void
WebGLContext::DrawBuffers(const dom::Sequence<GLenum>& buffers)
{
    const size_t buffersLength = buffers.Length();

    if (buffersLength == 0) {
        return ErrorInvalidValue("drawBuffers: invalid <buffers> (buffers must not be empty)");
    }

    if (mBoundFramebuffer == 0)
    {
        // OK: we are rendering in the default framebuffer

        /* EXT_draw_buffers :
         If the GL is bound to the default framebuffer, then <buffersLength> must be 1
         and the constant must be BACK or NONE. When draw buffer zero is
         BACK, color values are written into the sole buffer for single-
         buffered contexts, or into the back buffer for double-buffered
         contexts. If DrawBuffersEXT is supplied with a constant other than
         BACK and NONE, the error INVALID_OPERATION is generated.
         */
        if (buffersLength != 1) {
            return ErrorInvalidValue("drawBuffers: invalid <buffers> (main framebuffer: buffers.length must be 1)");
        }

        MakeContextCurrent();

        if (buffers[0] == LOCAL_GL_NONE) {
            const GLenum drawBuffersCommand = LOCAL_GL_NONE;
            gl->fDrawBuffers(1, &drawBuffersCommand);
            return;
        }
        else if (buffers[0] == LOCAL_GL_BACK) {
            const GLenum drawBuffersCommand = LOCAL_GL_COLOR_ATTACHMENT0;
            gl->fDrawBuffers(1, &drawBuffersCommand);
            return;
        }
        return ErrorInvalidOperation("drawBuffers: invalid operation (main framebuffer: buffers[0] must be GL_NONE or GL_BACK)");
    }

    // OK: we are rendering in a framebuffer object

    if (buffersLength > size_t(mGLMaxDrawBuffers)) {
        /* EXT_draw_buffers :
         The maximum number of draw buffers is implementation-dependent. The
         number of draw buffers supported can be queried by calling
         GetIntegerv with the symbolic constant MAX_DRAW_BUFFERS_EXT. An
         INVALID_VALUE error is generated if <buffersLength> is greater than
         MAX_DRAW_BUFFERS_EXT.
         */
        return ErrorInvalidValue("drawBuffers: invalid <buffers> (buffers.length > GL_MAX_DRAW_BUFFERS)");
    }

    for (uint32_t i = 0; i < buffersLength; i++)
    {
        /* EXT_draw_buffers :
         If the GL is bound to a draw framebuffer object, the <i>th buffer listed
         in <bufs> must be COLOR_ATTACHMENT<i>_EXT or NONE. Specifying a
         buffer out of order, BACK, or COLOR_ATTACHMENT<m>_EXT where <m> is
         greater than or equal to the value of MAX_COLOR_ATTACHMENTS_EXT,
         will generate the error INVALID_OPERATION.
         */
        /* WEBGL_draw_buffers :
         The value of the MAX_COLOR_ATTACHMENTS_WEBGL parameter must be greater than or equal to that of the MAX_DRAW_BUFFERS_WEBGL parameter.
         */
        if (buffers[i] != LOCAL_GL_NONE &&
            buffers[i] != GLenum(LOCAL_GL_COLOR_ATTACHMENT0 + i)) {
            return ErrorInvalidOperation("drawBuffers: invalid operation (buffers[i] must be GL_NONE or GL_COLOR_ATTACHMENTi)");
        }
    }

    MakeContextCurrent();

    gl->fDrawBuffers(buffersLength, buffers.Elements());
}

void
WebGL2Context::InvalidateSubFramebuffer(GLenum target, const dom::Sequence<GLenum>& attachments,
                                        GLint x, GLint y, GLsizei width, GLsizei height,
                                        ErrorResult& rv)
{
    const char funcName[] = "invalidateSubFramebuffer";

    if (IsContextLost())
        return;

    MakeContextCurrent();

    if (!ValidateFramebufferTarget(target, funcName))
        return;

    if (width < 0 || height < 0) {
        ErrorInvalidValue("%s: width and height must be >= 0.", funcName);
        return;
    }

    const WebGLFramebuffer* fb;
    bool isDefaultFB;
    switch (target) {
    case LOCAL_GL_FRAMEBUFFER:
    case LOCAL_GL_DRAW_FRAMEBUFFER:
        fb = mBoundDrawFramebuffer;
        isDefaultFB = gl->Screen()->IsDrawFramebufferDefault();
        break;

    case LOCAL_GL_READ_FRAMEBUFFER:
        fb = mBoundReadFramebuffer;
        isDefaultFB = gl->Screen()->IsReadFramebufferDefault();
        break;

    default:
        MOZ_CRASH("Bad target.");
    }

    const bool badColorAttachmentIsInvalidOp = true;
    for (size_t i = 0; i < attachments.Length(); i++) {
        if (!ValidateFramebufferAttachment(fb, attachments[i], funcName,
                                           badColorAttachmentIsInvalidOp))
        {
            return;
        }
    }

    // InvalidateFramebuffer is a hint to the driver. Should be OK to
    // skip calls if not supported, for example by OSX 10.9 GL
    // drivers.
    if (!gl->IsSupported(gl::GLFeature::invalidate_framebuffer))
        return;

    if (!fb && !isDefaultFB) {
        dom::Sequence<GLenum> tmpAttachments;
        if (!TranslateDefaultAttachments(attachments, &tmpAttachments)) {
            rv.Throw(NS_ERROR_OUT_OF_MEMORY);
            return;
        }

        gl->fInvalidateSubFramebuffer(target, tmpAttachments.Length(),
                                      tmpAttachments.Elements(), x, y, width, height);
    } else {
        gl->fInvalidateSubFramebuffer(target, attachments.Length(),
                                      attachments.Elements(), x, y, width, height);
    }
}

void
WebGLContext::DrawBuffers(const dom::Sequence<GLenum>& buffers)
{
    const char funcName[] = "drawBuffers";
    if (IsContextLost())
        return;

    if (!mBoundDrawFramebuffer) {
        // GLES 3.0.4 p186:
        // "If the GL is bound to the default framebuffer, then `n` must be 1 and the
        //  constant must be BACK or NONE. [...] If DrawBuffers is supplied with a
        //  constant other than BACK and NONE, or with a value of `n` other than 1, the
        //  error INVALID_OPERATION is generated."
        if (buffers.Length() != 1) {
            ErrorInvalidOperation("%s: For the default framebuffer, `buffers` must have a"
                                  " length of 1.",
                                  funcName);
            return;
        }

        switch (buffers[0]) {
        case LOCAL_GL_NONE:
        case LOCAL_GL_BACK:
            break;

        default:
            ErrorInvalidOperation("%s: For the default framebuffer, `buffers[0]` must be"
                                  " BACK or NONE.",
                                  funcName);
            return;
        }

        mDefaultFB_DrawBuffer0 = buffers[0];
        gl->Screen()->SetDrawBuffer(buffers[0]);
        return;
    }

    // Framebuffer object (not default framebuffer)

    if (buffers.Length() > mImplMaxDrawBuffers) {
        // "An INVALID_VALUE error is generated if `n` is greater than MAX_DRAW_BUFFERS."
        ErrorInvalidValue("%s: `buffers` must have a length <= MAX_DRAW_BUFFERS.",
                          funcName);
        return;
    }

    for (size_t i = 0; i < buffers.Length(); i++) {
        // "If the GL is bound to a draw framebuffer object, the `i`th buffer listed in
        //  bufs must be COLOR_ATTACHMENTi or NONE. Specifying a buffer out of order,
        //  BACK, or COLOR_ATTACHMENTm where `m` is greater than or equal to the value of
        // MAX_COLOR_ATTACHMENTS, will generate the error INVALID_OPERATION.

        // WEBGL_draw_buffers:
        // "The value of the MAX_COLOR_ATTACHMENTS_WEBGL parameter must be greater than or
        //  equal to that of the MAX_DRAW_BUFFERS_WEBGL parameter."
        // This means that if buffers.Length() isn't larger than MaxDrawBuffers, it won't
        // be larger than MaxColorAttachments.
        if (buffers[i] != LOCAL_GL_NONE &&
            buffers[i] != LOCAL_GL_COLOR_ATTACHMENT0 + i)
        {
            ErrorInvalidOperation("%s: `buffers[i]` must be NONE or COLOR_ATTACHMENTi.",
                                  funcName);
            return;
        }
    }

    MakeContextCurrent();

    const GLenum* ptr = nullptr;
    if (buffers.Length()) {
        ptr = buffers.Elements();
    }

    gl->fDrawBuffers(buffers.Length(), ptr);

    const auto end = ptr + buffers.Length();
    mBoundDrawFramebuffer->mDrawBuffers.assign(ptr, end);
}