Revert buggy exec optimisation (re: d6c9821c)

As of 2022-06-18, ksh 93u+m is not capable of being used as /bin/sh
while building GNU binutils. The execution of some of its build
system's dot scripts is incorrectly aborted as an external 'sed'
command is execve(2)'d without forking. This means that incorrect
exec optimization was happening.

Unfortunately I have not been able to derive a minimal reproducer
of the problem yet because the GNU binutils build scripts are very
complex. Pending further research, the optimisation is reverted.
Even if a way to make it work is found, it will not be reintroduced
to the 1.0 branch.

Thanks to @atheik for finding the problem and identifying the
commit that introduced it.

Resolves: https://github.com/ksh93/ksh/issues/507

NEWS

@@ -2,6 +2,13 @@ This documents significant changes in the 1.0 branch of ksh 93u+m.
 For full details, see the git log at: https://github.com/ksh93/ksh/tree/1.0
 Uppercase BUG_* IDs are shell bug IDs as used by the Modernish shell library.
 
+2022-08-05:
+
+- Reverted a buggy exec optimisation introduced on 2022-06-18. It is known
+  to make the build scripts of GNU binutils produce corrupted results.
+
+- Release 1.0.1.
+
 2022-08-01:
    _        _        ___ _____                          ___   ___   ___
   | | _____| |__    / _ \___ / _   _   _   _ __ ___    / / | / _ \ / _ \

src/cmd/ksh93/include/version.h

@@ -17,8 +17,8 @@
 #include <releaseflags.h>
 
 #define SH_RELEASE_FORK	"93u+m"		/* only change if you develop a new ksh93 fork */
-#define SH_RELEASE_SVER	"1.0.0"		/* semantic version number: https://semver.org */
-#define SH_RELEASE_DATE	"2022-08-01"	/* must be in this format for $((.sh.version)) */
+#define SH_RELEASE_SVER	"1.0.1"		/* semantic version number: https://semver.org */
+#define SH_RELEASE_DATE	"2022-08-05"	/* must be in this format for $((.sh.version)) */
 #define SH_RELEASE_CPYR	"(c) 2020-2022 Contributors to ksh " SH_RELEASE_FORK
 
 /* Scripts sometimes field-split ${.sh.version}, so don't change amount of whitespace. */

src/cmd/ksh93/sh/xec.c

@@ -1927,11 +1927,7 @@ int sh_exec(register const Shnode_t *t, int flags)
 				t = t->lst.lstrit;
 			}
 			while(t->tre.tretyp == TLST);
-			/*
-			 * if sh_state(SH_FORKED) was turned on in the meantime, mix it in to the flags to allow a last-command
-			 * no_fork optimization via execflg2 -- this happens when a subshell forks mid-execution (sh_subfork())
-			 */
-			sh_exec(t,flags|sh_isstate(SH_FORKED));
+			sh_exec(t,flags);
 			break;
 		    }
 

src/cmd/ksh93/tests/basic.sh

@@ -927,15 +927,6 @@ esac
 # ======
 # Test exec optimization of last command in script or subshell
 
-(
-	ulimit -t unlimited 2>/dev/null  # fork subshell
-	print "${.sh.pid:-$("$SHELL" -c 'echo "$PPID"')}"  # fallback for pre-93u+m ksh without ${.sh.pid}
-	"$SHELL" -c 'print "$$"'
-) >out
-pid1= pid2=
-{ read pid1 && read pid2; } <out && let "pid1 == pid2" \
-|| err_exit "last command in forked subshell not exec-optimized ($pid1 != $pid2)"
-
 got=$(
 	ulimit -t unlimited 2>/dev/null  # fork subshell
 	print "${.sh.pid:-$("$SHELL" -c 'echo "$PPID"')}"  # fallback for pre-93u+m ksh without ${.sh.pid}

src/cmd/ksh93/tests/functions.sh

@@ -1034,9 +1034,22 @@ function foo
 	typeset pid
 	$tmp1 > $tmp2 & pid=$!
 	wait $!
-	[[ $(< $tmp2) == $pid ]] || err_exit 'wrong PID for & job in function'
+	got=$(< $tmp2)
+	[[ $got == "$pid" ]] || err_exit 'wrong PID for & job in function' \
+		"(expected $(printf %q "$pid"), got $(printf %q "$got"))"
 }
 foo
+foo()
+{
+	$tmp1 > $tmp2 & pid=$!
+	wait $!
+	got=$(< $tmp2)
+	[[ $got == "$pid" ]] || err_exit 'wrong PID for & job in POSIX function' \
+		"(expected $(printf %q "$pid"), got $(printf %q "$got"))"
+}
+foo
+unset pid
+unset -f foo
 # make sure compiled functions work
 [[ $(tmp=$tmp $SHELL <<- \++++
 	cat > $tmp/functions <<- \EOF