From 5f8b0512f015fad4ca1eb368b84c2ed9947cd741 Mon Sep 17 00:00:00 2001
From: Martijn Dekker <martijn@inlv.org>
Date: Mon, 8 Jun 2020 01:05:27 +0200
Subject: [PATCH] POSIX compliance fix: apply 'set -u' to $!

POSIX requires[*] that expanding any unset parameter other than $@
and $* is an error when 'set -u'/'set -o nounset' is active.
However, on ksh93, $! was exempt as well. That is a bug.
[*] https://pubs.opengroup.org/onlinepubs/9699919799/utilities/V3_chap02.html#tag_18_25

src/cmd/ksh93/sh/macro.c:
- special(): Handle 'set -u' for special parameters if/when it is
  about to return NIL. That code path is currently only possible to
  reach for "$!", but this is future-proof and will do the right
  thing if any other special parameter can ever have no value.

src/cmd/ksh93/tests/options.sh:
- Add and tweak 'set -u' regression tests.

(cherry picked from commit 75cc7a38cafe3a9929e1ed17d8b952babda22a09)
---
 NEWS                           |  3 +++
 src/cmd/ksh93/sh/macro.c       | 13 +++++++++++++
 src/cmd/ksh93/tests/options.sh | 11 ++++++++++-
 3 files changed, 26 insertions(+), 1 deletion(-)

diff --git a/NEWS b/NEWS
index 2b9911b03..b2484cede 100644
--- a/NEWS
+++ b/NEWS
@@ -10,6 +10,9 @@ Any uppercase BUG_* names are modernish shell bug IDs.
   nonexistent positional parameter such as $1, $2, ... is accessed, as other
   shells do and POSIX requires. (This does *not* apply to "$@" and "$*".)
 
+- If 'set -u'/'set -o nounset' is active, then the shell now errors out if $!
+  is accessed before the shell has launched any background process.
+
 2020-06-06:
 
 - The 'times' command is now a builtin command that conforms to POSIX
diff --git a/src/cmd/ksh93/sh/macro.c b/src/cmd/ksh93/sh/macro.c
index d789d1941..2899908ba 100644
--- a/src/cmd/ksh93/sh/macro.c
+++ b/src/cmd/ksh93/sh/macro.c
@@ -2790,6 +2790,19 @@ static char *special(Shell_t *shp,register int c)
 		else
 			return(shp->st.cmdname);
 	}
+	/* Handle 'set -u'/'set -o nounset' for special parameters */
+	if(sh_isoption(SH_NOUNSET))
+	{
+		int d=fcget();
+		fcseek(-1);
+		if(!(d && strchr(":+-?=",d)))
+		{
+			char c_str[2];
+			c_str[0]=(char)c;
+			c_str[1]='\0';
+			errormsg(SH_DICT,ERROR_exit(1),e_notset,c_str);
+		}
+	}
 	return(NIL(char*));
 }
 
diff --git a/src/cmd/ksh93/tests/options.sh b/src/cmd/ksh93/tests/options.sh
index 1375440e4..a8abb2d86 100755
--- a/src/cmd/ksh93/tests/options.sh
+++ b/src/cmd/ksh93/tests/options.sh
@@ -511,10 +511,19 @@ $SHELL -uc 'var=foo;unset var;: ${var%foo}' >/dev/null 2>&1 && err_exit '${var%f
 $SHELL -uc 'var=foo;unset var;: ${!var}' >/dev/null 2>&1 && err_exit '${!var} should fail with set -u'
 $SHELL -uc 'var=foo;unset var;: ${#var}' >/dev/null 2>&1 && err_exit '${#var} should fail with set -u'
 $SHELL -uc 'var=foo;unset var;: ${var-OK}' >/dev/null 2>&1 || err_exit '${var-OK} should not fail with set -u'
-$SHELL -uc 'var=foo;nset var;: ${var:-OK}' >/dev/null 2>&1 || err_exit '${var:-OK} should not fail with set -u'
+$SHELL -uc 'var=foo;unset var;: ${var:-OK}' >/dev/null 2>&1 || err_exit '${var:-OK} should not fail with set -u'
 (set -u -- one two; : $2) 2>/dev/null || err_exit "an unset PP failed with set -u"
 (set -u -- one two; : $3) 2>/dev/null && err_exit "a set PP failed to fail with set -u"
+(set -u -- one two; : ${3%foo}) 2>/dev/null && err_exit '${3%foo} failed to fail with set -u'
+(set -u -- one two; : ${3-OK}) 2>/dev/null || err_exit '${3-OK} wrongly failed with set -u'
+(set -u -- one two; : ${3:-OK}) 2>/dev/null || err_exit '${3:-OK} wrongly failed with set -u'
+(set -u -- one two; : ${#3}) 2>/dev/null && err_exit '${#3} failed to fail with set -u'
 (set -u --; : $@ $*) 2>/dev/null || err_exit '$@ and/or $* fail to be exempt from set -u'
+$SHELL -uc ': $!' 2>/dev/null && err_exit '$! failed to fail with set -u'
+$SHELL -uc ': ${!%foo}' >/dev/null 2>&1 && err_exit '${!%foo} should fail with set -u'
+$SHELL -uc ': ${#!}' >/dev/null 2>&1 && err_exit '${#!} should fail with set -u'
+$SHELL -uc ': ${!-OK}' >/dev/null 2>&1 || err_exit '${!-OK} should not fail with set -u'
+$SHELL -uc ': ${!:-OK}' >/dev/null 2>&1 || err_exit '${!:-OK} should not fail with set -u'
 
 z=$($SHELL 2>&1 -uc 'print ${X23456789012345}')
 [[ $z == *X23456789012345:* ]] || err_exit "error message garbled with set -u got $z"