bug#49524: 28.0.50; make-serial-process is not portable

[Ken Brown]

On 7/12/2021 9:27 AM, Ken Brown wrote:
> On 7/11/2021 12:24 PM, Eli Zaretskii wrote:
> > > From: Ken Brown <kbrown@cornell.edu>
> > > Date: Sun, 11 Jul 2021 11:24:58 -0400
> > >
> > > Fmake_serial_process calls Fserial_process_configure, which calls
> > > serial_configure, which calls cfsetspeed with the speed argument equal to the
> > > numerical baud rate (e.g., 9600).  But the documentation of cfsetspeed says that
> > > the speed argument must be one of the Bnnn constants defined in termios.h (e.g.,
> > > B9600).  See, for example,
> > >
> > >     https://man7.org/linux/man-pages/man3/termios.3.html
> > >
> > > This incorrect call of cfsetspeed happens to succeed on GNU/Linux because
> > > glibc's cfsetspeed allows the argument to be the numerical baud rate, which it
> > > converts to the appropriate Bnnn constant.  But I don't think emacs should be
> > > relying on this undocumented behavior.  In particular, this doesn't work on
> > > Cygwin.  And it wouldn't even work on GNU/Linux if emacs used the cfsetspeed
> > > replacement defined in sysdep.c instead of glibc's cfsetspeed.
> > >
> > > I think the way to fix this is to imitate the glibc code that converts the baud
> > > rate to a Bnnn constant, but maybe someone has a better idea.
> >
> > Converting in sysdep.c:serial_configure sounds TRT to me.
>
> Patch attached.

BTW, we've decided to change Cygwin's cfsetspeed to be compatible with glibc's, 
so the problem fixed by my patch won't exist on Cygwin going forward.  And I 
checked FreeBSD out of curiosity and found that there's no issue there either 
because of the way they define the Bnnn constants:

#define B0      0
#define B50     50
#define B75     75
...

So I should probably remove the reference to non-glibc platforms in my commit 
message.

Ken