Attaching an image to many color attachment points is unsupported

gpu/command_buffer/service/framebuffer_manager.cc

Index: gpu/command_buffer/service/framebuffer_manager.cc
diff --git a/gpu/command_buffer/service/framebuffer_manager.cc b/gpu/command_buffer/service/framebuffer_manager.cc
index cec5b104b7ab678b71a164d4da999293e99df8a8..b870f0a8b029a3d601a05f4d444905f72386b792 100644
--- a/gpu/command_buffer/service/framebuffer_manager.cc
+++ b/gpu/command_buffer/service/framebuffer_manager.cc
@@ -89,6 +89,15 @@ class RenderbufferAttachment
     return renderbuffer_.get() == renderbuffer;
   }
 
+  bool IsSameAttachment(const Attachment* attachment) const override {
+    if (attachment->IsRenderbufferAttachment()) {
+      const RenderbufferAttachment* other =
+          reinterpret_cast<const RenderbufferAttachment*>(attachment);
+      return IsRenderbuffer(other->renderbuffer());
+    }
+    return false;
+  }
+
   bool Is3D() const override { return false; }
 
   bool CanRenderTo(const FeatureInfo*) const override { return true; }
@@ -220,6 +229,18 @@ class TextureAttachment
     return texture == texture_ref_.get();
   }
 
+  bool IsSameAttachment(const Attachment* attachment) const override {
+    if (attachment->IsTextureAttachment()) {
+      const TextureAttachment* other =
+          reinterpret_cast<const TextureAttachment*>(attachment);
+      return IsTexture(other->texture()) &&
+          layer_ == other->layer() &&
+          target_ == other->target() &&
+          level_ == other->level();
+    }
+    return false;
+  }
+
   bool IsRenderbuffer(Renderbuffer* /* renderbuffer */) const override {
     return false;
   }
@@ -711,6 +732,21 @@ GLenum Framebuffer::IsPossiblyComplete(const FeatureInfo* feature_info) const {
     if (!attachment->CanRenderTo(feature_info)) {
       return GL_FRAMEBUFFER_INCOMPLETE_ATTACHMENT;
     }
+
+    // In WebGL 2.0, attaching an image to more than one color attachment point
+    // should return FRAMEBUFFER_UNSUPPORTED.
+    if (feature_info->context_type() == CONTEXT_TYPE_WEBGL2) {

[qiankun, 2016/07/20 08:53:33]

Is it right only for webgl 2 context? I think gles should not apply this
restriction.

[Zhenyao Mo, 2016/07/20 13:55:44]

We need to apply it to ES3 contexts as well.  If they are on top of D3D9, this
limit will apply.

You should remove the if condition here, since this should apply in ES2 / WEBGL1
for EXT_draw_buffers extension cases - we will need to update the WebGL
extension spec also.

[qiankun, 2016/07/20 15:03:26]

I see. You told me to update extension case before. I will do it tomorrow.
So, we still need to check if EXT_draw_buffers extension is available for ES2? 
ES2 doesn't have this limitation. Right?

[Zhenyao Mo, 2016/07/20 16:12:48]

On the command buffer side, we don't need to check for the extension.  If it's
not there, then there will be no more than one color attachment, and this check
should just pass. This is not on a perf critical path, so this is OK.

+      if (it->first >= GL_COLOR_ATTACHMENT0 &&
+          it->first < GL_COLOR_ATTACHMENT0 + manager_->max_color_attachments_) {
+        for (GLenum i = it->first + 1;
+             i < GL_COLOR_ATTACHMENT0 + manager_->max_color_attachments_; i++) {
+          const Attachment* other = GetAttachment(i);
+          if (other && attachment->IsSameAttachment(other)) {
+            return GL_FRAMEBUFFER_UNSUPPORTED;
+          }
+        }
+      }
+    }
   }
 
   // This does not mean the framebuffer is actually complete. It just means our