Fix "/dev/null: cannot create [Bad file descriptor]" (re: feeb62d1)

Intermittent regression test failure:

test coprocess begins at 2022-02-08+23:41:52
/usr/local/src/ksh93/ksh/src/cmd/ksh93/tests/coprocess.sh[116]: /dev/null: cannot create [Bad file descriptor]
	coprocess.sh[118]: FAIL: /bin/cat coprocess hung after 'exec 5<&p 6>&p; exec 5<&- 6>&-'
test coprocess failed at 2022-02-08+23:41:56 with exit code 1 [ 33 tests 1 error ]

The coprocess didn't hang; the 2>/dev/null redirection in the
regression test somehow failed.

Running the regression test in a loop 10000 times is a fairly
reliable way of reproducing the failure on my system. This has
allowed me to find the commit that introduced it. It is: feeb62d1

How odd. That commit merely sets errno to EBADF if sh_close() is
called with a file descriptor < 0. I don't understand how this
causes a race condition.

But calling abort() in the location of the feeb62d1 patch did allow
me to trace all the sh_close() calls with a negative file
descriptor. Those shouldn't be happening in any case.

All those sh_close() calls were related to co-processes, which that
regression test also tests. A co-process pipe that is not in use is
set to -1 so we need to avoid calling sh_close() if that happened.

This commit does that (as well as making some consistency tweaks)
and it reliably makes the race condition go away on my system. The
"why" of this remains a mystery as the only difference is that
errno is not set to EBADF in these situations, and both
sh_redirect() and sh_open() explicitly set errno to 0.

Resolves: https://github.com/ksh93/ksh/issues/483

src/cmd/ksh93/sh/io.c

@@ -1045,9 +1045,9 @@ static Sfoff_t	file_offset(int fn, char *fname)
  */
 void sh_pclose(register int pv[])
 {
-	if(pv[0]>=2)
+	if(pv[0] > -1)
 		sh_close(pv[0]);
-	if(pv[1]>=2)
+	if(pv[1] > -1)
 		sh_close(pv[1]);
 	pv[0] = pv[1] = -1;
 }

src/cmd/ksh93/sh/jobs.c

@@ -396,10 +396,11 @@ int job_reap(register int sig)
 			/* check for coprocess completion */
 			if(pid==sh.cpid)
 			{
-				sh_close(sh.coutpipe);
-				sh_close(sh.cpipe[1]);
-				sh.cpipe[1] = -1;
-				sh.coutpipe = -1;
+				if(sh.coutpipe > -1)
+					sh_close(sh.coutpipe);
+				if(sh.cpipe[1] > -1)
+					sh_close(sh.cpipe[1]);
+				sh.coutpipe = sh.cpipe[1] = -1;
 			}
 			else if(sh.subshell)
 				sh_subjobcheck(pid);

src/cmd/ksh93/sh/path.c

@@ -1275,10 +1275,13 @@ static noreturn void exscript(register char *path,register char *argv[],char **e
 	sh.bckpid = 0;
 	sh.st.ioset=0;
 	/* clean up any cooperating processes */
-	if(sh.cpipe[0]>0)
+	if(sh.cpipe[0] > -1)
 		sh_pclose(sh.cpipe);
-	if(sh.cpid && sh.outpipe)
+	if(sh.cpid && sh.outpipe && *sh.outpipe > -1)
+	{
 		sh_close(*sh.outpipe);
+		*sh.outpipe = -1;
+	}
 	sh.cpid = 0;
 	if(sp=fcfile())
 		while(sfstack(sp,SF_POPSTACK));

src/cmd/ksh93/sh/subshell.c

@@ -455,8 +455,10 @@ void sh_subjobcheck(pid_t pid)
 	{
 		if(sp->cpid==pid)
 		{
-			sh_close(sp->coutpipe);
-			sh_close(sp->cpipe);
+			if(sp->coutpipe > -1)
+				sh_close(sp->coutpipe);
+			if(sp->cpipe > -1)
+				sh_close(sp->cpipe);
 			sp->coutpipe = sp->cpipe = -1;
 			return;
 		}