Do not exit shell on Ctrl+C with SIGINT ignored (re: 7e5fd3e)

The killpg(getpgrp(),SIGINT) call added to ed_getchar() in that
commit caused the interactive shell to exit on ^C even if SIGINT is
being ignored. We cannot revert or remove that call without
breaking job control. This commit applies a new fix instead.

Reproducers fixed by this commit:

  SIGINT ignored by child:

    $ PS1='childshell$ ' ksh
    childshell$ trap '' INT
    childshell$ (press Ctrl+C)
    $

  SIGINT ignored by parent:

    $ (trap '' INT; ENV=/./dev/null PS1='childshell$ ' ksh)
    childshell$ (press Ctrl+C)
    $

  SIGINT ignored by parent, trapped in child:

    $ (trap '' INT; ENV=/./dev/null PS1='childshell$ ' ksh)
    childshell$ trap 'echo test' INT
    childshell$ (press Ctrl+C)
    $

I've experimentally determined that, under these conditions, the
SFIO stream error state is set to 256 == 0400 == SH_EXITSIG.

src/cmd/ksh93/sh/main.c: exfile():
- On EOF or error, do not return (exiting the shell) if the shell
  state is interactive and if sferror(iop)==SH_EXITSIG.
- Refactor that block a little to make the new check fit in nicely.

src/cmd/ksh93/tests/pty.sh:
- Test the above three reproducers.

Fixes: #343

NEWS

@@ -9,6 +9,9 @@ Any uppercase BUG_* names are modernish shell bug IDs.
   command to: basename, cat, cp, cut, dirname, getconf, ln, mktemp, mv.
   Add /opt/ast/bin to your $PATH to use these. Type 'cp --man', etc. for info.
 
+- A bug introduced on 2020-09-17 was fixed that caused interactive ksh to exit
+  if Ctrl+C was pressed while SIGINT was being ignored (as in "trap '' INT").
+
 2021-12-13:
 
 - Fixed a bug introduced on 2020-08-09 that prevented '.' and '..' from

src/cmd/ksh93/sh/main.c

@@ -535,20 +535,31 @@ static void	exfile(register Shell_t *shp, register Sfio_t *iop,register int fno)
 		errno = 0;
 		if(tdone || !sfreserve(iop,0,0))
 		{
+			int	sferr;
 		eof_or_error:
-			if(sh_isstate(SH_INTERACTIVE) && !sferror(iop)) 
+			sferr = sferror(iop);
+			if(sh_isstate(SH_INTERACTIVE))
 			{
-				if(--maxtry>0 && sh_isoption(SH_IGNOREEOF) &&
-					 !sferror(sfstderr) && (shp->fdstatus[fno]&IOTTY))
+				if(!sferr)
 				{
-					sfclrerr(iop);
-					errormsg(SH_DICT,0,e_logout);
-					continue;
+					if(--maxtry>0 && sh_isoption(SH_IGNOREEOF)
+					&& !sferror(sfstderr) && (shp->fdstatus[fno]&IOTTY))
+					{
+						sfclrerr(iop);
+						errormsg(SH_DICT,0,e_logout);
+						continue;
+					}
+					else if(job_close(shp)<0)
+						continue;
 				}
-				else if(job_close(shp)<0)
+				else if(sferr==SH_EXITSIG)
+				{
+					/* Ctrl+C with SIGINT ignored */
+					sfputc(sfstderr,'\n');
 					continue;
+				}
 			}
-			if(errno==0 && sferror(iop) && --maxtry>0)
+			if(errno==0 && sferr && --maxtry>0)
 			{
 				sfclrlock(iop);
 				sfclrerr(iop);

src/cmd/ksh93/tests/pty.sh

@@ -948,5 +948,47 @@ w : ..\t
 r : \.\./\r\n$
 !
 
+# err_exit #
+tst $LINENO <<"!"
+L Ctrl+C with SIGINT ignored
+# https://github.com/ksh93/ksh/issues/343
+
+d 15
+
+# SIGINT ignored by child
+p :test-1:
+w PS1=':child-!: ' "$SHELL"
+p :child-1:
+w trap '' INT
+p :child-2:
+c \\\cC
+r :child-2:
+w echo "OK $PS1"
+u ^OK :child-!: \r\n$
+w exit
+
+# SIGINT ignored by parent
+p :test-2:
+w (trap '' INT; ENV=/./dev/null PS1=':child-!: ' "$SHELL")
+p :child-1:
+c \\\cC
+r :child-1:
+w echo "OK $PS1"
+u ^OK :child-!: \r\n$
+w exit
+
+# SIGINT ignored by parent, trapped in child
+p :test-3:
+w (trap '' INT; ENV=/./dev/null PS1=':child-!: ' "$SHELL")
+p :child-1:
+w trap 'echo test' INT
+p :child-2:
+c \\\cC
+r :child-2:
+w echo "OK $PS1"
+u ^OK :child-!: \r\n$
+w exit
+!
+
 # ======
 exit $((Errors<125?Errors:125))