Prevent crash in robust access mode when buffer has no data

src/libANGLE/renderer/d3d/d3d11/Buffer11.cpp

@@ -348,6 +348,14 @@ gl::Error Buffer11::setData(const gl::Context *context,
 
 angle::Result Buffer11::getData(const gl::Context *context, const uint8_t **outData)
 {
+    if (mSize == 0)
+    {
+        // TODO(http://anglebug.com/2840): This ensures that we don't crash or assert in robust
+        // buffer access behavior mode if there are buffers without any data. However, technically
+        // it should still be possible to draw, with fetches from this buffer returning zero.
+        return angle::Result::Stop();
+    }
+
     SystemMemoryStorage *systemMemoryStorage = nullptr;
     ANGLE_TRY(getBufferStorage(context, BUFFER_USAGE_SYSTEM_MEMORY, &systemMemoryStorage));
 

src/tests/gl_tests/RobustBufferAccessBehaviorTest.cpp

@@ -302,6 +302,25 @@ TEST_P(RobustBufferAccessBehaviorTest, VeryLargeVertexCountWithDynamicVertexData
     // This may or may not generate an error, but it should not crash.
 }
 
+// Test that robust access works even if there's no data uploaded to the vertex buffer at all.
+TEST_P(RobustBufferAccessBehaviorTest, NoBufferData)
+{
+    ANGLE_SKIP_TEST_IF(!initExtension());
+    ANGLE_GL_PROGRAM(program, essl1_shaders::vs::Simple(), essl1_shaders::fs::Red());
+    glUseProgram(program);
+
+    glEnableVertexAttribArray(0);
+    GLBuffer buf;
+    glBindBuffer(GL_ARRAY_BUFFER, buf);
+
+    glVertexAttribPointer(0, 1, GL_FLOAT, false, 0, nullptr);
+    ASSERT_GL_NO_ERROR();
+
+    std::array<GLubyte, 1u> indices = {0};
+    glDrawElements(GL_POINTS, indices.size(), GL_UNSIGNED_BYTE, indices.data());
+    ASSERT_GL_NO_ERROR();
+}
+
 ANGLE_INSTANTIATE_TEST(RobustBufferAccessBehaviorTest,
                        ES2_D3D9(),
                        ES2_D3D11_FL9_3(),