_color_format_) & 0x0008) != 0 ? 0 : 1);
       state_.SetDeviceColorMask(GL_TRUE, GL_TRUE, GL_TRUE, GL_TRUE);
       glClearStencil(0);
       glStencilMask(-1);
       glClearDepth(1.0f);
       state_.SetDeviceDepthMask(GL_TRUE);
       state_.SetDeviceCapabilityState(GL_SCISSOR_TEST, false);
+      bool reset_draw_buffer = false;
+      if ((backbuffer_needs_clear_bits_ | GL_COLOR_BUFFER_BIT) != 0 &&
+          group_->draw_buffer() == GL_NONE) {
+        reset_draw_buffer = true;
+        GLenum buf = GL_BACK;
+        if (GetBackbufferServiceId() != 0)  // emulated backbuffer
+          buf = GL_COLOR_ATTACHMENT0;
+        glDrawBuffersARB(1, &buf);
+      }
       glClear(backbuffer_needs_clear_bits_);
+      if (reset_draw_buffer) {
+        GLenum buf = GL_NONE;
+        glDrawBuffersARB(1, &buf);
+      }
       backbuffer_needs_clear_bits_ = 0;
       RestoreClearState();
     }
     return true;
   }
 
   if (framebuffer_manager()->IsComplete(framebuffer)) {
     return true;
   }
 
@@ skipping 1925 matching lines @@
     GLenum target, Framebuffer* framebuffer) {
   if (target == GL_READ_FRAMEBUFFER_EXT) {
     // bind this to the DRAW point, clear then bind back to READ
     // TODO(gman): I don't think there is any guarantee that an FBO that
     //   is complete on the READ attachment will be complete as a DRAW
     //   attachment.
     glBindFramebufferEXT(GL_READ_FRAMEBUFFER_EXT, 0);
     glBindFramebufferEXT(GL_DRAW_FRAMEBUFFER_EXT, framebuffer->service_id());
   }
   GLbitfield clear_bits = 0;
-  if (framebuffer->HasUnclearedAttachment(GL_COLOR_ATTACHMENT0)) {
+  if (framebuffer->HasUnclearedColorAttachments()) {
     glClearColor(
         0.0f, 0.0f, 0.0f,
         (GLES2Util::GetChannelsForFormat(
              framebuffer->GetColorAttachmentFormat()) & 0x0008) != 0 ? 0.0f :
                                                                        1.0f);
     state_.SetDeviceColorMask(GL_TRUE, GL_TRUE, GL_TRUE, GL_TRUE);
     clear_bits |= GL_COLOR_BUFFER_BIT;
+    framebuffer->PrepareDrawBuffersForClear();
   }
 
   if (framebuffer->HasUnclearedAttachment(GL_STENCIL_ATTACHMENT) ||
       framebuffer->HasUnclearedAttachment(GL_DEPTH_STENCIL_ATTACHMENT)) {
     glClearStencil(0);
     glStencilMask(-1);
     clear_bits |= GL_STENCIL_BUFFER_BIT;
   }
 
   if (framebuffer->HasUnclearedAttachment(GL_DEPTH_ATTACHMENT) ||
       framebuffer->HasUnclearedAttachment(GL_DEPTH_STENCIL_ATTACHMENT)) {
     glClearDepth(1.0f);
     state_.SetDeviceDepthMask(GL_TRUE);
     clear_bits |= GL_DEPTH_BUFFER_BIT;
   }
 
   state_.SetDeviceCapabilityState(GL_SCISSOR_TEST, false);
   glClear(clear_bits);
 
+  if ((clear_bits | GL_COLOR_BUFFER_BIT) != 0)
+    framebuffer->RestoreDrawBuffersAfterClear();
+
   framebuffer_manager()->MarkAttachmentsAsCleared(
       framebuffer, renderbuffer_manager(), texture_manager());
 
   RestoreClearState();
 
   if (target == GL_READ_FRAMEBUFFER_EXT) {
     glBindFramebufferEXT(GL_READ_FRAMEBUFFER_EXT, framebuffer->service_id());
     Framebuffer* draw_framebuffer =
         GetFramebufferInfoForTarget(GL_DRAW_FRAMEBUFFER_EXT);
     GLuint service_id = draw_framebuffer ? draw_framebuffer->service_id() :
@@ skipping 5713 matching lines @@
   }
 }
 
 // Include the auto-generated part of this file. We split this because it means
 // we can easily edit the non-auto generated parts right here in this file
 // instead of having to edit some template or the code generator.
 #include "gpu/command_buffer/service/gles2_cmd_decoder_autogen.h"
 
 }  // namespace gles2
 }  // namespace gpu