Fix POSIX mode IFS behaviour if xtrace on (re: 9e2a8c6, 8f14514)

Reproducer script:

  IFS=$'\t\t' val=$'\tone\t\ttwo\tthree\t'
  set -o posix; set $val; print -n $#
  set +o posix; set $val; print -n $#
  set -o posix; set $val; print -n $#
  set +o posix; set $val; print $#

The output is normally correct but is wrong when xtrace is on:

  $ arch/darwin.arm64-64/bin/ksh test6.sh
  3535
  $ arch/darwin.arm64-64/bin/ksh -x test6.sh 2>/dev/null
  5353

Analysis: In sh_argopts() in args.c, whenever we're going to change
the posix (SH_POSIX) option, sh_invalidate_ifs() is called to
trigger a regeneration of the IFS state table (in get_ifs(),
init.c); this is necessary to enable or disable the effect of the
doubled-up whitespace character. The 'set' command is then traced
with a sh_trace() call before the new options take effect. The
trace involves expanding PS4, which involes reading IFS, which
causes get_ifs() to regenerate the IFS state table before the posix
option change takes effect.

src/cmd/ksh93/sh/args.c: sh_argopts():
- Instead of calling sh_invalidate_ifs() immediately, remember the
  need to call it in a flag variable.
- Use that flag to delay the actual sh_invalidate_ifs() call until
  after sh_trace() has been called.

Re-reading the sh_invalidate_ifs() function caused me to realise
that this function has a bug, too. For np pointing to IFS, it
assumes that the IFS discipline IFS_disc is pointed to by
np->nvfun. This is not correct if another discipline is added to
it, e.g. by defining an IFS.get() shell discipline function. Doing
that causes an invalid read in sh_invalidate_ifs() and incorrect
behaviour.

src/cmd/ksh93/sh/init.c: sh_invalidate_ifs():
- Move the function to after the IFS_disc declaration.
- Use nv_hasdisc() (nvdisc.c) to find the pointer to IFS_disc
  starting from np->nvfun, which is a linked list.

src/cmd/ksh93/tests/posix.sh:
- Set a dummy IFS.get() to test the above fix.

NEWS

@@ -2,6 +2,13 @@ This documents significant changes in the 1.0 branch of ksh 93u+m.
 For full details, see the git log at: https://github.com/ksh93/ksh/tree/1.0
 Uppercase BUG_* IDs are shell bug IDs as used by the Modernish shell library.
 
+2024-03-05:
+
+- Fixed a corner case bug causing incorrect field splitting behaviour of a
+  repeated whitespace character in the IFS variable. The behaviour was
+  incorrect when the posix option was changed while xtrace was on, and when a
+  shell discipline function was defined for IFS. Bug introduced on 2022-03-05.
+
 2024-03-03:
 
 - Fixed incorrect behaviour of {n1..n2..incr} range brace expansions, which

src/cmd/ksh93/include/version.h

@@ -18,7 +18,7 @@
 
 #define SH_RELEASE_FORK	"93u+m"		/* only change if you develop a new ksh93 fork */
 #define SH_RELEASE_SVER	"1.0.9-beta"	/* semantic version number: https://semver.org */
-#define SH_RELEASE_DATE	"2024-04-03"	/* must be in this format for $((.sh.version)) */
+#define SH_RELEASE_DATE	"2024-04-05"	/* must be in this format for $((.sh.version)) */
 #define SH_RELEASE_CPYR	"(c) 2020-2024 Contributors to ksh " SH_RELEASE_FORK
 
 /* Scripts sometimes field-split ${.sh.version}, so don't change amount of whitespace. */

src/cmd/ksh93/sh/args.c

@@ -119,6 +119,7 @@ int sh_argopts(int argc,char *argv[])
 #endif
 	Shopt_t		newflags;
 	int		defaultflag=0, setflag=0, action=0, trace=(int)sh_isoption(SH_XTRACE);
+	int		invalidate_ifs = 0;
 	Namval_t	*np = NULL;
 	const char	*cp;
 	int		verbose, f;
@@ -175,7 +176,7 @@ int sh_argopts(int argc,char *argv[])
 					{
 						off_option(&newflags,o);
 						if(o==SH_POSIX)
-							sh_invalidate_ifs();
+							invalidate_ifs = 1;
 					}
 				}
 			}
@@ -251,7 +252,7 @@ int sh_argopts(int argc,char *argv[])
 				off_option(&newflags,SH_BRACEEXPAND);
 #endif
 				on_option(&newflags,SH_LETOCTAL);
-				sh_invalidate_ifs();
+				invalidate_ifs = 1;
 			}
 			on_option(&newflags,o);
 			off_option(&sh.offoptions,o);
@@ -269,7 +270,7 @@ int sh_argopts(int argc,char *argv[])
 				on_option(&newflags,SH_BRACEEXPAND);
 #endif
 				off_option(&newflags,SH_LETOCTAL);
-				sh_invalidate_ifs();
+				invalidate_ifs = 1;
 			}
 			if(o==SH_XTRACE)
 				trace = 0;
@@ -294,6 +295,9 @@ int sh_argopts(int argc,char *argv[])
 	}
 	if(trace)
 		sh_trace(argv,1);
+	/* Invalidating the IFS state table must be done after sh_trace, because xtrace reads IFS */
+	if(invalidate_ifs)
+		sh_invalidate_ifs();
 	argc -= opt_info.index;
 	argv += opt_info.index;
 	if(action==PRINT)

src/cmd/ksh93/sh/init.c

@@ -554,18 +554,6 @@ static void put_ifs(Namval_t* np,const char *val,int flags,Namfun_t *fp)
 	}
 }
 
-/* Invalidate IFS state table */
-void sh_invalidate_ifs(void)
-{
-	Namval_t *np = sh_scoped(IFSNOD);
-	if(np)
-	{
-		struct ifs *ip = (struct ifs*)np->nvfun;
-		if(ip)
-			ip->ifsnp = 0;
-	}
-}
-
 /*
  * This is the lookup function for IFS
  * It keeps the sh.ifstable up to date
@@ -1015,6 +1003,19 @@ static const Namdisc_t SH_VERSION_disc	= {  0, 0, get_version, nget_version };
 
 
 static const Namdisc_t IFS_disc		= {  sizeof(struct ifs), put_ifs, get_ifs };
+
+/* Invalidate IFS state table */
+void sh_invalidate_ifs(void)
+{
+	Namval_t *np = sh_scoped(IFSNOD);
+	if(np)
+	{
+		struct ifs *ip = (struct ifs*)nv_hasdisc(np, &IFS_disc);
+		if(ip)
+			ip->ifsnp = 0;
+	}
+}
+
 const Namdisc_t RESTRICTED_disc	= {  sizeof(Namfun_t), put_restricted };
 static const Namdisc_t CDPATH_disc	= {  sizeof(Namfun_t), put_cdpath }; 
 #if SHOPT_VSH || SHOPT_ESH

src/cmd/ksh93/tests/posix.sh

@@ -118,6 +118,7 @@ got=$("$SHELL" -c 'typeset -p testint')
 set --posix
 
 # disables the special handling of repeated isspace class characters in the IFS variable;
+IFS.get() { :; }  # tests if sh_invalidate_ifs() in init.c correctly gets IFS_disc
 IFS=$'x\t\ty' val=$'\tun\t\tduo\ttres\t'
 got=$(set $val; echo "$#")
 exp=3
@@ -131,6 +132,7 @@ got=$(set --default; set $val; echo "$#")
 [[ $got == "$exp" ]] || err_exit "repeated IFS whitespace char (default): incorrect number of fields" \
 	"(expected $(printf %q "$exp"), got $(printf %q "$got"))"
 IFS=$' \t\n' # default
+unset -f IFS.get
 
 # causes file descriptors > 2 to be left open when invoking another program;
 exp='ok'