[bug #61116] [troff] clamp output line length to horizontal resolution

[G. Branden Robinson]

URL:
  <https://savannah.gnu.org/bugs/?61116>

                 Summary: [troff] clamp output line length to horizontal
resolution
                 Project: GNU troff
            Submitted by: gbranden
            Submitted on: Sat 04 Sep 2021 04:52:15 PM UTC
                Category: Core
                Severity: 3 - Normal
              Item Group: Incorrect behaviour
                  Status: In Progress
                 Privacy: Public
             Assigned to: gbranden
             Open/Closed: Open
         Discussion Lock: Any
         Planned Release: None

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Details:

This issue arose during analysis of bug #61089.

Observe the following Heirloom Doctools troff behavior.


$ cat ll.roff
.tm horizontal resolution .H is \n(.Hu
.nr ll 49u
.de m1
.       ll \\n(llu
.       tm ll: \\n(llu; .l: \\n(.lu
.       nr ll -1
.       if \\n(llu>=0 .m1
..
.m1
$ ./bin/nroff ./ll.roff
horizontal resolution .H is 24u
ll: 49u; .l: 48u
ll: 48u; .l: 48u
ll: 47u; .l: 48u
ll: 46u; .l: 48u
ll: 45u; .l: 48u
ll: 44u; .l: 48u
ll: 43u; .l: 48u
ll: 42u; .l: 48u
ll: 41u; .l: 48u
ll: 40u; .l: 48u
ll: 39u; .l: 48u
ll: 38u; .l: 48u
ll: 37u; .l: 48u
ll: 36u; .l: 24u
ll: 35u; .l: 24u
ll: 34u; .l: 24u
ll: 33u; .l: 24u
ll: 32u; .l: 24u
ll: 31u; .l: 24u
ll: 30u; .l: 24u
ll: 29u; .l: 24u
ll: 28u; .l: 24u
ll: 27u; .l: 24u
ll: 26u; .l: 24u
ll: 25u; .l: 24u
ll: 24u; .l: 24u
ll: 23u; .l: 24u
ll: 22u; .l: 24u
ll: 21u; .l: 24u
ll: 20u; .l: 24u
ll: 19u; .l: 24u
ll: 18u; .l: 24u
ll: 17u; .l: 24u
ll: 16u; .l: 24u
ll: 15u; .l: 24u
ll: 14u; .l: 24u
ll: 13u; .l: 24u
ll: 12u; .l: 24u
ll: 11u; .l: 24u
ll: 10u; .l: 24u
ll: 9u; .l: 24u
ll: 8u; .l: 24u
ll: 7u; .l: 24u
ll: 6u; .l: 24u
ll: 5u; .l: 24u
ll: 4u; .l: 24u
ll: 3u; .l: 24u
ll: 2u; .l: 24u
ll: 1u; .l: 24u
ll: 0u; .l: 24u


The output line length is not permitted to be less than the output device's
horizontal resolution, and moreover it is not allowed to be zero.

groff has, historically, permitted this, but it does not appear to have been a
deliberate design decision.  The code was not written to handle this case (see
bug #61089 and bug #44018).

John Gardner found himself exploring in this part of the design space, as
shown in bug #61089.

If we're going to define semantics for an output line length of zero as a
groff extension, we should do so after consulting the mailing list and in
conjunction with appropriate documentation.  That'll be a separate ticket.

For now, we should make groff behave like Heirloom Doctools (unless I find
enough contradictory practice in other implementations, but that will take a
bit of time).






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