[bug #66339] [ms] cannot start a document with a `DS` static display

[G. Branden Robinson]

URL:
  <https://savannah.gnu.org/bugs/?66339>

                 Summary: [ms] cannot start a document with a `DS` static
display
                   Group: GNU roff
               Submitter: gbranden
               Submitted: Wed 16 Oct 2024 05:50:36 PM UTC
                Category: Macro package ms
                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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Follow-up Comments:


-------------------------------------------------------
Date: Wed 16 Oct 2024 05:50:36 PM UTC By: G. Branden Robinson <gbranden>
Affects _groff_ 1.23.0.


$ cat ATTIC/start-with-display.ms
.DS
Hello, world!  .s=\n(.s, nl=\n(nl
.DE
$ /usr/bin/groff --version | head -n 1
GNU groff version 1.22.4
$ /usr/bin/groff -a -ms ATTIC/start-with-display.ms
<beginning of page>
 Hello, world! .s=10, nl=72000
  
$ ~/groff-stable/bin/groff --version | head -n 1
GNU groff version 1.23.0
$ ~/groff-stable/bin/groff -a -ms ATTIC/start-with-display.ms
<beginning of page>
s.tmac:ATTIC/start-with-display.ms:3: error: must be in top-level environment,
not 'nf', when first page is started
Hello, world! .s=1, nl=-1
  


Analysis from a
[https://lists.gnu.org/archive/html/groff/2024-10/msg00090.html post to the
groff mailing list]:


It appears to me that this was a bug I introduced.[1]  I bisected it
down to commit 1887bbe68c in November 2021.

The fix is simple, and causes no automated test failures (but I'll be
writing a new one); here's what I have in my working copy.

$ git diff
diff --git a/tmac/s.tmac b/tmac/s.tmac
index dd682e5b1..adf18e0c7 100644
--- a/tmac/s.tmac
+++ b/tmac/s.tmac
@@ -1071,6 +1071,7 @@ .de ds*end!2
 .el .@error-recover mismatched .DE
 ..
 .de DS
+.br
 .if '\\n(.z'ds*div' .@error-recover cannot begin display within display
 .nr ds*badarg 0
 .di ds*div

...

[1] The lack of `br` meant that a document that started with a `DS`
    macro call did not "sweep" the drawing position past vertical
    position 0, which springs a trap that calls a macro called
    `cov*first-page-init`,[2] which does much essential initialization,
    including (indirectly) setup of the `PS` register storing the type
    size.  Consequently, the text of the display would render at 1
    point(!) instead of the formatter's default of 10.  The diagnostic
    about the `nf` environment might warn the user that the formatting
    is likely to go awry, but we can do better than that by simply
    restoring the break.

[2]
https://git.savannah.gnu.org/cgit/groff.git/tree/tmac/s.tmac?id=198346d187de9e340bbf9d4f80c2dc4d42f5f74e#n203









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