[bug #61089] [troff] assertion failed: 'desired_space.is_zero() && nspaces == 0'

[G. Branden Robinson]

Update of bug #61089 (project groff):

                  Status:               Need Info => In Progress            

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Follow-up Comment #13:

This experimental patch was instructive.


diff --git a/src/roff/troff/env.cpp b/src/roff/troff/env.cpp
index 4af8741e1..1b6a15755 100644
--- a/src/roff/troff/env.cpp
+++ b/src/roff/troff/env.cpp
@@ -2053,14 +2053,15 @@ static node *node_list_reverse(node *n)
 static void distribute_space(node *n, int nspaces, hunits desired_space,
                             int force_reverse = 0)
 {
+  hunits em = curenv->get_size();
+  double Ems = (double)desired_space.to_units() / nspaces
+              / (em.is_zero() ? hresolution : em.to_units());
+  debug("spreading %1m per space among %2 spaces", Ems, nspaces);
   static int reverse = 0;
   if (force_reverse || reverse)
     n = node_list_reverse(n);
   if (!force_reverse && nspaces > 0 && spread_limit >= 0
       && desired_space.to_units() > 0) {
-    hunits em = curenv->get_size();
-    double Ems = (double)desired_space.to_units() / nspaces
-                / (em.is_zero() ? hresolution : em.to_units());
     if (Ems > spread_limit)
       output_warning(WARN_BREAK, "spreading %1m per space", Ems);
   }


Input (pretty much John's reduced reproducer):


$ cat EXPERIMENTS/61089.roff 
.de _
.       na
.       nh
.       ll 0
.       di A
.tm line length in diversion is \\n(.l
\&\\$1
.       di
.       br
..
._ " XYZ"
.tm line length is \n(.l
.A


Output:


line length in diversion is 0
troff: backtrace: 'EXPERIMENTS/61089.roff':7: macro '_'
troff: backtrace: file 'EXPERIMENTS/61089.roff':11
troff: EXPERIMENTS/61089.roff:11: debug: spreading -nanm per space among 0
spaces
troff: backtrace: 'EXPERIMENTS/61089.roff':7: macro '_'
troff: backtrace: file 'EXPERIMENTS/61089.roff':11
troff: EXPERIMENTS/61089.roff:11: warning [p 0, 0.0i, div 'A', 0.0i]: line has
non-positive width 0m
line length is 0
troff: warning [p 1, 0.0i]: can't break line
troff: debug: spreading 0m per space among 1 spaces
troff: ../src/roff/troff/env.cpp:2074: distribute_space(): assertion failed:
'desired_space.is_zero() && nspaces == 0'
/home/branden/src/GIT/groff/build/groff: error: troff: Aborted (core dumped)


Yeaaaaah.  Check that out.  NaNs (division by zero) and an attempt to spread
0m of space.

I think resolving this mess will involve three steps.

(1) Make distribute_space() more irritable about its inputs.  If either
`nspaces` or `desired_space` is zero, return early.  I might instrument this
check with a diagnostic, too, but maybe not because...

(2) In the short run I think we should do as Heirloom Doctools troff does and
clamp the line length to the horizontal resolution.  This will require checks
on the implementation of the `ll` request and the code that reads the DESC
file in the first place.  I'm reasonably familiar with these areas of the
codebase.

(3) Bring John's proposed zero-line-length semantics to the groff mailing
list.  I have no particular objection to it but since that list is effectively
the entire troff community's sounding board, I think it's prudent to notify
and seek the feedback of other implementors and experts.  If it survives
scrutiny there, implement it.

Whether (3) survives or not, I think either (1) or (2) should make it
unnecessary to add checks at the call sites of `distribute_space()`.  (And if
I'm wrong, there is only a handful of such places.)

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