Reset arithmetic recursion level on all errors (re: 264ba48b)

The recursion level for arithmetic expressions is kept track of in
a static 'level' variable in streval.c. It is reset when arithmetic
expressions throw an error.

But an error for an arithmetic expression may also occur elsewhere
-- at least in one case: when an arithmetic expression attempts to
change a read-only variable. In that case, the recursion level is
never reset because that code does not have access to the static
'level' variable.

If many such conditions occur (as in the new readonly.sh regression
tests), an arithmetic command like 'i++' may eventually fail with a
'recursion too deep' error.

To mitigate the problem, MAXLEVEL in streval.c was changed from 9
to 1024 in 264ba48b (as in the ksh 93v- beta). This commit leaves
that increase, but adds a proper fix.

src/cmd/ksh93/include/defs.h:
- Add global sh.arithrecursion (a.k.a. shp->arithrecursion)
  variable to keep track of the arithmetic recursion level,
  replacing the static 'level' variable in streval.c.

src/cmd/ksh93/sh/xec.c: sh_exec():
- Reset sh.arithrecursion before starting a new simple command
  (TCOM), a new subshell with parentheses (TPAR), a new pipe
  (TFIL), or a new [[ ... ]] command (TTST). These are the same
  places where 'echeck' is set to 1 for --errexit and ERR trap
  checks, so it should cover everything.

src/cmd/ksh93/sh/streval.c:
- Change all uses of 'level' to sh.arithrecursion.
- _seterror, aritherror(): No longer bother to reset the level
  to zero here; xec.c should have this covered for all cases now.

src/cmd/ksh93/tests/arith.sh:
- Add tests for main shell and subshell.

NEWS

@@ -3,6 +3,13 @@ For full details, see the git log at: https://github.com/ksh93/ksh
 
 Any uppercase BUG_* names are modernish shell bug IDs.
 
+2021-04-10:
+
+- Fixed: the internal count of the recursion level for arithmetic expressions
+  was not reset when certain errors occurred in a virtual subshell. This could
+  cause an erroneous "recursion to deep" error when a loop executed many
+  subshells containing arithmetic expressions with errors, e.g. for testing.
+
 2021-04-09:
 
 - Fixed a bug that caused ksh to enable -c during the shell's initialization

src/cmd/ksh93/include/defs.h

@@ -203,6 +203,7 @@ struct shared
 	char		universe; \
 	char		winch; \
 	char		inarith; 	/* set when in ((...)) */ \
+	short		arithrecursion;	/* current arithmetic recursion level */ \
 	char		indebug; 	/* set when in debug trap */ \
 	unsigned char	ignsig;		/* ignored signal in subshell */ \
 	unsigned char	lastsig;	/* last signal received */ \

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	"2021-04-09"	/* must be in this format for $((.sh.version)) */
+#define SH_RELEASE_DATE	"2021-04-10"	/* 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/streval.c

@@ -65,8 +65,6 @@
 				*((type*)stakptr((v)->offset)) = (val)),(v)->offset)
 #define roundptr(ep,cp,type)	(((unsigned char*)(ep))+round(cp-((unsigned char*)(ep)),pow2size(sizeof(type))))
 
-static int level;
-
 struct vars				/* vars stacked per invocation */
 {
 	Shell_t		*shp;
@@ -125,14 +123,12 @@ static int _seterror(struct vars *vp,const char *msg)
 		vp->errmsg.value = (char*)msg;
 	vp->errchr = vp->nextchr;
 	vp->nextchr = "";
-	level = 0;
 	return(0);
 }
 
 
 static void arith_error(const char *message,const char *expr, int mode)
 {
-        level = 0;
 	mode = (mode&3)!=0;
         errormsg(SH_DICT,ERROR_exit(mode),message,expr);
 }
@@ -177,7 +173,7 @@ Sfdouble_t	arith_exec(Arith_t *ep)
 	node.nosub = 0;
 	node.ptr = 0;
 	node.eflag = 0;
-	if(level++ >=MAXLEVEL)
+	if(shp->arithrecursion++ >= MAXLEVEL)
 	{
 		arith_error(e_recursive,ep->expr,ep->emode);
 		return(0);
@@ -249,7 +245,7 @@ Sfdouble_t	arith_exec(Arith_t *ep)
 			if(node.flag = c)
 				lastval = 0;
 			node.isfloat=0;
-			node.level = level;
+			node.level = shp->arithrecursion;
 			node.nosub = 0;
 			num = (*ep->fun)(&ptr,&node,VALUE,num);
 			if(node.emode&ARITH_ASSIGNOP)
@@ -497,8 +493,8 @@ Sfdouble_t	arith_exec(Arith_t *ep)
 		*sp = num;
 		*tp = type;
 	}
-	if(level>0)
+	if(shp->arithrecursion>0)
-		level--;
+		shp->arithrecursion--;
 	if(type==0 && !num)
 		num = 0;
 	return(num);
@@ -1022,7 +1018,6 @@ Sfdouble_t strval(Shell_t *shp,const char *s,char **end,Sfdouble_t(*conv)(const
 	    default:
 		return(1);
 	}
-	level=0;
 	errormsg(SH_DICT,ERROR_exit(1),message,ep->name);
 	UNREACHABLE();
     }

src/cmd/ksh93/sh/xec.c

@@ -1020,6 +1020,7 @@ int sh_exec(register const Shnode_t *t, int flags)
 			error_info.line = t->com.comline-shp->st.firstline;
 			com = sh_argbuild(shp,&argn,&(t->com),OPTIMIZE);
 			echeck = 1;
+			shp->arithrecursion = 0;
 			if(t->tre.tretyp&COMSCAN)
 			{
 				argp = t->com.comarg;
@@ -1903,6 +1904,7 @@ int sh_exec(register const Shnode_t *t, int flags)
 
 		    case TPAR:
 			echeck = 1;
+			shp->arithrecursion = 0;
 			flags &= ~OPTIMIZE_FLAG;
 			if(!shp->subshell && !shp->st.trapcom[0] && !shp->st.trap[SH_ERRTRAP] && (flags&sh_state(SH_NOFORK)))
 			{
@@ -1966,6 +1968,7 @@ int sh_exec(register const Shnode_t *t, int flags)
 			int	*exitval=0,*saveexitval = job.exitval;
 			pid_t	savepgid = job.curpgid;
 			echeck = 1;
+			shp->arithrecursion = 0;
 			job.exitval = 0;
 			job.curjobid = 0;
 			if(shp->subshell)
@@ -2649,6 +2652,7 @@ int sh_exec(register const Shnode_t *t, int flags)
 				skipexitset++;
 			error_info.line = t->tst.tstline-shp->st.firstline;
 			echeck = 1;
+			shp->arithrecursion = 0;
 			if((type&TPAREN)==TPAREN)
 			{
 				sh_exec(t->lst.lstlef,OPTIMIZE);

src/cmd/ksh93/tests/arith.sh

@@ -822,5 +822,31 @@ got=$(set +x; { var="x\]+b\[\$(echo INJECTION >&2)"; typeset -A a; ((a[\$var]++)
 got=$(set +x; { var="x\]+b\[\$(uname>&2)"; typeset -A a; ((a["\$var"]++)); typeset -p a; } 2>&1)
 [[ $got == "$exp" ]] || err_exit "Array subscript quoting test 5M: expected $(printf %q "$exp"), got $(printf %q "$got")"
 
+# ======
+# Arithmetic recursion level was not reset on encountering readonly error in combination with a recursive arithmetic expression.
+# (arithmetic array subscripts inside arithmetic expressions are one example of such recursion)
+# https://github.com/ksh93/ksh/pull/239#discussion_r606874442
+srcdir=${SHTESTS_COMMON%/tests/*}
+integer maxlevel=$(grep $'^#define MAXLEVEL\t' "$srcdir/sh/streval.c" | sed 's/.*MAXLEVEL//')
+if	((!maxlevel))
+then	err_exit "could not get maximum arithmetic recursion level from source code; update this test"
+	maxlevel=1024
+fi
+integer loopcount=maxlevel+10
+got=$(
+	typeset -r -A -i ro_arr=([a]=10 [b]=20 [c]=30)
+	for ((i=0; i<loopcount; i++)); do
+		let "ro_arr[i+1] += 5"
+	done 2>&1
+)
+[[ $got == *recursion* ]] && err_exit "recursion level not reset on readonly error (main shell)"
+got=$(
+	typeset -r -A -i ro_arr=([a]=10 [b]=20 [c]=30)
+	for ((i=0; i<loopcount; i++)); do
+		( ((ro_arr[i+1] += 5)) )
+	done 2>&1
+)
+[[ $got == *recursion* ]] && err_exit "recursion level not reset on readonly error (subshell)"
+
 # ======
 exit $((Errors<125?Errors:125))