[luatex] behavior of buildpage_filter ?

[Frank Mittelbach]

Hi there,

the way I read the documentation on the buildpage_filter callback it is 
supposed to be called whenever LuaTeX is ready to move material to the 
main vertical list.

My understanding was that this means the cases that are documented in 
the TeX book as "...exercises the pagebuilder"

However, it seems to be that the callback is run far more often and in 
cases where it is definitely clear that there is no way whatsoever to 
move something to the main vertical list, e.g. it seems to be called 
whenever a box is packaged by tex.linebreak regardless of what happens 
with the output of the linebreaking

Of course in such a case the recent contributions are empty so nothing 
much happens but the following simple input


\input{ltluatex}

\directlua{
function buildpage ( context )
   print ( "************* buildpage at: " .. context .. " at " .. 
tex.inputlineno)
   return true
end
luatexbase.add_to_callback("buildpage_filter", buildpage, "new buildpage 
callback")
}

\input tufte

\bye