Fix shell exit on function call redirection error (re: 23f2e23)

This regression also exists on ksh 93v- and ksh2020, from which it
was backported.

Reproducer:

$ (fn() { true; }; fn >/dev/null/ne; true) 2>/dev/null; echo $?
1

Expected output: 0 (as on ksh 93u+).

FreeBSD sh and NetBSD sh are the only other known shells that share
this behaviour. POSIX currently allows both behaviours, but may
require the ksh 93u+ behaviour in future. In any case, this causes
an incompatibility with established ksh behaviour that could easily
break existing ksh scripts.

src/cmd/ksh93/sh/xec.c: sh_exec():
- Commit 23f2e23 introduced a check for jmpval > SH_JMPIO (5).
  When a function call pushes context for a redirection, this is
  done with the jmpval exit value of SH_JMPCMD (6). Change that to
  SH_JMPIO to avoid triggering that check.

src/cmd/ksh93/tests/exit.sh:
- Add regression tests for exit behaviour on various kinds of
  shell errors as listed in the POSIX standard, including an error
  in a redirection of a function call.

Fixes: https://github.com/ksh93/ksh/issues/310

NEWS

@@ -7,6 +7,9 @@ Any uppercase BUG_* names are modernish shell bug IDs.
 
 - Fixed SHLVL so that replacing ksh by itself (exec ksh) will not increase it.
 
+- Fixed a regression introduced on 2020-08-05 that caused a non-interactive
+  shell to exit if an I/O redirection of a function call encountered an error.
+
 2021-05-13:
 
 - Fixed a bug with 'test -t 1' that was introduced on 2021-04-26:

src/cmd/ksh93/sh/xec.c

@@ -1550,7 +1550,7 @@ int sh_exec(register const Shnode_t *t, int flags)
 					if(io)
 					{
 						indx = shp->topfd;
-						sh_pushcontext(shp,buffp,SH_JMPCMD);
+						sh_pushcontext(shp,buffp,SH_JMPIO);
 						jmpval = sigsetjmp(buffp->buff,0);
 					}
 					if(jmpval == 0)

src/cmd/ksh93/tests/exit.sh

@@ -127,5 +127,57 @@ exp=1
 [[ $exp == $status ]] || err_exit 'bare exit after false' \
 	"(expected '$exp', got '$status')"
 
+# ======
+# Exit behaviour for commands, expansions and assignments
+# https://pubs.opengroup.org/onlinepubs/9699919799/utilities/V3_chap02.html#tag_18_08_01
+
+builtin -s | while read cmd
+do
+	[[ $cmd == : ]] && continue  # ':' never errors out
+	("$cmd" --badoption; true) 2>/dev/null \
+	&& err_exit "Special built-in utility error does not cause shell to exit ($cmd)"
+	(command "$cmd" --badoption; true) 2>/dev/null \
+	|| err_exit "Error in special built-in utility prefixed by 'command' causes shell to exit ($cmd)"
+done
+
+(command shift 10; true) 2>/dev/null \
+|| err_exit "Regular built-in utility error causes shell to exit"
+
+(readonly foo=bar; foo=baz; true) 2>/dev/null \
+&& err_exit "Variable assignment error does not cause shell to exit"
+
+(command true ${foo@bar}; true) 2>/dev/null \
+&& err_exit "Expansion error does not cause shell to exit"
+
+(/dev/null/nonexistent; true) 2>/dev/null \
+|| err_exit "Command not found causes shell to exit"
+
+# Exit behaviour for redirections
+
+(>/dev/null/nonexistent; true) 2>/dev/null \
+|| err_exit 'Error in lone redirection causes shell to exit'
+
+(: >/dev/null/nonexistent; true) 2>/dev/null \
+&& err_exit 'Redirection error with special built-in utility does not cause shell to exit'
+
+(false >/dev/null/nonexistent; true) 2>/dev/null \
+|| err_exit 'Redirection error with regular built-in utility causes shell to exit'
+
+("$(whence -p false)" >/dev/null/nonexistent; true) 2>/dev/null \
+|| err_exit 'Redirection error with external command causes shell to exit'
+
+(/dev/null/nonexistent >/dev/null/nonexistent; true) 2>/dev/null \
+|| err_exit 'Redirection error with nonexistent command causes shell to exit'
+
+(foo=bar >/dev/null/nonexistent; true) 2>/dev/null \
+|| err_exit 'Redirection error with variable assignment causes shell to exit'
+
+({ false; }; >/dev/null/nonexistent; true) 2>/dev/null \
+|| err_exit 'Redirection error with compound command causes shell to exit'
+
+# https://github.com/ksh93/ksh/issues/310
+(fn() { false; }; fn >/dev/null/nonexistent; true) 2>/dev/null \
+|| err_exit 'Redirection error with function execution causes shell to exit'
+
 # ======
 exit $((Errors<125?Errors:125))