BufferSubData(gl.COPY_READ_BUFFER, gl.COPY_WRITE_BUFFER, 0, 0, 8);
        await p;
    }
    checkGetBufferSubData(gl.NO_ERROR, "srcData");

    debug("");
    debug("write-fence-transformfeedback-wait-read");
    gl.copyBufferSubData(gl.ARRAY_BUFFER, gl.COPY_WRITE_BUFFER, 0, 0, 8);
    {
        const p = fence();
        gl.bindBuffer(gl.COPY_WRITE_BUFFER, null);
        copyBufferUsingTransformFeedback(srcBuffer, readbackBuffer);
        gl.bindBuffer(gl.COPY_WRITE_BUFFER, readbackBuffer);
        await