Backport bugfix for BUG_CSUBSTDO from ksh93v- 2012-08-24 (#259)

This commit fixes BUG_CSUBSTDO, which could break stdout inside of
non-forking command substitutions. The breakage only occurred when
stdout was closed outside of the command substitution and a file
descriptor other than stdout was redirected in the command substitution
(such as stderr). Thanks to the ast-open-history repo, I was able to
identify and backport the bugfix from ksh93v- 2012-08-24.

This backport may fix other bugs as well. On 93v- 2012-08-24 it
fixed the regression below, though it was not triggered on 93u+(m).
  src/cmd/ksh93/tests/heredoc.sh
  487 print foo > $tmp/foofile 
  488 x=$( $SHELL 2> /dev/null 'read <<< $(<'"$tmp"'/foofile) 2> /dev/null;print -r "$REPLY"') 
  489 [[ $x == foo ]] || err_exit '<<< $(<file) not working' 

src/cmd/ksh93/sh/io.c: sh_open():
- If the just-opened file descriptor exists in sftable and is
  flagged with SF_STRING (as in non-forking command substitutions,
  among other situations), then move the file descriptor to a
  number >= 10.

src/cmd/ksh93/tests/io.sh:
- Add a regression test for BUG_CSUBSTDO, adapted from the one in
  modernish.

NEWS

@@ -8,6 +8,10 @@ Any uppercase BUG_* names are modernish shell bug IDs.
 - The $LC_TIME variable is now recognized by ksh and if set to an invalid
   locale will show an error.
 
+- Fixed BUG_CSUBSTDO: If standard output is closed before running a command
+  substitution, redirecting any other file descriptor no longer closes standard
+  output inside of the command substitution.
+
 2021-04-05:
 
 - Fixed a regression, introduced in ksh 93t+ 2009-07-31, that caused a command

TODO

@@ -23,12 +23,6 @@ https://github.com/modernish/modernish/tree/0.16/lib/modernish/cap/
   initial ! (and, on some shells, ^) retains the meaning of negation, even
   in quoted strings within bracket patterns, including quoted variables.
 
-- BUG_CSUBSTDO: If standard output (file descriptor 1) is closed before
-  entering a $(command substitution), and any other file descriptors are
-  redirected within the command substitution, commands such as 'echo' will
-  not work within the command substitution, acting as if standard output is
-  still closed.
-
 - BUG_IFSGLOBS: In glob pattern matching (as in case or parameter
   substitution with # and %), if IFS starts with ? or * and the "$*"
   parameter expansion inserts any IFS separator characters, those characters

src/cmd/ksh93/sh/io.c

@@ -755,6 +755,7 @@ onintr(struct addrinfo* addr, void* handle)
 int sh_open(register const char *path, int flags, ...)
 {
 	Shell_t			*shp = sh_getinterp();
+	Sfio_t			*sp;
 	register int		fd = -1;
 	mode_t			mode;
 	char			*e;
@@ -869,6 +870,16 @@ int sh_open(register const char *path, int flags, ...)
 		mode = IOREAD;
 	if(fd >= shp->gd->lim.open_max)
 		sh_iovalidfd(shp,fd);
+	if((sp = shp->sftable[fd]) && (sfset(sp,0,0) & SF_STRING))
+	{
+		int n,err=errno;
+		if((n = sh_fcntl(fd,F_DUPFD,10)) >= 10)
+		{
+			while(close(fd) < 0 && errno == EINTR)
+				errno = err;
+			fd = n;
+		}
+	}
 	shp->fdstatus[fd] = mode;
 	return(fd);
 }

src/cmd/ksh93/tests/io.sh

@@ -785,5 +785,15 @@ then
 		"(expected $(printf %q "$expect"), got $(printf %q "$actual"))"
 fi
 
+# ======
+# Test for BUG_CSUBSTDO: If stdout is closed before running a command substitution,
+# redirecting any file descriptor in the command substitution would break stdout
+# inside of the command substitution. This only occurred when redirecting any other
+# file descriptor inside of the command substitution.
+exp='Foo bar'
+{ got=$(echo 'Foo bar' 2>/dev/null); } >&-
+[[ $exp == $got ]] || err_exit "BUG_CSUBSTDO: Closing stdout outside of command substitution breaks stdout inside of command substitution" \
+	"(expected $(printf %q "$exp"), got $(printf %q "$got"))"
+
 # ======
 exit $((Errors<125?Errors:125))