Fix undefined behaviour (re: 4592859, 23b7a16)

The previous commit exposed undefined behaviour elsewhere that is
the result of old ksh bugs. My 2021 attempt to fix it in 23b7a16
was also wrong.

src/cmd/ksh93/sh/init.c: put_rand(), put_lineno(), put_lastarg():
- An incorrect typecast to double instead of Sfdouble_t (which
  translates to 'long double' on most systems, but not on macOS M1)
  caused arithmetic assignments to RANDOM, LINENO and _ to produce
  undefined results where sizeof(long double)!=sizeof(double). As
  ksh has been using Sfdouble_t for all arithmetic operations for
  years, fix the bug by typecasting to Sfdouble_t instead.

src/cmd/ksh93/tests/arith.sh:
- Fix the tests added in the previous commit to tolerate
  system-dependent rounding errors.

NEWS

@@ -8,9 +8,11 @@ Uppercase BUG_* IDs are shell bug IDs as used by the Modernish shell library.
   functions of a variable to be triggered when performing an assignment in
   an arithmetic expression; only the 'set' discipline is now triggered when
   assigning (as was always the documented behaviour). This applies to the
-  shell's internal disciplines as well. For example, seeding the $RANDOM
-  generator via an arithmetic expression assignment now no longer takes the
-  first value away from the reproducible pseudorandom sequence.
+  shell's internal disciplines as well. For example, $RANDOM is no longer
+  spuriously read from while seeding it via an arithmetic expression.
+
+- Fixed: assigning to LINENO, RANDOM, or _ via an arithmetic expression gave
+  undefined results on systems where sizeof(long double)!=sizeof(double).
 
 2024-01-16:
 

src/cmd/ksh93/sh/init.c

@@ -673,7 +673,7 @@ static Sfdouble_t nget_seconds(Namval_t* np, Namfun_t *fp)
 static void put_rand(Namval_t* np,const char *val,int flags,Namfun_t *fp)
 {
 	struct rand *rp = (struct rand*)fp;
-	long n;
+	Sfdouble_t n;
 	sh_save_rand_seed(rp, 0);
 	if(!val)
 	{
@@ -684,7 +684,7 @@ static void put_rand(Namval_t* np,const char *val,int flags,Namfun_t *fp)
 		return;
 	}
 	if(flags&NV_INTEGER)
-		n = *(double*)val;
+		n = *(Sfdouble_t*)val;
 	else
 		n = sh_arith(val);
 	srand(rp->rand_seed = (unsigned int)n);
@@ -753,7 +753,7 @@ static void put_lineno(Namval_t* np,const char *val,int flags,Namfun_t *fp)
 		return;
 	}
 	if(flags&NV_INTEGER)
-		n = (Sfdouble_t)(*(double*)val);
+		n = *(Sfdouble_t*)val;
 	else
 		n = sh_arith(val);
 	sh.st.firstline += (int)(nget_lineno(np,fp) + 1 - n);
@@ -778,7 +778,7 @@ static void put_lastarg(Namval_t* np,const char *val,int flags,Namfun_t *fp)
 {
 	if(flags&NV_INTEGER)
 	{
-		sfprintf(sh.strbuf,"%.*g",12,*((double*)val));
+		sfprintf(sh.strbuf,"%.*Lg",12,*((Sfdouble_t*)val));
 		val = sfstruse(sh.strbuf);
 	}
 	if(val)

src/cmd/ksh93/tests/arith.sh

@@ -1000,9 +1000,10 @@ got=$(PATH=/dev/null; typeset -i z; redirect 2>&1; z='add(2 , 3)'; echo $z)
 # value should still be cast to the type of the variable that is assigned to
 
 float x
-x.getn() { .sh.value=999.99; }
-let "(got = x = 1234.56) == 1234.56" || err_exit "arithmetic assignment triggers getn discipline (got $got)"
-let "(got = x) == 999.99" || err_exit "arithmetic comparison fails to trigger getn discipline (got $got)"
+x.getn() { .sh.value=987.65; }
+let "got = x = 1234.56"
+[[ $got == 1234.56* ]] || err_exit "arithmetic assignment triggers getn discipline (got $got)"
+[[ $x == 987.65* ]] || err_exit "arithmetic comparison fails to trigger getn discipline (got $x)"
 unset x
 whence -q x.getn && err_exit "unset x fails to unset -f x.getn"
 
@@ -1015,7 +1016,8 @@ whence -q x.getn && err_exit "unset x fails to unset -f x.getn"
 		then	err_exit "arithmetic assignment does not return properly typecast value (-${sz}i, got $got)"
 		fi
 		typeset -${sz}F x=0
-		if	! let "(got = x = 123.95) == 123.95"
+		let "got = x = 123.95"
+		if	[[ $got != 123.95* ]]  # ignore OS-dependent rounding error
 		then	err_exit "arithmetic assignment does not return properly typecast value (-${sz}F, got $got)"
 		fi
 	done
@@ -1029,5 +1031,15 @@ if	! let "(got = RANDOM = 123.95) == 123"
 then	err_exit "arithmetic assignment to RANDOM does not return typecast of assigned value (got $got)"
 fi
 
+let "_ = 123.95, got = _"
+if	[[ $got != '123.95' ]]
+then	err_exit "arithmetic assignment to _ fails (got $got)"
+fi
+
+got=$(let "LINENO = 123"; print $LINENO )
+if	[[ $got != '122' ]]   # TODO: should be 123
+then	err_exit "arithmetic assignment to LINENO fails (got $got)"
+fi
+
 # ======
 exit $((Errors<125?Errors:125))