[osg-users] how safe is to add nodes in cull traversal

[Robert Osfield]

On 10 September 2016 at 12:42, Trajce Nikolov NICK
<trajce.nikolov.nick at gmail.com> wrote:
> Hi Community,
>
> me again :-) ..  As the subject says, I an having the viewer setup with
> CullDrawThreadPerContext and I want to add nodes in the cull traversal. Is
> it safe to do so?

How long have you been using the OSG now?

Short answer. NO.  Longer answer, NO it's not safe.

Full answer: It's *not* generally safe to add nodes during any
traversal, especially the cull traversal.  There are very specific
circumstances when it can be done during during a traversal but you
have to be fully aware of what can go wrong and make sure you don't
fall foul of any of the potential problems.

A major gotcha with changing the scene graph in the cull traversal is
that multiple camera's can have multiple threads running cull
traversals on them on parallel.  If you wanted to go change the scene
graph when threading is happened then you'd need to lock the node in
some way to prevent problems happening.  There are lots of threading
pitfalls here - both for performance and obscure threading crashes.

Another major issue is invalidation of iterators.  If you have code
that traverses through a vector of children and as you handle one of
the children you decide it's time to add/remove/insert and new child
into the list being iterated then any reallocation of the vector being
iterator will invalidate the iterators.  Invalidate the iterators and
bang you're app will go down in flames.

So.... in almost all instances trying to add/remove/insert nodes
during the cull traversal is REALLY BAD IDEA.

If you really want to do it. Go do it, you have the source code, you
have debuggers, you can go shoot yourself in the foot or if your lucky
work out a really narrow and safe way to do what you want to do.  I
won't nurse you along in this route though.  Strong recommendation,
don't play silly buggers, don't try it. It'll save you lots of time.

Far better is do what other multi-threaded parts of the OSG do, do
what the DatabasePager does, load data in a background thread,
incremental compile new objects, when compilation is complete merge
the subgraph with the main scene graph as an update operation prior to
the update and event traversals.    This approach is safe, scales
well, is well established to work well for multi-threaded and
applications that need to target a solid 60hz.

Robert.