When should a nested server first post a buffer?

[Christopher James Halse Rogers]

On Fri, May 29, 2015 at 2:35 AM, Alan Griffiths 
<alan.griffiths at canonical.com> wrote:
> There's a comment in unity-system-compositor (src/window_manager.cpp)
> 
>     // We need to wait for the second frame before marking the session
>     // as ready. The first frame posted from sessions is a blank 
> frame.
>     // TODO: Solve this issue at its root and remove this workaround
> 
> that led me to do some experimenting. And what I discovered is that a 
> nested server posts a frame when the first surface is *created*.
> 
> That is...
> 
> 1. start a host session: sudo mir_demo_server --vt 2 --arw-file
> 2. Start a nested session that will be obvious: mir_demo_server 
> --host-socket /tmp/mir_socket --background-color purple 
> --custom-compositor adorning --window-manager tiling
>> At this point vt2 will have a black display because the nested 
>> server has yet to post a buffer. (Although there's a titlebar 
>> visible for its window - which is wrong but not the issue I want to 
>> raise.)
> 
> 3. Now create a surface but don't post a buffer: mir_demo_client_basic
>> At this point the purple background from the nested server appears. 
>> (And yes, I checked it is the surface appearing that causes the 
>> post: "tiling" doesn't paint titlebar surfaces and sticking a 
>> sleep() after mir_surface_create() makes it clear this is the 
>> relevant call.)
> 
> This seems inconsistent. I could imagine it making sense to post a 
> frame when starting (but the above comment suggests this isn't 
> desirable).
> 
> Since we don't post on startup we really ought to wait until we 
> actually have content from a client.

A nested server should post a frame as soon as it's got something to 
draw. For most servers that should happen at startup, because they're 
drawing a shell. Presumably Unity8 falls into this category?

For mir_demo_server_basic we probably shouldn't draw anything before 
the first client posts a buffer. For mir_demo_server we probably should 
at least draw a background on startup.