POSIX: 'command': don't disable declaration proprts (re: b9d10c5a)

Following the resolution of Austin Group bug 1393[*] that is set to
be included in the next version of the POSIX standard, the
'command' prefix in POSIX mode (set -o posix) no longer disables
the declaration properties of declaration built-ins.
[*] https://austingroupbugs.net/view.php?id=1393

src/cmd/ksh93/sh/parse.c: lex():
- Skip the 'command' prefix even in POSIX mode so that any
  declaration commands prefixed by it are treated as such in xec.c
  (sh_exec()).

src/cmd/ksh93/sh/xec.c: sh_exec():
- The foregoing change reintroduced a variant of BUG_CMDSPEXIT: the
  shell exits on something like 'command export readonlyvar=foo'.
  This now fixes that bug for both POSIX and non-POSIX mode. When
  calling nv_setlist() to process true shell assignments, and there
  is a 'command' prefix, push a shell context and use sigsetjmp to
  intercept any errors in assignments and stop the shell exiting.

src/cmd/ksh93/tests/builtins.sh:
- Borrow the BUG_CMDSPEXIT regression test from modernish and adapt
  it for ksh. (I'm the author so yes, I can do this.) Original:
  https://github.com/modernish/modernish/blob/ae8fe9c3/lib/modernish/tst/builtin.t#L80-L109

NEWS

@@ -17,6 +17,12 @@ Any uppercase BUG_* names are modernish shell bug IDs.
 - Completed the 2021-04-30 fix for ${var<OP>'{}'} where <OP> is '-', '+',
   ':-' or ':+' by fixing a bug that caused an extra '}' to be output.
 
+- Following the resolution of Austin Group bug 1393[*] that is set to be
+  included in the next version of the POSIX standard, the 'command' prefix
+  in POSIX mode (set -o posix) no longer disables the declaration properties
+  of declaration built-ins. This reverts a change introduced on 2020-09-11.
+  [*] https://austingroupbugs.net/view.php?id=1393
+
 2021-04-30:
 
 - The emacs 'ESC .' (M-.) and vi '_' commands now take shell quoting into

src/cmd/ksh93/sh.1

@@ -5913,13 +5913,6 @@ executing the command as a regular built-in.
 .I "option-name"
 prevents a script from terminating when an invalid
 option name is given.)
-If
-.I name\^
-refers to a declaration built-in,
-as marked with \fB\(dd\fR in this manual,
-and the \fBposix\fR shell option is on,
-then the declaration properties are removed so that arguments containing
-\fB=\fR are not treated specially.
 .IP
 The
 .B \-p
@@ -7246,9 +7239,6 @@ disables the \fB&>\fR redirection shorthand;
 .IP \[bu]
 makes the \fB<>\fR redirection operator default to redirecting standard input
 if no file descriptor number precedes it;
-.IP \[bu]
-causes the \fBcommand\fR utility to disable declaration command properties of
-any declaration commands it invokes;
 and
 .IP \[bu]
 disables a hack that makes \fBtest -t\fR (\fB[ -t ]\fR) equivalent to

src/cmd/ksh93/sh/parse.c

@@ -1456,7 +1456,7 @@ static Shnode_t *simple(Lex_t *lexp,int flag, struct ionod *io)
 							lexp->intypeset = 1;
 						key_on = 1;
 					}
-					else if(np==SYSCOMMAND && !sh_isoption(SH_POSIX))
+					else if(np==SYSCOMMAND)	/* treat 'command typeset', etc. as declaration command */
 						cmdarg++;
 					else if(np==SYSEXEC || np==SYSREDIR)
 						lexp->inexec = 1;

src/cmd/ksh93/sh/xec.c

@@ -1172,7 +1172,24 @@ int sh_exec(register const Shnode_t *t, int flags)
 						flgs |= NV_IDENT;
 					else
 						flgs |= NV_VARNAME;
-					nv_setlist(argp,flgs,tp);
+					/* execute the list of assignments */
+					if((!np || nv_isattr(np,BLT_SPC)) && !command)
+					{
+						/* bare assignment(s) or special builtin, and no 'command' prefix: exit on error */
+						nv_setlist(argp,flgs,tp);
+					}
+					else
+					{
+						/* avoid exit on error from nv_setlist, e.g. read-only variable */
+						struct checkpt *chkp = (struct checkpt*)stakalloc(sizeof(struct checkpt));
+						sh_pushcontext(shp,chkp,SH_JMPCMD);
+						jmpval = sigsetjmp(chkp->buff,1);
+						if(!jmpval)
+							nv_setlist(argp,flgs,tp);
+						sh_popcontext(shp,chkp);
+						if(jmpval)	/* error occurred */
+							goto setexit;
+					}
 					if(np==shp->typeinit)
 						shp->typeinit = 0;
 					shp->envlist = argp;

src/cmd/ksh93/tests/builtins.sh

@@ -1241,5 +1241,30 @@ got=$("$SHELL" -c 'cd /; echo "$OLDPWD"' 2>&1)
 	"(expected $(printf %q "$exp"), got $(printf %q "$got"))"
 cd "$tmp"
 
+# ======
+# BUG_CMDSPEXIT
+exp='ok1ok2ok3ok4ok5ok6ok7ok8ok9ok10ok11ok12end'
+got=$(	readonly v=foo
+	exec 2>/dev/null
+	# All the "special builtins" below should fail, and not exit, so 'print end' is reached.
+	# Ref.: http://pubs.opengroup.org/onlinepubs/9699919799/utilities/contents.html
+	# Left out are 'command exec /dev/null/nonexistent', where no shell follows the standard,
+	# as well as 'command exit' and 'command return', because, well, obviously.
+	command : </dev/null/no		|| print -n ok1
+	command . /dev/null/no		|| print -n ok2
+	command set +o bad@option	|| print -n ok3
+	command shift $(($# + 1))	|| print -n ok4
+	(unalias times; PATH=/dev/null; eval 'command times foo bar >/dev/null || print -n ok5')
+	command trap foo bar baz quux	|| print -n ok6
+	command unset v			|| print -n ok7
+	command eval "("		|| print -n ok8
+	command export v=baz		|| print -n ok9
+	command readonly v=bar		|| print -n ok10
+	command break			&& print -n ok11  # 'break' and 'continue' are POSIXly allowed to quietly...
+	command continue		&& print -n ok12  # ..."succeed" if they are used outside of a loop :-/
+	print end)
+[[ $got == "$exp" ]] || err_exit "prefixing special builtin with 'command' does not stop it from exiting the shell on error" \
+	"(expected $(printf %q "$exp"), got $(printf %q "$got"))"
+
 # ======
 exit $((Errors<125?Errors:125))