Fix unsetting aliases in subshells

Aliases can now be correctly unset within subshell environments
(such as ( ... ), $(command substitutions), etc), as well as
non-subshell "shared" command substitutions (${ ...; }). Before,
attempts to unset aliases within these were silently ignored.

Prior discussion: https://github.com/att/ast/issues/108

Subshell alias trees are only referenced in a few places in the
code, *and* have always been broken, so this commit gets rid of the
whole notion of a subshell alias tree. Instead, there is now just
one flat alias tree, and subshells fork into a separate process
when aliases are set or unset within them. It is not really
conceivable that this could be a performance-sensitive operation,
or even a common one, so this is a clean fix with no downside.

src/cmd/ksh93/include/defs.h:
- Remove sh_subaliastree() definition.

src/cmd/ksh93/sh/subshell.c:
- Remove salias element (pointer to subshell alias tree) from
  subshell struct.
- Remove sh_subaliastree() function.
- sh_subshell(): Remove alias subshell tree cleanup.

src/cmd/ksh93/bltins/typeset.c:
- b_alias(): If in subshell, fork before setting alias.
- b_unalias(): If in subshell, fork before unsetting alias.
- unall(): Remove sh_subaliastree() call.

src/cmd/ksh93/sh/name.c:
- nv_open(): Remove sh_subaliastree() call.

src/cmd/ksh93/tests/subshell.sh:
- Add regression tests for unsetting or redefining aliases within
  subshells.

(cherry picked from commit 12a15605b9521a2564a6e657905705a060e89095)

NEWS

@@ -18,6 +18,9 @@ Any uppercase BUG_* names are modernish shell bug IDs.
   This now correctly lists your directory and then prints "ls executed",
   instead of printing "ls executed" twice.
 
+- Fix a similar bug with aliases. These can now be correctly unset
+  in subshell environments.
+
 2020-05-21:
 
 - Fix truncating of files with the combined redirections '<>;file' and

src/cmd/ksh93/bltins/typeset.c

@@ -197,6 +197,8 @@ int    b_alias(int argc,register char *argv[],Shbltin_t *context)
 			troot = tdata.sh->track_tree;
 		}
 	}
+	if(context->shp->subshell && !context->shp->subshare)
+		sh_subfork();
 	return(setall(argv,flag,troot,&tdata));
 }
 
@@ -1142,6 +1144,8 @@ int    b_set(int argc,register char *argv[],Shbltin_t *context)
 int    b_unalias(int argc,register char *argv[],Shbltin_t *context)
 {
 	Shell_t *shp = context->shp;
+	if(shp->subshell && !shp->subshare)
+		sh_subfork();
 	return(unall(argc,argv,shp->alias_tree,shp));
 }
 
@@ -1161,11 +1165,7 @@ static int unall(int argc, char **argv, register Dt_t *troot, Shell_t* shp)
 	struct checkpt buff;
 	NOT_USED(argc);
 	if(troot==shp->alias_tree)
-	{
 		name = sh_optunalias;
-		if(shp->subshell)
-			troot = sh_subaliastree(0);
-	}
 	else
 		name = sh_optunset;
 	while(r = optget(argv,name)) switch(r)

src/cmd/ksh93/include/defs.h

@@ -423,7 +423,6 @@ extern void		sh_printopts(Shopt_t,int,Shopt_t*);
 extern int 		sh_readline(Shell_t*,char**,volatile int,int,ssize_t,long);
 extern Sfio_t		*sh_sfeval(char*[]);
 extern void		sh_setmatch(Shell_t*,const char*,int,int,int[],int);
-extern Dt_t		*sh_subaliastree(int);
 extern void             sh_scope(Shell_t*, struct argnod*, int);
 extern Namval_t		*sh_scoped(Shell_t*, Namval_t*);
 extern Dt_t		*sh_subfuntree(int);

src/cmd/ksh93/sh/name.c

@@ -1390,8 +1390,6 @@ Namval_t *nv_open(const char *name, Dt_t *root, int flags)
 		msg = e_aliname;
 		while((c= *(unsigned char*)cp++) && (c!='=') && (c!='/') &&
 			(c>=0x200 || !(c=sh_lexstates[ST_NORM][c]) || c==S_EPAT || c==S_COLON));
-		if(shp->subshell && c=='=')
-			root = sh_subaliastree(1);
 		if(c= *--cp)
 			*cp = 0;
 		np = nv_search(name, root, (flags&NV_NOADD)?0:NV_ADD); 

src/cmd/ksh93/sh/subshell.c

@@ -71,7 +71,6 @@ static struct subshell
 	Dt_t		*var;	/* variable table at time of subshell */
 	struct Link	*svar;	/* save shell variable table */
 	Dt_t		*sfun;	/* function scope for subshell */
-	Dt_t		*salias;/* alias scope for subshell */
 	Pathcomp_t	*pathlist; /* for PATH variable */
 #if (ERROR_VERSION >= 20030214L)
 	struct Error_context_s *errcontext;
@@ -375,24 +374,6 @@ static void nv_restore(struct subshell *sp)
 	sp->shpwd=save;
 }
 
-/*
- * return pointer to alias tree
- * create new one if in a subshell and one doesn't exist and create is non-zero
- */
-Dt_t *sh_subaliastree(int create)
-{
-	register struct subshell *sp = subshell_data;
-	if(!sp || sp->shp->curenv==0)
-		return(sh.alias_tree);
-	if(!sp->salias && create)
-	{
-		sp->salias = dtopen(&_Nvdisc,Dtoset);
-		dtview(sp->salias,sp->shp->alias_tree);
-		sp->shp->alias_tree = sp->salias;
-	}
-	return(sp->salias);
-}
-
 /*
  * return pointer to function tree
  * create new one if in a subshell and one doesn't exist and create is non-zero
@@ -710,12 +691,6 @@ Sfio_t *sh_subshell(Shell_t *shp,Shnode_t *t, volatile int flags, int comsub)
 	{
 		int n;
 		shp->options = sp->options;
-		if(sp->salias)
-		{
-			shp->alias_tree = dtview(sp->salias,0);
-			table_unset(sp->salias,0);
-			dtclose(sp->salias);
-		}
 		if(sp->sfun)
 		{
 			shp->fun_tree = dtview(sp->sfun,0);

src/cmd/ksh93/tests/subshell.sh

@@ -652,5 +652,34 @@ v=$("$SHELL" -c "$(cat "$a")") && [[ $v == ok ]] || err_exit 'fail: more fun 2'
 v=$("$SHELL" -c 'eval "$(cat "$1")"' x "$a") && [[ $v == ok ]] || err_exit "fail: more fun 3"
 v=$("$SHELL" -c '. "$1"' x "$a") && [[ $v == ok ]] || err_exit "fail: more fun 4"
 
+# ======
+# Unsetting or redefining aliases within subshells
+
+# ...alias can be unset in subshell
+
+alias al="echo 'mainalias'"
+
+(unalias al; alias al >/dev/null) && err_exit 'alias fails to be unset in subshell'
+
+v=$(unalias al; alias al >/dev/null) && err_exit 'alias fails to be unset in comsub'
+
+[[ $(eval 'al') == 'mainalias' ]] || err_exit 'main alias fails to survive unset in subshell(s)'
+
+v=${ eval 'al'; unalias al 2>&1; } && [[ $v == 'mainalias' ]] && ! alias al >/dev/null \
+|| err_exit 'main shell alias wrongly survives unset within ${ ...; }'
+
+# ...alias can be redefined in subshell
+
+alias al="echo 'mainalias'"
+
+(alias al='echo sub'; [[ $(eval 'al') == sub ]]) || err_exit 'alias fails to be redefined in subshell'
+
+v=$(alias al='echo sub'; eval 'al') && [[ $v == sub ]] || err_exit 'alias fails to be redefined in comsub'
+
+[[ $(eval 'al') == 'mainalias' ]] || err_exit 'main alias fails to survive redefinition in subshell(s)'
+
+v=${ eval 'al'; alias al='echo subshare'; } && [[ $v == 'mainalias' && $(eval 'al') == subshare ]] \
+|| err_exit 'alias redefinition fails to survive ${ ...; }'
+
 # ======
 exit $((Errors<125?Errors:125))