WebGL 2: validate read buffer attachment when reading from FBO

Source/core/html/canvas/WebGLRenderingContextBase.cpp

Index: Source/core/html/canvas/WebGLRenderingContextBase.cpp
diff --git a/Source/core/html/canvas/WebGLRenderingContextBase.cpp b/Source/core/html/canvas/WebGLRenderingContextBase.cpp
index 1f1288f7f0fcd95bffbb33106e102646e8991c1f..45f3cef0d9ceead3578ec91c3814a2583e1d1b0e 100644
--- a/Source/core/html/canvas/WebGLRenderingContextBase.cpp
+++ b/Source/core/html/canvas/WebGLRenderingContextBase.cpp
@@ -724,6 +724,9 @@ void WebGLRenderingContextBase::initializeNewContext()
 
     m_backDrawBuffer = GL_BACK;
 
+    // set the default read color buffer for FBO
+    m_readbufferOfFBO = GL_COLOR_ATTACHMENT0;
+
     if (isWebGL2OrHigher()) {
         m_defaultVertexArrayObject = WebGLVertexArrayObject::create(this, WebGLVertexArrayObjectBase::VaoTypeDefault);
     } else {
@@ -1539,6 +1542,18 @@ bool WebGLRenderingContextBase::validateSettableTexFormat(const char* functionNa
     return true;
 }
 
+bool WebGLRenderingContextBase::validateReadBufferAttachment(WebGLFramebuffer* readFramebufferBinding)
+{
+    if (readFramebufferBinding) {
+        WebGLSharedObject* attachmentObject = readFramebufferBinding->getAttachmentObject(m_readbufferOfFBO);
+        if (!attachmentObject) {
+            synthesizeGLError(GL_INVALID_OPERATION, "copyTexImage2D", "no image attached to read buffer");
+            return false;
+        }
+    }
+    return true;
+}
+
 void WebGLRenderingContextBase::copyTexImage2D(GLenum target, GLint level, GLenum internalformat, GLint x, GLint y, GLsizei width, GLsizei height, GLint border)
 {
     if (isContextLost())
@@ -1567,6 +1582,8 @@ void WebGLRenderingContextBase::copyTexImage2D(GLenum target, GLint level, GLenu
         synthesizeGLError(GL_INVALID_FRAMEBUFFER_OPERATION, "copyTexImage2D", reason);
         return;
     }
+    if (!validateReadBufferAttachment("copyTexImage2D", readFramebufferBinding))

[Ken Russell (switch to Gerrit), 2015/06/24 00:24:38]

Has this code path been tested? It looks to me like it will do the wrong thing
in this situation:

 - The user calls readBuffer(gl.BACK)
 - The user attempts to copyTexImage2D from the "default" back buffer (which is
actually the DrawingBuffer) to a texture

The problem is that we need to translate the readBuffer from BACK to
COLOR_ATTACHMENT0 when the DrawingBuffer is bound. Is this being done anywhere?

If there aren't conformance tests for this then let's please add them.

[yunchao, 2015/06/24 08:50:51]

Good catch. I have updated the patch to translate GL_BACK to
GL_COLOR_ATTACHMENT0 against default frame buffer of WebGL in readBuffer API. 

The conformance test of readbuffer and APIs related to reading from FBO is in
preparation. 

+        return;
     clearIfComposited();
     ScopedDrawingBufferBinder binder(drawingBuffer(), m_framebufferBinding.get());
     webContext()->copyTexImage2D(target, level, internalformat, x, y, width, height, border);
@@ -1610,6 +1627,8 @@ void WebGLRenderingContextBase::copyTexSubImage2D(GLenum target, GLint level, GL
         synthesizeGLError(GL_INVALID_FRAMEBUFFER_OPERATION, "copyTexSubImage2D", reason);
         return;
     }
+    if (!validateReadBufferAttachment("copyTexSubImage2D", readFramebufferBinding))
+        return;
     clearIfComposited();
     ScopedDrawingBufferBinder binder(drawingBuffer(), m_framebufferBinding.get());
     webContext()->copyTexSubImage2D(target, level, xoffset, yoffset, x, y, width, height);
@@ -3343,6 +3362,9 @@ void WebGLRenderingContextBase::readPixels(GLint x, GLint y, GLsizei width, GLsi
         return;
     }
 
+    if (!validateReadBufferAttachment("readPixels", readFramebufferBinding))
+        return;
+
     clearIfComposited();
     void* data = pixels->baseAddress();