Fork in case of negative pwdfd while inside of cd(1)

Preempting a fork is now unnecessary, as cd can just fork if it knows
the current pwdfd is invalid.

src/cmd/ksh93/bltins/cd_pwd.c

@@ -136,7 +136,7 @@ int	b_cd(int argc, char *argv[],Shbltin_t *context)
 	 * If sh_subshell() in subshell.c cannot use fchdir(2) to restore the PWD using a saved file descriptor,
 	 * we must fork any virtual subshell now to avoid the possibility of ending up in the wrong PWD on exit.
 	 */
-	if(sh.subshell && !sh.subshare && !test_inode(sh.pwd,e_dot))
+	if(sh.subshell && !sh.subshare && (!sh_validate_subpwdfd() || !test_inode(sh.pwd,e_dot)))
 		sh_subfork();
 	/*
 	 * Do $CDPATH processing, except if the path is absolute or the first component is '.' or '..'

src/cmd/ksh93/include/defs.h

@@ -143,6 +143,7 @@ extern void		sh_subjobcheck(pid_t);
 extern int		sh_subsavefd(int);
 extern void		sh_subtmpfile(void);
 extern void		sh_pwdupdate(int);
+extern int		sh_validate_subpwdfd(void);
 extern char 		*sh_substitute(const char*,const char*,char*);
 extern void		sh_timetraps(void);
 extern const char	*_sh_translate(const char*);

src/cmd/ksh93/sh/subshell.c

@@ -458,6 +458,16 @@ void sh_pwdupdate(int fd)
 	sh.pwdfd = fd;
 }
 
+/*
+ * Check if the parent shell has a valid PWD fd to return to.
+ * Only for use by cd inside of virtual subshells.
+ */
+int sh_validate_subpwdfd(void)
+{
+	struct subshell *sp = subshell_data;
+	return sp->pwdfd > 0;
+}
+
 /*
  * Run command tree <t> in a virtual subshell
  * If comsub is not null, then output will be placed in temp file (or buffer)
@@ -597,8 +607,6 @@ Sfio_t *sh_subshell(Shnode_t *t, volatile int flags, int comsub)
 		if(sh.savesig < 0)
 		{
 			sh.savesig = 0;
-			if(sp->pwdfd < 0 && !sh.subshare)	/* if we don't have a file descriptor to our PWD ... */
-				sh_subfork();			/* ...we have to fork, as we cannot fchdir back to it. */
 			/* Virtual subshells are not safe to suspend (^Z, SIGTSTP) in the interactive main shell. */
 			if(sh_isstate(SH_INTERACTIVE))
 			{