[bug #63366] groff.texi: clarify handling of special fonts

[G. Branden Robinson]

Update of bug #63366 (project groff):

                  Status:                    None => Need Info              

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Follow-up Comment #1:


[comment #0 original submission:]
> The "Special Fonts" section of the Texinfo documentation is ambiguous on one
point and seemingly missing info on another.
> 
> This section was last touched in 2014 (by commit e3c63595
<http://git.savannah.gnu.org/cgit/groff.git/commit/?id=e3c63595>, fixing a
recurring grammatical nit throughout the file), so has remained unchanged
since 1.22.3 (and therefore hasn't been Brandenified).

Hi Dave,

Yes, I haven't yet revised our Texinfo manual in the "fonts and symbols"
department because it's going to take some significant effort to reform.  Also
there are many points of detail that I need to learn and confirm.
 
> This section says both "The Symbol font is usually a special font" and "Use
the 'special' request to define special fonts.  Initially, this list is
empty."  So the initial list of special fonts both is empty and contains
Symbol.

There is a data structure in GNU troff's node.cpp called
"global_special_fonts" that is presumably the list that this refers to, and
this is indeed empty on startup.  (I quickly implemented a `pft` request to
dump some font state to stderr.)
fonts 9 0 0 0 0 0 SS S ZD ZDR
".special BOGUS" produces a diagnostic now (as we already knew from earlier
this year), but more tellingly (for the "ps" device) ".special TR" does indeed
add Times Roman to this formerly empty list, and ".special" with no arguments
_does_ clear it out again.

> That Symbol is in fact a default special font is easily demonstrable.  The
groff input characters \[HE] and \[CL] have glyphs in Symbol but not Times:
> 
> $ egrep 'HE|CL' /usr/share/groff/current/font/devps/TR
> $ egrep 'HE|CL' /usr/share/groff/current/font/devps/S
> CL      753,533,26      3       167     club
> HE      753,532,33      3       169     heart
> fonts 9 0 0 0 0 0 SS S ZD ZDR
> And groff input that specifies no font information, defaulting to Times,
also finds Symbol when needed:
> 
> $ printf 'I \[HE] my \[CL]\n' | groff -Z | fgrep font
> x font 5 TR
> wx font 11 S
> 

I think something else is going on here.  Our documentation also says this:

"special
              The font is special: when a glyph is requested that is not
present in the current font, it is sought in any mounted fonts that bear      
        this property."

...and when we consult the 'ps' device's DESC file, we see the line

fonts 9 0 0 0 0 0 SS S ZD ZDR

...which means that the Symbol font is _mounted_.  (In fact, _all_ of these
named fonts are special.)

Now, why this vector of special fonts isn't reported as the
global_special_fonts list, I don't know.  I'm not certain they're _supposed_
to be the same thing.

(There might be some C++ variable scoping issues.  global_special_fonts is
local to the node.cpp file, but DESC file parsing and the actions that are
taken consequent to the directives found there happen in libgroff.) 

> This section of the manual also states, "Previous calls to 'special' or
'fspecial' are overwritten; without arguments, the particular list of special
fonts is set to empty."  So to remove the Symbol font from the list of special
fonts, one ought to be able to give these requests empty lists.

This statement seems to accurately describe the way the requests are
implemented.
 
> But this doesn't behave as might be expected from the above-quoted
sentence:
> 
> $ printf '.special\n.fspecial TR\nI \[HE] my \[CL]\n' | groff -Z | fgrep
font
> x font 5 TR
> wx font 11 S
> 
> That the two ".*special" requests in this sample input are essentially
no-ops can be explicitly demonstrated via:
> 
> $ diff <(printf 'I \[HE] my \[CL]\n' | groff -Z) <(printf
'.special\n.fspecial TR\nI \[HE] my \[CL]\n' | groff -Z) | wc
>       0       0       0
> 
> I can only conclude that the Symbol font is an _extra_-special special font
that exists outside the _ordinary_ "special" mechanism.  The documentation
could be clearer about this.
> 
> Additionally, I'm puzzled by the word "usually" up there.  How does one
overturn this usual case, if the documented method of emptying the lists of
special fonts exempts the extra-special Symbol font?

If there were a way to un-mount a font...well, actually there is.

Try your experiments after doing this.

.fp 11 TR

This will clobber the "S" font's slot in the table with a copy of Times Roman.
 This is an ugly hack, but I suspect it will work.

I _imagine_ that the problem here is a bug: the `global_special_fonts`
variable in troff is not wired up to the outcome of DESC file processing as it
should be.  You might ask on the list; it seems to be the season for the
oldsters to wearily sigh and attempt to correct the wayward youths.

Font and particularly character/glyph resolution are pretty complex in groff
and I have not mastered the details; probably the only way that will happen is
if I undertake that documentation revision.

But we should be prepared to reclassify this as yet another decades-old bug.


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