Fix process substitution infinite loops in virtual machines

src/cmd/ksh93/sh/xec.c:
- An infinite loop may occur if an orphaned process from a process
  substitution isn't adopted by PID 1. Rather than close the FIFO
  when the parent process is init, check for the existence of the
  the parent shell or parent forked subshell by using kill(2).

src/cmd/ksh93/include/defs.h,
src/cmd/ksh93/sh/init.c,
src/cmd/ksh93/sh/subshell.c:
- Add a new 'parent_pid' variable to store the correct PPID. The
  PPID cannot be obtained after creating the process substitution
  (as that causes intermittent looping) and 'shgd->current_pid'
  can't be used as it's the process substitution's PID. As it's
  only used for FIFO process substitutions, hide it behind
  !SHOPT_DEVFD.

src/cmd/ksh93/tests/io.sh:
- Test for the infinite looping bug by using a forked
  command substitution.

Fixes: ksh93#213

NEWS

@@ -3,6 +3,10 @@ For full details, see the git log at: https://github.com/ksh93/ksh
 
 Any uppercase BUG_* names are modernish shell bug IDs.
 
+2021-03-10:
+- Fixed an intermittent bug that caused process substitutions to infinitely
+  loop in Linux virtual machines that use systemd.
+
 2021-03-09:
 
 - The ${!foo@} and ${!foo*} expansions yield variable names beginning with foo,

src/cmd/ksh93/include/defs.h

@@ -138,6 +138,9 @@ struct shared
 	pid_t		pid;		/* $$, the main shell's PID (invariable) */
 	pid_t		ppid;		/* $PPID, the main shell's parent's PID */
 	pid_t		current_pid;	/* ${.sh.pid}, PID of current ksh process (updates when subshell forks) */
+#if !SHOPT_DEVFD
+	pid_t		parent_pid;	/* PID of the parent shell or subshell (used in process substitutions) */
+#endif
 	int		realsubshell;	/* ${.sh.subshell}, actual subshell level (including virtual and forked) */
 	unsigned char	sigruntime[2];
 	Namval_t	*bltin_nodes;

src/cmd/ksh93/sh/init.c

@@ -1238,7 +1238,11 @@ Shell_t *sh_init(register int argc,register char *argv[], Shinit_f userinit)
 		sh_regress_init(shp);
 #endif
 		shgd = sh_newof(0,struct shared,1,0);
+#if !SHOPT_DEVFD
+		shgd->parent_pid = shgd->current_pid = shgd->pid = getpid();
+#else
 		shgd->current_pid = shgd->pid = getpid();
+#endif
 		shgd->ppid = getppid();
 		shgd->userid=getuid();
 		shgd->euserid=geteuid();
@@ -1634,7 +1638,11 @@ int sh_reinit(char *argv[])
 	shp->errtrap = 0;
 	shp->end_fn = 0;
 	/* update ${.sh.pid}, $$, $PPID */
+#if !SHOPT_DEVFD
+	shgd->parent_pid = shgd->current_pid = shgd->pid = getpid();
+#else
 	shgd->current_pid = shgd->pid = getpid();
+#endif
 	shgd->ppid = getppid();
 	return(1);
 }

src/cmd/ksh93/sh/subshell.c

@@ -215,6 +215,9 @@ void sh_subfork(void)
 		sp->subpid=0;
 		shp->st.trapcom[0] = (comsub==2 ? NULL : trap);
 		shp->savesig = 0;
+#if !SHOPT_DEVFD
+		shgd->parent_pid = shgd->current_pid;
+#endif
 		/* sh_fork() increases ${.sh.subshell} but we forked an existing virtual subshell, so undo */
 		shgd->realsubshell--;
 	}

src/cmd/ksh93/sh/xec.c

@@ -85,8 +85,7 @@ struct funenv
     static void fifo_check(void *handle)
     {
 	Shell_t	*shp = (Shell_t*)handle;
-	pid_t pid = getppid();
-	if(pid==1)
+	if(kill(shp->gd->parent_pid,0) < 0)
 	{
 		unlink(shp->fifo);
 		sh_done(shp,0);
@@ -1894,7 +1893,11 @@ int sh_exec(register const Shnode_t *t, int flags)
 					_sh_fork(shp,pid,0,0);
 				if(pid==0)
 				{
+#if !SHOPT_DEVFD
+					shgd->parent_pid = shgd->current_pid = getpid();
+#else
 					shgd->current_pid = getpid();
+#endif
 					sh_exec(t->par.partre,flags);
 					shp->st.trapcom[0]=0;
 					sh_done(shp,0);

src/cmd/ksh93/tests/io.sh

@@ -715,5 +715,18 @@ got=$(export tmp; "$SHELL" -ec \
 (redirect {v}>$tmp/v.out; echo ok2 >&$v) 2>/dev/null
 [[ -r $tmp/v.out && $(<$tmp/v.out) == ok2 ]] || err_exit 'redirect {varname}>file not working in a subshell'
 
+# ======
+# Test for process substitution infinite looping
+procsub_pid="$(
+	ulimit -t unlimited  # fork the subshell
+	true >(true)
+	echo "$!"
+)"
+sleep 2 # wait for process to close (long wait to avoid false failures)
+
+kill -0 $procsub_pid 2> /dev/null &&
+	kill -KILL $procsub_pid && # don't leave around what is effectively a zombie process
+	err_exit "process substitutions loop after parent shell finishes"
+
 # ======
 exit $((Errors<125?Errors:125))