nvdisc.c: Fix crash after an error or signal in discipline function

[JohnoKing]

This patch fixes the crashes experienced when a discipline function (such as .sh.tilde.set() or PS2.get()) exits because of a signal or an error from a special builtin. The crashes were caused by ksh entering an inconsistent state after performing a longjmp away from the assign() and lookup() functions in nvdisc.c. Fixing the crash requires entering a new context, then setting a nonlocal goto with sigsetjmp. Any longjmps that happen while running the discipline function will go back to assign/lookup, allowing ksh to do a proper cleanup afterwards.

I haven't added the regression test for this bug to pty.sh. The test I made does work when ran with the bugfix applied to ksh, but it causes CPU-hungry hangups in unpatched versions of ksh. I couldn't create a good workaround for the hangups, but I'll leave the test here in case anyone manages to fix it:

--- a/src/cmd/ksh93/tests/pty.sh
+++ b/src/cmd/ksh93/tests/pty.sh
@@ -907,5 +907,26 @@ w true); echo "Exit status is $?"
 u Exit status is 0
 !
 
+# err_exit #
+tst $LINENO << "!"
+L crash when discipline functions exit with an error
+# https://github.com/ksh93/ksh/issues/346
+
+w PS1.get() {; printf '$ '; trap --invalid-flag 2>/dev/null; }
+w PS2.get() {; printf '> '; trap --invalid-flag 2>/dev/null; }
+w .sh.tilde.set() {
+w case ${.sh.value} in
+w '~ret') .sh.value='Exit status is' ;;
+w esac
+w trap --invalid-flag 2>/dev/null
+w }
+w echo ~
+w echo ~ret
+s 50
+w echo ~
+w echo ~ret $?
+u Exit status is 0
+!
+
 # ======
 exit $((Errors<125?Errors:125))

Resolves: #346

[McDutchie]

Thanks, works for me.

[McDutchie]

I'll try to take a look at the regression test later. But I think it's nonessential as this seems to be a straightforward fix.