Fix restoring DEBUG trap upon exiting virtual subshell

This trap failed to be restored correctly when being trapped in
a subshell, causing corruption or a crash when restoring the
parent shell environment's trap upon leaving the subshell.

Thanks to Koichi Nakashima for the report and reproducer.

src/cmd/ksh93/sh/fault.c: sh_sigreset():
- Fix an off-by-one error in the loop that restores the
  pseudosignal traps.

src/cmd/ksh93/tests/basic.sh:
- Test overwriting the main shell trap in a subshell for all
  pseudosignals.

Makes progress on: https://github.com/ksh93/ksh/issues/155

NEWS

@@ -3,6 +3,11 @@ For full details, see the git log at: https://github.com/ksh93/ksh
 
 Any uppercase BUG_* names are modernish shell bug IDs.
 
+2021-01-23:
+
+- Fixed: when the DEBUG trap was redefined in a subshell, the DEBUG trap in
+  the parent environment was corrupted or the shell crashed.
+
 2021-01-22:
 
 - Compile-time shell options can now be edited in src/cmd/ksh93/SHOPT.sh

src/cmd/ksh93/include/version.h

@@ -20,7 +20,7 @@
 
 #define SH_RELEASE_FORK	"93u+m"		/* only change if you develop a new ksh93 fork */
 #define SH_RELEASE_SVER	"1.0.0-alpha"	/* semantic version number: https://semver.org */
-#define SH_RELEASE_DATE	"2021-01-20"	/* must be in this format for $((.sh.version)) */
+#define SH_RELEASE_DATE	"2021-01-23"	/* must be in this format for $((.sh.version)) */
 #define SH_RELEASE_CPYR	"(c) 2020-2021 Contributors to ksh " SH_RELEASE_FORK
 
 /* Scripts sometimes field-split ${.sh.version}, so don't change amount of whitespace. */

src/cmd/ksh93/sh/fault.c

@@ -344,7 +344,7 @@ void	sh_sigreset(register int mode)
 			sh.sigflag[sig] = flag;
 		}
 	}
-	for(sig=SH_DEBUGTRAP-1;sig>=0;sig--)
+	for(sig=SH_DEBUGTRAP; sig>=0; sig--)
 	{
 		if(trap=sh.st.trap[sig])
 		{

src/cmd/ksh93/tests/basic.sh

@@ -744,6 +744,22 @@ got=$(eval 'x=${ for i in test; do case $i in test) true;; esac; done; }' 2>&1)
 got=$(eval 'x=`for i in test; do case $i in test) true;; esac; done`' 2>&1) \
 || err_exit "case in a for loop inside a \`comsub\` caused syntax error (got $(printf %q "$got"))"
 
+# ======
+# The DEBUG trap crashed when re-trapping inside a subshell
+# https://github.com/ksh93/ksh/issues/155 (#2)
+exp=$'trap -- \': main\' EXIT\ntrap -- \': main\' ERR\ntrap -- \': main\' KEYBD\ntrap -- \': main\' DEBUG'
+got=$({ "$SHELL" -c '
+	PATH=/dev/null
+	for sig in EXIT ERR KEYBD DEBUG
+	do	trap ": main" $sig
+		( trap ": LEAK : $sig" $sig )
+		trap
+		trap - "$sig"
+	done
+'; } 2>&1)
+((!(e = $?))) && [[ $got == "$exp" ]] || err_exit 'Pseudosignal trap failed when re-trapping in subshell' \
+	"(got status $e$( ((e>128)) && print -n / && kill -l "$e"), $(printf %q "$got"))"
+
 # ======
 # The DEBUG trap had side effects on the exit status
 # https://github.com/ksh93/ksh/issues/155 (#4)