How would I best interrupt a particular frame from rendering.

I'm thinking if in the mantra node's Pre-Frame Script's script I put…
import os

rn = hou.node('/out/mantra1')
fname = rn.parm('vm_picture')
if os.path.exists(fname.eval()) == True :
print(“Skip this Frame then”)

What would follow to skip rendering the frame (and not damage the existing frame or waste render time)?

break;?
hou.setFrame(hou.frame() + inc) ?
hou.incrementFrame()?

Oh it officially says in docs that this one is not yet implemented incrementFrame()?
which is slightly odd since you'd only need to get the increment frame out.
inc = rn.parm('f3').eval()
as now noted for my second suggestion? or do you? It's potentially more complicated then that no doubt.

I tried the idea of changing the increment value for the next frame but it didn't affect the current in process render. The GUI only updates after interrupt/completion though.

Ultimately not only do you need a flag for skipping rendered frames but in this case it would be nice to also allow it to cook everything (scripts generating trails etc.) without the actual render.

I wondered if there's a way to pass to the script (or callbacks etc.) the in-processing mantra node and not just the one that it won't see changes on until it completes.
Is there any way to Override the variables of the currently rendering node? It seems spawned.

It looks like the only way is from the command-line via hscript and calling out each frame… lame.

The thing is you'd have to cache geo or output IFD files because the cooking won't happen properly calling single frames out. Well actually I suppose you can walk each frame and then render the current needed frames in hscript command-line mode.

The basically seems like an intentional oversight in preference for concerns over distributed rendering, licensing and renderfarm tools or something. I mean seriously how can you miss “Skip already rendered frames”?