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: #217

NEWS

@@ -5,6 +5,11 @@ Any uppercase BUG_* names are modernish shell bug IDs.
 
 2021-03-16:
 
+- Tilde expansion can now be extended or modified by defining a .sh.tilde.get
+  or .sh.tilde.set discipline function. This replaces a 2004 undocumented
+  attempt to add this functionality via a .sh.tilde built-in, which never
+  worked and crashed the shell. See the manual for details on the new method.
+
 - Fixed a bug in interactive shells: if a variable used by the shell called
   a discipline function (such as PS1.get() or COLUMNS.set()), the value of $?
   was set to the exit status of the discipline function instead of the last

src/cmd/ksh93/TYPES

@@ -17,7 +17,7 @@ To define a type, use
 where definition contains assignment commands, declaration commands,
 and function definitions.  A declaration command (for example typeset,
 readonly, and export), is a built-in that differs from other builtins in
-that tilde substitution is performed on arguments after an =, assignments
+that tilde expansion is performed on arguments after an =, assignments
 do not have to precede the command name, and field splitting and pathname
 expansion is not performed on the arguments.
 For example,

src/cmd/ksh93/bltins/typeset.c

@@ -1373,7 +1373,7 @@ static int print_namval(Sfio_t *file,register Namval_t *np,register int flag, st
 		return(0);
 	if(nv_isattr(np,NV_NOPRINT|NV_INTEGER)==NV_NOPRINT)
 	{
-		if(is_abuiltin(np) && strcmp(np->nvname,".sh.tilde"))
+		if(is_abuiltin(np))
 			sfputr(file,nv_name(np),'\n');
 		return(0);
 	}

src/cmd/ksh93/data/variables.c

@@ -102,6 +102,7 @@ const struct shtable2 shtab_variables[] =
 	".sh.math",	0,				(char*)0,
 	".sh.pool",	0,				(char*)0,
 	".sh.pid",	NV_INTEGER|NV_NOFREE,		(char*)0,
+	".sh.tilde",	0,				(char*)0,
 	"SHLVL",	NV_INTEGER|NV_NOFREE|NV_EXPORT,	(char*)0,
 #if SHOPT_MULTIBYTE
 	"CSWIDTH",	0,				(char*)0,

src/cmd/ksh93/include/variables.h

@@ -91,6 +91,7 @@
 #define SH_MATHNOD	(shgd->bltin_nodes+61)
 #define SH_JOBPOOL	(shgd->bltin_nodes+62)
 #define SH_PIDNOD	(shgd->bltin_nodes+63)
-#define SHLVL		(shgd->bltin_nodes+64)
+#define SH_TILDENOD	(shgd->bltin_nodes+64)
+#define SHLVL		(shgd->bltin_nodes+65)
 
 #endif /* SH_VALNOD */

src/cmd/ksh93/sh.1

@@ -782,22 +782,24 @@ Preset aliases can be unset or redefined.
 .B "r=\(fmhist \-s\(fm"
 .PD
 .RE
-.SS Tilde Substitution.
+.SS Tilde Expansion.
 After alias substitution is performed, each word
 is checked to see if it begins with an unquoted
 .BR \(ap .
-For tilde substitution,
+For tilde expansion,
 .I word\^
 also refers to the
 .I word\^
 portion of parameter expansion
 (see
 .I "Parameter Expansion\^"
 below).
-If it does, then the word up to a
+If a
+.I word\^
+is preceded by a tilde, then it is checked up to a
 .B /
-is checked to see if it matches a user name in the
-password database (See
+to see if it matches a user name in the
+password database (see
 .IR getpwname (3).)
 If a match is found, the
 .B \(ap
@@ -810,38 +812,74 @@ by itself, or in front of a
 .BR / ,
 is replaced by
 .SM
-.BR $HOME .
+.BR $HOME ,
+unless the
+.B HOME
+variable is unset, in which case
+the current user's home directory as configured in the operating system
+is used.
 A
 .B \(ap
 followed by a
 .B +
 or
 .B \-
-is replaced by the value of
+is replaced by
 .B
 .SM $PWD
-and
+or
 .B
 .SM $OLDPWD
 respectively.
 .PP
 In addition,
 when expanding a
-.IR "variable assignment" ,
-.I tilde
-substitution is attempted when
+variable assignment (see
+.I Variable Assignments
+above),
+tilde expansion is attempted when
 the value of the assignment
 begins with a
 .BR \(ap ,
 and when a
 .B \(ap
 appears after a
 .BR : .
-The
+A
 .B :
-also terminates a
-.B \(ap
-login name.
+also terminates a user name following a
+.BR \(ap .
+.PP
+The tilde expansion mechanism may be extended or modified
+by defining one of the discipline functions
+.B .sh.tilde.set
+or
+.B .sh.tilde.get
+(see
+.I Functions
+and
+.I Discipline Functions
+below).
+If either exists,
+then upon encountering a tilde word to expand,
+that function is called with the tilde word assigned to either
+.B .sh.value
+(for the
+.B .sh.tilde.set
+function) or
+.B .sh.tilde
+(for the
+.B .sh.tilde.get
+function).
+Performing tilde expansion within a discipline function will not recursively
+call that function, but default tilde expansion remains active,
+so literal tildes should still be quoted where required.
+Either function may assign a replacement string to
+.BR .sh.value .
+If this value is non-empty and does not start with a
+.BR \(ap ,
+it replaces the default tilde expansion when the function terminates.
+Otherwise, the tilde expansion is left unchanged.
 .SS Command Substitution.
 The standard output from a command list enclosed in
 parentheses preceded by a dollar sign (
@@ -5549,7 +5587,7 @@ Commands that are preceded by a \(dd symbol below are
 Any following words
 that are in the format of a variable assignment
 are expanded with the same rules as a variable assignment.
-This means that tilde substitution is performed after the
+This means that tilde expansion is performed after the
 .B =
 sign, array assignments of the form
 \f2varname\^\fP\f3=(\fP\f2assign_list\^\fP\f3)\fP

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);
 }
 
 /*

src/cmd/ksh93/tests/builtins.sh

@@ -689,9 +689,6 @@ v=$($SHELL 2> /dev/null +o rc -ic $'getopts a:bc: opt --man\nprint $?')
 read baz <<< 'foo\\\\bar'
 [[ $baz == 'foo\\bar' ]] || err_exit 'read of foo\\\\bar not getting foo\\bar'
 
-: ~root
-[[ $(builtin) == *.sh.tilde* ]] &&  err_exit 'builtin contains .sh.tilde'
-
 # ======
 # Check that I/O errors are detected <https://github.com/att/ast/issues/1093>
 actual=$(

src/cmd/ksh93/tests/pty.sh

@@ -40,8 +40,6 @@ Darwin | FreeBSD | Linux )
 	exit 0 ;;
 esac
 
-integer lineno=1
-
 # On some systems, the stty command does not appear to work correctly on a pty pseudoterminal.
 # To avoid false regressions, we have to set 'erase' and 'kill' on the real terminal.
 if	test -t 0 2>/dev/null </dev/tty && stty_restore=$(stty -g </dev/tty)
@@ -81,7 +79,7 @@ function tst
 	do	if	[[ $text == *debug* ]]
 		then	print -u2 -r -- "$text"
 		else	offset=${text/*: line +([[:digit:]]):*/\1}
-			err_exit "${text/: line $offset:/: line $(( lineno + offset)):}"
+			err\_exit "$lineno" "${text/: line $offset:/: line $(( lineno + offset)):}"
 		fi
 	done
 }
@@ -92,7 +90,7 @@ unset EDITOR
 export VISUAL=vi PS1=':test-!: ' PS2='> ' PS4=': ' ENV=/./dev/null EXINIT= HISTFILE= TERM=dumb
 
 if	! pty $bintrue < /dev/null
-then	err_exit pty command hangs on $bintrue -- tests skipped
+then	warning "pty command hangs on $bintrue -- tests skipped"
 	exit 0
 fi
 
@@ -734,6 +732,7 @@ w echo "Exit status is: $?"
 u Exit status is: 1
 !
 
+# err_exit #
 ((SHOPT_ESH)) && ((SHOPT_VSH)) && tst $LINENO <<"!"
 L crash after switching from emacs to vi mode
 
@@ -754,5 +753,24 @@ r ^:test-2: echo Success\r\n$
 r ^Success\r\n$
 !
 
+# err_exit #
+((SHOPT_VSH || SHOPT_ESH)) && tst $LINENO <<"!"
+L value of $? after tilde expansion in tab completion
+
+# Make sure that a .sh.tilde.set discipline function
+# cannot influence the exit status.
+
+w [[ -o ?vi ]] || set -o emacs
+w .sh.tilde.set() { true; }
+w HOME=/tmp
+w false ~\t
+u false /tmp
+w echo "Exit status is: $?"
+u Exit status is: 1
+w (exit 42)
+w echo $? ~\t
+u 42 /tmp
+!
+
 # ======
 exit $((Errors<125?Errors:125))