Fix ${ comsub; } killing job control

Another longstanding whopper of a bug in basic ksh93 functionality:
run a ${ shared-state; } command substitution twice and job control
promptly loses track of all your running jobs. New jobs are tracked
again until you run another two shared-state command substitutions.
This is in at least 93t+, 93u-, 93u+, 93v- and ksh2020.

$ sleep 300 &
[1]	56883
$ jobs						# OK
[1] +  Running                 sleep 300 &
$ v=${ echo hi1; }
$ jobs						# OK
[1] +  Running                 sleep 300 &
$ v=${ echo hi2; }
$ jobs						# Nothing!
$ fg
ksh: fg: no such job

src/cmd/ksh93/sh/subshell.c: sh_subshell():
- The current environment number shp->curenv (a.k.a. sh.curenv) was
  not being restored if the virtual subshell we're leaving is of
  the shared-state command substitution variety as it was wrongly
  considered to be part of the environment that didn't need
  restoring. This caused it to be out of sync with shp->jobenv
  (a.k.a. sh.jobenv) which did get restored from savedcurenv.
  Restore both from savedcurenv at the same time for any subshell.
  (How these numbers are used exactly remains to be discovered.)

src/cmd/ksh93/tests/jobs.sh:
- Added, with a test for this bug to start it off. There is no
  other test script where job control fits, and a lot more related
  fixes are anticipated: https://github.com/ksh93/ksh/issues/119

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.
 
+2021-02-11:
+
+- Fixed a bug that caused ksh to lose track of all running background jobs if
+  a shared-state command substitution of the form v=${ cmd; } was used twice.
+
 2021-02-05:
 
 - Fixed a longstanding bug that caused redirections that store a file

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	"2021-02-05"	/* must be in this format for $((.sh.version)) */
+#define SH_RELEASE_DATE	"2021-02-11"	/* 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/subshell.c

@@ -752,7 +752,7 @@ Sfio_t *sh_subshell(Shell_t *shp,Shnode_t *t, volatile int flags, int comsub)
 	path_delete((Pathcomp_t*)shp->pathlist);
 	shp->pathlist = (void*)sp->pathlist;
 	job_subrestore(sp->jobs);
-	shp->jobenv = savecurenv;
+	shp->curenv = shp->jobenv = savecurenv;
 	job.curpgid = savejobpgid;
 	job.exitval = saveexitval;
 	shp->bckpid = sp->bckpid;
@@ -794,7 +794,6 @@ Sfio_t *sh_subshell(Shell_t *shp,Shnode_t *t, volatile int flags, int comsub)
 		if(n>0)
 			memset(&shp->st.trapcom[savst.trapmax],0,n*sizeof(char*));
 		shp->st = savst;
-		shp->curenv = savecurenv;
 		shp->st.otrap = 0;
 		if(nsig)
 		{

src/cmd/ksh93/tests/jobs.sh

@@ -0,0 +1,44 @@
+########################################################################
+#                                                                      #
+#              This file is part of the ksh 93u+m package              #
+#             Copyright (c) 2021 Contributors to ksh 93u+m             #
+#                    <https://github.com/ksh93/ksh>                    #
+#                      and is licensed under the                       #
+#                 Eclipse Public License, Version 1.0                  #
+#                                                                      #
+#                A copy of the License is available at                 #
+#          http://www.eclipse.org/org/documents/epl-v10.html           #
+#         (with md5 checksum b35adb5213ca9657e911e9befb180842)         #
+#                                                                      #
+#                  Martijn Dekker <martijn@inlv.org>                   #
+#                                                                      #
+########################################################################
+function err_exit
+{
+	print -u2 -n "\t"
+	print -u2 -r "${Command}[$1]: ${@:2}"
+	let Errors++
+}
+alias err_exit='err_exit $LINENO'
+
+Command=${0##*/}
+integer Errors=0
+
+[[ -d $tmp && -w $tmp && $tmp == "$PWD" ]] || { err\_exit "$LINENO" '$tmp not set; run this from shtests. Aborting.'; exit 1; }
+
+# All the tests here should run with job control on
+set -o monitor
+
+# ======
+# Before 2021-02-11, using a shared-state ${ command substitution; } twice caused ksh to lose track of all running jobs
+sleep 1 & p1=$!
+sleep 2 & p2=$!
+j1=${ jobs; }
+[[ $j1 == $'[2] +  Running '*$'\n[1] -  Running '* ]] || err_exit "sleep jobs not registered (got $(printf %q "$j1"))"
+: ${ :; } ${ :; }
+j2=${ jobs; }
+kill $p1 $p2
+[[ $j2 == "$j1" ]] || err_exit "jobs lost after shared-state command substitution ($(printf %q "$j2") != $(printf %q "$j1"))"
+
+# ======
+exit $((Errors<125?Errors:125))