Fix BUG_KBGPID: $! was not updated under certain conditions

The $! special parameter was not set if a background job
(somecommand &) or co-process (somecommand |&) was launched as the
only command within a braces block with an attached redirection,
for example:
	{
		somecommand &
	} >&2
With the bug, $! was unchanged; now it contains the PID of
somecommand.

Ref.: https://github.com/att/ast/issues/1357

src/cmd/ksh93/sh/parse.c: item():
- When processing redirections following a compound command, always
  create a parent node with the TSETIO (I/O redirection) token.
     Before this commit, if the last command was of type TFORK (and
  the last command only tested as TFORK if the bg job or coprocess
  was the only command in a braces block, because the ksh parser
  optimises away the braces in that case), then the parent node was
  created with the TFORK token instead.
     I have no idea what David Korn's intention was with that, but
  this is clearly very wrong. Creating another TFORK node when
  parsing the redirection caused sh_exec() in sh/xec.c to execute
  the redirection in an extra forked, non-background subshell.
  Since redirections are executed before anything else, this
  subshell is what then launched the background job between the
  braces, so $! (a.k.a. shp->bckpid) was updated in that subshell
  only, and never in the main shell. The extra subshell also
  prevented the background job from being noticed by job control
  on interactive shells.
     So, the fix is simply to remove the broken test for TFORK.

src/cmd/ksh93/tests/variables.sh:
- Add regression tests for a bg job and a co-process as the only
  command within a braces block with attached redirection.

(cherry picked from commit ffe5df30e69f7b596941a98498014d8e838861f2)

NEWS

@@ -4,6 +4,16 @@ For full details, see the git log at:
 
 Any uppercase BUG_* names are modernish shell bug IDs.
 
+2020-06-04:
+
+- Fix BUG_KBGPID: the $! special parameter was not set if a background job
+  (somecommand &) or co-process (somecommand |&) was launched as the only
+  command within a braces block with an attached redirection, for example:
+	{
+		somecommand &
+	} >&2
+  With the bug, $! was unchanged; now it contains the PID of somecommand.
+
 2020-05-31:
 
 - Fix a bug in autoloading functions. Directories in the path search list

src/cmd/ksh93/include/version.h

@@ -17,4 +17,4 @@
 *                  David Korn <dgk@research.att.com>                   *
 *                                                                      *
 ***********************************************************************/
-#define SH_RELEASE	"93u+m 2020-05-31"
+#define SH_RELEASE	"93u+m 2020-06-04"

src/cmd/ksh93/sh/parse.c

@@ -1360,11 +1360,10 @@ static Shnode_t	*item(Lex_t *lexp,int flag)
 		return(t);
 	}
 	sh_lex(lexp);
+	/* redirection(s) following a compound command */
 	if(io=inout(lexp,io,0))
 	{
-		if((tok=t->tre.tretyp&COMMSK) != TFORK)
-			tok = TSETIO;
-		t=makeparent(lexp,tok,t);
+		t=makeparent(lexp,TSETIO,t);
 		t->tre.treio=io;
 	}
 done:

src/cmd/ksh93/tests/variables.sh

@@ -558,6 +558,15 @@ set -- $x
 : & pid=$!
 ( : & )
 [[ $pid == $! ]] || err_exit '$! value not preserved across subshells'
+
+pid=$!
+{ : & } >&2
+[[ $pid == $! ]] && err_exit '$! value not updated after bg job in braces+redir'
+
+pid=$!
+{ : |& } >&2
+[[ $pid == $! ]] && err_exit '$! value not updated after co-process in braces+redir'
+
 unset foo
 typeset -A foo
 function foo.set