Allow proper tilde expansion overrides (#225)

Until now, when performing any tilde expansion like ~/foo or
~user/foo, ksh added a placeholder built-in command called
'.sh.tilde', ostensibly with the intention to allow users to
override it with a shell function or custom builtin. The multishell
ksh93 repo <https://github.com/multishell/ksh93/> shows this was
added sometime between 2002-06-28 and 2004-02-29. However, it has
never worked and crashed the shell.

This commit replaces that with something that works. Specific tilde
expansions can now be overridden using .set or .get discipline
functions associated with the .sh.tilde variable (see manual,
Discipline Functions).

For example, you can use either of:

.sh.tilde.set()
{
        case ${.sh.value} in
        '~tmp') .sh.value=${XDG_RUNTIME_DIR:-${TMPDIR:-/tmp}} ;;
        '~doc') .sh.value=~/Documents ;;
        '~ksh') .sh.value=/usr/local/src/ksh93/ksh ;;
        esac
}

.sh.tilde.get()
{
        case ${.sh.tilde} in
        '~tmp') .sh.value=${XDG_RUNTIME_DIR:-${TMPDIR:-/tmp}} ;;
        '~doc') .sh.value=~/Documents ;;
        '~ksh') .sh.value=/usr/local/src/ksh93/ksh ;;
        esac
}

src/cmd/ksh93/include/variables.h,
src/cmd/ksh93/data/variables.c:
- Add SH_TILDENOD for a new ${.sh.tilde} predefined variable.
  It is initially unset.

src/cmd/ksh93/sh/macro.c:
- sh_btilde(): Removed.
- tilde_expand2(): Rewritten. I started out with the tiny version
  of this function from the 2002-06-28 version of ksh. It uses the
  stack instead of sfio, which is more efficient. A bugfix for
  $HOME == '/' was retrofitted so that ~/foo does not become
  //foo instead of /foo. The rest is entirely new code.
     To implement the override functionality, it now checks if
  ${.sh.tilde} has any discipline function associated with it.
  If it does, it assigns the tilde expression to ${.sh.tilde} using
  nv_putval(), triggering the .set discipline, and then reads it
  back using nv_getval(), triggering the .get discipline. The
  resulting value is used if it is nonempty and does not still
  start with a tilde.

src/cmd/ksh93/bltins/typeset.c,
src/cmd/ksh93/tests/builtins.sh:
- Since ksh no longer adds a dummy '.sh.tilde' builtin, remove the
  ad-hoc hack that suppressed it from the output of 'builtin'.

src/cmd/ksh93/tests/tilde.sh:
- Add tests verifying everything I can think of, as well as tests
  for bugs found and fixed during this rewrite.

src/cmd/ksh93/tests/pty.sh:
- Add test verifying that the .sh.tilde.set() discipline does not
  modify the exit status value ($?) when performing tilde expansion
  as part of tab completion.

src/cmd/ksh93/sh.1:
- Instead of "tilde substitution", call the basic mechanism "tilde
  expansion", which is the term used everywhere else (including the
  1995 Bolsky/Korn ksh book).
- Document the new override feature.

Resolves: https://github.com/ksh93/ksh/issues/217

src/cmd/ksh93/sh/macro.c

@@ -2619,64 +2619,44 @@ static int	charlen(const char *string,int len)
 	}
 }
 
-/*
- * This is the default tilde discipline function
- */
-static int sh_btilde(int argc, char *argv[], Shbltin_t *context)
-{
-	Shell_t *shp = context->shp;
-	char *cp = sh_tilde(shp,argv[1]);
-	NOT_USED(argc);
-	if(!cp)
-		cp = argv[1];
-	sfputr(sfstdout, cp, '\n');
-	return(0);
-}
- 
 /*
  * <offset> is byte offset for beginning of tilde string
  */
 static void tilde_expand2(Shell_t *shp, register int offset)
 {
-	char		shtilde[10], *av[3], *ptr=stkfreeze(shp->stk,1);
-	Sfio_t		*iop, *save=sfstdout;
-	Namval_t	*np;
-	static int	beenhere=0;
-	strcpy(shtilde,".sh.tilde");
-	np = nv_open(shtilde,shp->fun_tree, NV_VARNAME|NV_NOARRAY|NV_NOASSIGN|NV_NOFAIL);
-	if(np && !beenhere)
+	char		*cp = NIL(char*);		/* character pointer for tilde expansion result */
+	char		*stakp = stakptr(0);		/* current stack object (&stakp[offset] is tilde string) */
+	int		curoff = staktell();		/* current offset of current stack object */
+	static char	block;				/* for disallowing tilde expansion in .get/.set to change ${.sh.tilde} */
+	/*
+	 * Allow overriding tilde expansion with a .sh.tilde.set or .get discipline function.
+	 */
+	if(!block && SH_TILDENOD->nvfun && SH_TILDENOD->nvfun->disc)
 	{
-		beenhere = 1;
-		sh_addbuiltin(shtilde,sh_btilde,0);
-		nv_onattr(np,NV_EXPORT);
+		stakfreeze(1);				/* terminate current stack object to avoid data corruption */
+		block++;
+		nv_putval(SH_TILDENOD, &stakp[offset], 0);
+		cp = nv_getval(SH_TILDENOD);
+		block--;
+		if(cp[0]=='\0' || cp[0]=='~')
+			cp = NIL(char*);		/* do not use empty or unexpanded result */
+		stakset(stakp,curoff);			/* restore stack to state on function entry */
 	}
-	av[0] = ".sh.tilde";
-	av[1] = &ptr[offset];
-	av[2] = 0;
-	iop = sftmp((IOBSIZE>PATH_MAX?IOBSIZE:PATH_MAX)+1);
-	sfset(iop,SF_READ,0);
-	sfstdout = iop;
-	if(np)
-		sh_fun(np, (Namval_t*)0, av);
-	else
-		sh_btilde(2, av, &shp->bltindata);
-	sfstdout = save;
-	stkset(shp->stk,ptr, offset);
-	sfseek(iop,(Sfoff_t)0,SEEK_SET);
-	sfset(iop,SF_READ,1);
-	if(ptr = sfreserve(iop, SF_UNBOUND, -1))
+	/*
+	 * Perform default tilde expansion unless overridden.
+	 * Write the result to the stack, if any.
+	 */
+	stakputc(0);
+	if(!cp)
+		cp = sh_tilde(shp,&stakp[offset]);
+	if(cp)
 	{
-		Sfoff_t n = sfvalue(iop);
-		while(ptr[n-1]=='\n')
-			n--;
-		if(n==1 && fcpeek(0)=='/' && ptr[n-1])
-			n--;
-		if(n)
-			sfwrite(shp->stk,ptr,n);
+		stakseek(offset);
+		if(!(cp[0]=='/' && !cp[1] && fcpeek(0)=='/'))
+			stakputs(cp);			/* for ~ == /, avoid ~/foo -> //foo */
 	}
 	else
-		sfputr(shp->stk,av[1],0);
-	sfclose(iop);
+		stakseek(curoff);
 }
 
 /*