Implement a better fix for unsetting special env vars

The regression this commit fixes was first introduced in ksh93t
2008-07-25. It was previously worked around in 6f0e008 by forking
subshells if any special environment variable is unset.

The reason why this problem doesn't occur in ksh93s+ is because in
that version of ksh sh_assignok never moves nodes, it only clones
them. The second argument doesn't set NV_MOVE, which makes
`sh_assignok(np,0)` very similar to `sh_assignok(np,1)`. In ksh93t and
higher, setting second argument to zero causes the node to be moved
with NV_MOVE, which causes the discipline function associated with
the variable node to be removed when `np->nvfun` is set to zero (i.e.
NULL). This is why a command like `(unset LC_NUMERIC; LC_NUMERIC=invalid)`
doesn't print a diagnostic, as it looses its discipline function.

This patch fixes the problem by cloning the node with sh_assignok
if it is a special variable with a discipline function. This allows
special variables to work as expected in virtual subshells. The
original workaround has been kept for the $PATH variable only, as
hash tables are still broken in virtual subshells. It has been updated
accordingly to only fork subshells if it detects the variable node
for PATH. I have added two more regression tests for changing the
PATH in subshells to make sure hash tables continue working as
expected with this fix.

src/cmd/ksh93/bltins/typeset.c:
 - Only fork virtual subshells if the PATH will be changed. If a
   variable is a special variable with a discipline function, it
   should be just be cloned, not moved.

src/cmd/ksh93/sh/nvdisc.c:
 - Add a comment to clarify that NV_MOVE will delete the discipline
   function associated with the node.

src/cmd/ksh93/tests/subshells.sh:
 - Add two more regression tests for unsetting the PATH in subshells,
   one for if PATH is being pointed to by a nameref. Condense the
   hash table tests by moving the main test into a single function.

src/cmd/ksh93/bltins/typeset.c

@@ -1208,11 +1208,9 @@ static int unall(int argc, char **argv, register Dt_t *troot, Shell_t* shp)
 	}
 
 	/*
-	 * Check for variables with internal trap/discipline functions (PATH, LANG, LC_*, LINENO, etc.).
-	 * Unsetting these in a virtual/non-forked subshell would cause them to lose their discipline actions,
-	 * so, for example, (unset PATH; PATH=/dev/null; ls) would run 'ls'! Until a fix is found, make the problem
-	 * go away by forking the subshell. To avoid crashing, this must be done before calling sh_pushcontext(),
-	 * so we need to loop through the args separately to check if any variable to unset has a discipline function.
+	 * Unsetting the PATH in a non-forked subshell will cause the parent shell's hash table to be reset,
+	 * so fork to work around the problem. To avoid crashing, this must be done before calling sh_pushcontext(),
+	 * so we need to loop through the args separately and check if any node is equal to PATHNOD.
 	 */
 	if(shp->subshell && !shp->subshare && troot==shp->var_tree)
 	{
@@ -1222,8 +1220,8 @@ static int unall(int argc, char **argv, register Dt_t *troot, Shell_t* shp)
 			np=nv_open(name,troot,NV_NOADD|nflag);
 			if(!np)
 				continue;
-			/* Has discipline function, and is not a nameref? */
-			if(np->nvfun && np->nvfun->disc && !nv_isref(np))
+			/* Are we changing the PATH? */
+			if(np==PATHNOD)
 			{
 				nv_close(np);
 				sh_subfork();
@@ -1276,7 +1274,18 @@ static int unall(int argc, char **argv, register Dt_t *troot, Shell_t* shp)
 				}
 
 				if(shp->subshell)
-					np=sh_assignok(np,0);
+				{
+					if(!nv_isattr(np,NV_NODISC|NV_ARRAY) && !nv_isvtree(np))
+					{
+						/*
+						 * Variables with internal trap/discipline functions (LC_*, LINENO, etc.) need to be
+						 * cloned, as moving them will remove the discipline function.
+						 */
+						np=sh_assignok(np,1);
+					}
+					else
+						np=sh_assignok(np,0);
+				}
 			}
 
 			if(!nv_isnull(np) || nv_size(np) || nv_isattr(np,~(NV_MINIMAL|NV_NOFREE)))

src/cmd/ksh93/sh/nvdisc.c

@@ -962,7 +962,7 @@ int nv_clone(Namval_t *np, Namval_t *mp, int flags)
 	{
 		if(nv_isattr(np,NV_INTEGER))
 			mp->nvalue.ip = np->nvalue.ip;
-		np->nvfun = 0;
+		np->nvfun = 0; /* This will remove the discipline function, if there is one */
 		np->nvalue.cp = 0;
 		if(!nv_isattr(np,NV_MINIMAL) || nv_isattr(mp,NV_EXPORT))
 		{

src/cmd/ksh93/tests/subshell.sh

@@ -710,13 +710,19 @@ v=${ eval 'al'; alias al='echo subshare'; } && [[ $v == 'mainalias' && $(eval 'a
 || err_exit 'alias redefinition fails to survive ${ ...; }'
 
 # Resetting a subshell's hash table should not affect the parent shell
-(hash -r)
-[[ $(hash) ]] || err_exit $'resetting the hash table in a subshell affects the parent shell\'s hash table'
-(PATH="$PATH")
-[[ $(hash) ]] || err_exit $'resetting the PATH in a subshell affects the parent shell\'s hash table'
+check_hash_table()
+{
+	[[ $(hash) ]] || err_exit $'resetting the hash table in a subshell affects the parent shell\'s hash table'
+	# Ensure the hash table isn't empty before the next test is run
+	hash -r chmod
+}
+
+(hash -r); check_hash_table
+(PATH="$PATH"); check_hash_table
+(unset PATH); check_hash_table
+(nameref PATH_TWO=PATH; unset PATH_TWO); check_hash_table
 
 # Adding a utility to a subshell's hash table should not affect the parent shell
-hash -r chmod
 (hash cat)
 [[ $(hash) == "chmod=$(whence -p chmod)" ]] || err_exit $'changes to a subshell\'s hash table affect the parent shell'