Fix ${.sh.subshell} counter to actually count level of subshells

This counter is documented as follows:
"The current depth for subshells and command substitution."

But before this commit, the actual behaviour was that the counter
was reset to zero whenever a subshell forked for any reason: a
pipe, background job, running 'ulimit', redirecting stdout in a
command substitution, and more. This behaviour was:

1. Not consistent with the documentation. Non-forked (a.k.a.
   virtual) subshells are an internal implementation detail which
   scripts should not have to be concerned with. The manual page
   doesn't mention them at all.

2. Inherently broken. Since a subshell may fork for any number of
   reasons, even mid-run, and those reasons may change with
   bugfixes and further development, scripts have never actually
   been able to rely on the value of ${.sh.subshell}.

So, this commit fixes the counter to count the levels of all
subshells, both virtual and forked.

src/cmd/ksh93/sh/xec.c: _sh_fork():
- Increase ${.sh.subshell} whenever we fork.

src/cmd/ksh93/sh/subshell.c:
- sh_subfork():
  * Fix comment to properly explain what it does. It doesn't
    "create" a subshell, it forks off an existing virtual subshell.
  * Don't zero ${.sh.subshell}. Instead, since sh_fork() increases
    it but we're forking an existing subshell, undo the increase.
- sh_subshell():
  * Remove 'int16_t subshell' variable. It was unnecessary and
    mostly unused. It was also the wrong type: it was assigned the
    value from shp->subshell which is of type short.
  * Increase and decrease the level of virtual subshells and
    ${.sh.subshell} independently.

src/cmd/ksh93/tests/variables.sh:
- Add regression tests for ${.sh.subshell} in virtual and forked
  subshells of several kinds: comsub, parentheses, pipe, bg job.
- Undo wrong error test count fix from 04b4aef0.

(cherry picked from commit a0e0e29e7e0dbf21e4b3958ae02bde6665fb2696)

NEWS

@@ -15,6 +15,10 @@ Any uppercase BUG_* names are modernish shell bug IDs.
   macOS, by eliminating an invalid/undefined use of memccpy() on overlapping
   buffers in the commonly used sfputr() function.
 
+- Fix the ${.sh.subshell} level counter; it is no longer reset to zero when a
+  non-forked subshell happens to fork into a separate process for some reason
+  (an internal implementation detail that should be unnoticeable to scripts).
+
 2020-06-04:
 
 - Fix BUG_KBGPID: the $! special parameter was not set if a background job

src/cmd/ksh93/sh/subshell.c

@@ -171,7 +171,7 @@ void	sh_subtmpfile(Shell_t *shp)
 
 
 /*
- * This routine creates a temp file if necessary and creates a subshell.
+ * This routine creates a temp file if necessary, then forks a virtual subshell into a real subshell.
  * The parent routine longjmps back to sh_subshell()
  * The child continues possibly with its standard output replaced by temp file
  */
@@ -210,10 +210,11 @@ void sh_subfork(void)
 		subshell_data = 0;
 		shp->subshell = 0;
 		shp->comsub = 0;
-		SH_SUBSHELLNOD->nvalue.s = 0;
 		sp->subpid=0;
 		shp->st.trapcom[0] = trap;
 		shp->savesig = 0;
+		/* sh_fork() increases ${.sh.subshell} but we forked an existing virtual subshell, so undo */
+		SH_SUBSHELLNOD->nvalue.s--;
 	}
 }
 
@@ -454,7 +455,6 @@ Sfio_t *sh_subshell(Shell_t *shp,Shnode_t *t, volatile int flags, int comsub)
 	int savecurenv = shp->curenv;
 	int savejobpgid = job.curpgid;
 	int *saveexitval = job.exitval;
-	int16_t subshell;
 	char *savsig;
 	Sfio_t *iop=0;
 	struct checkpt buff;
@@ -475,9 +475,8 @@ Sfio_t *sh_subshell(Shell_t *shp,Shnode_t *t, volatile int flags, int comsub)
 	shp->curenv = ++subenv;
 	savst = shp->st;
 	sh_pushcontext(shp,&buff,SH_JMPSUB);
-	subshell = shp->subshell+1;
-	SH_SUBSHELLNOD->nvalue.s = subshell;
-	shp->subshell = subshell;
+	shp->subshell++;		/* increase level of virtual subshells */
+	SH_SUBSHELLNOD->nvalue.s++;	/* increase ${.sh.subshell} */
 	sp->prev = subshell_data;
 	sp->shp = shp;
 	sp->sig = 0;
@@ -757,8 +756,10 @@ Sfio_t *sh_subshell(Shell_t *shp,Shnode_t *t, volatile int flags, int comsub)
 	shp->coshell = sp->coshell;
 #endif /* SHOPT_COSHELL */
 	if(shp->subshell)
-		SH_SUBSHELLNOD->nvalue.s = --shp->subshell;
-	subshell = shp->subshell;
+	{
+		shp->subshell--;		/* decrease level of virtual subshells */
+		SH_SUBSHELLNOD->nvalue.s--;	/* decrease ${.sh.subshell} */
+	}
 	subshell_data = sp->prev;
 	if(!argsav  ||  argsav->dolrefcnt==argcnt)
 		sh_argfree(shp,argsav,0);

src/cmd/ksh93/sh/xec.c

@@ -3195,7 +3195,8 @@ pid_t _sh_fork(Shell_t *shp,register pid_t parent,int flags,int *jobid)
 	/* except for those `lost' by trap   */
 	if(!(flags&FSHOWME))
 		sh_sigreset(2);
-	shp->subshell = 0;
+	SH_SUBSHELLNOD->nvalue.s++;	/* increase ${.sh.subshell} */
+	shp->subshell = 0;		/* zero virtual subshells */
 	shp->comsub = 0;
 	shp->spid = 0;
 	if((flags&FAMP) && shp->coutpipe>1)

src/cmd/ksh93/tests/variables.sh

@@ -621,8 +621,25 @@ function a.set
 	[[ ${.sh.subshell} == 1 ]] || err_exit '${.sh.subshell} should be 1'
 	(
 		[[ ${.sh.subshell} == 2 ]] || err_exit '${.sh.subshell} should be 2'
+		exit $Errors
 	)
 )
+Errors=$?	# ensure error count survives subshell
+
+actual=$(
+	{
+		(
+			echo ${.sh.subshell} | cat	# left element of pipe should increase ${.sh.subshell}
+			echo ${.sh.subshell}
+			ulimit -t unlimited		# fork
+			echo ${.sh.subshell}		# should be same after forking existing virtual subshell
+		)
+		echo ${.sh.subshell}			# a background job should also increase ${.sh.subshell}
+	} & wait "$!"
+	echo ${.sh.subshell}
+)
+expect=$'4\n3\n3\n2\n1'
+[[ $actual == "$expect" ]] || err_exit "\${.sh.subshell} failure (expected $(printf %q "$expect"), got $(printf %q "$actual"))"
 
 set -- {1..32768}
 (( $# == 32768 )) || err_exit "\$# failed -- expected 32768, got $#"
@@ -657,7 +674,7 @@ set --
 
 	exit $Errors
 )
-let "Errors += $?"  # ensure error count survives subshell
+Errors=$?  # ensure error count survives subshell
 
 cd $tmp