[bug #65962] input.cpp: reduce stack limit to <=100

[G. Branden Robinson]

Update of bug #65962 (group groff):

                Severity:              3 - Normal => 1 - Wish               
                  Status:                    None => Postponed              

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Follow-up Comment #1:

No defect is reported here; this is a wish list item.  Setting severity.

> I have not seen the reasoning for this high value.

Consider that the only way to write a loop in A&T _troff_ was to write a
recursive macro.


     An often better solution--and one that is more portable, since AT&T
     'troff' lacked the 'while' request--is to instead write a recursive
     macro.  It will be parsed only once.(2)  (*note while-Footnote-2::)

          .de yy
          .  if (\\n(nm > 0) \{\
          .    \" many lines of code
          .    nr nm -1
          .    yy
          .  \}
          ..
          .
          .de xx
          .  nr nm 10
          .  yy
          ..

     To prevent infinite loops, the default number of available
     recursion levels is 1,000 or somewhat less.(3)  (*note
     while-Footnote-3::) You can disable this protective measure, or
     raise the limit, by setting the 'slimit' register.  *Note
     Debugging::.


Next consider the sorts of things that one might loop over.  A list of
references, say.  In longer documents there could easily be more than 100 of
these.  More than 1000?  Less likely.  (Though I do recall that my college
psychology textbook seemed to flirt with that order of magnitude.)

> I have run the build of groff with reduced values of the constant
"DEFAULT_INPUT_STACK_LIMIT"
> and it needs > 25 < 50 stack elements.

You could have just set the `slimit` register in the "tmac/troffrc", then
cleaned the build tree's "doc" directory and remade...


 -- Register: \n[slimit]
     If greater than 0, sets the maximum quantity of objects on GNU
     'troff''s internal input stack.  If less than or equal to 0, there
     is no limit: recursion can continue until program memory is
     exhausted.  The default is 1,000.


Running experiments on short, dummy documents--or empty ones--is not going to
resemble the rendering of real documents.

I'm pleased to hear that you had difficulty getting _groff_ to grow the stack
beyond 50 "frames" just using its own facilities (macro packages, documents).

But that doesn't make 50 a reasonable limit for users' scenarios.

I'll postpone this ticket for now in case any further discussion occurs but my
intention at this point is to close it as rejected.


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