shell/math: fix the order of variable resolution in binops

function                                             old     new   delta
arith_apply                                         1134    1143      +9

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>

shell/hush_test/hush-arith/arith-assign-in-varexp.right

@@ -0,0 +1,3 @@
+20:20
+a=b=10
+b=10

shell/hush_test/hush-arith/arith-assign-in-varexp.tests

@@ -0,0 +1,8 @@
+exec 2>&1
+a='b=10'
+b=3
+# The variables should evaluate left-to-right,
+# thus b is set to 10 _before_ addition
+echo 20:$((a + b))
+echo "a=$a"
+echo "b=$b"

shell/hush_test/hush-arith/arith-comma1.right

@@ -0,0 +1,3 @@
+10:10
+a=b=10
+b=10

shell/hush_test/hush-arith/arith-comma1.tests

@@ -0,0 +1,6 @@
+exec 2>&1
+a='b=10'
+b=3
+echo 10:$((a,b))
+echo "a=$a"
+echo "b=$b"

shell/hush_test/hush-arith/arith-ternary2.right

@@ -0,0 +1,3 @@
+6:6
+a=b=+err+
+b=6

shell/hush_test/hush-arith/arith-ternary2.tests

@@ -0,0 +1,7 @@
+exec 2>&1
+a='b=+err+'
+b=5
+# The not-taken branch should not parse variables
+echo 6:$((0 ? a : ++b))
+echo "a=$a"
+echo "b=$b"

shell/hush_test/hush-arith/arith-ternary_nested.right

@@ -0,0 +1 @@
+5:5

shell/hush_test/hush-arith/arith-ternary_nested.tests

@@ -0,0 +1,2 @@
+exec 2>&1
+echo 5:$((1?2?3?4?5:6:7:8:9))

shell/math.c

@@ -340,7 +340,20 @@ arith_apply(arith_state_t *math_state, operator op, var_or_num_t *numstack, var_
 	if (op == TOK_CONDITIONAL) /* Example: $((a ? b)) */
 		return "malformed ?: operator";
 
-	/* Resolve name to value, if needed */
+	if (PREC(op) < UNARYPREC) {
+		/* In binops a ~ b, variables are resolved left-to-right,
+		 * resolve top_of_stack[-1] _before_ resolving top_of_stack[0]
+		*/
+		if (top_of_stack == numstack) /* need two arguments */
+			goto syntax_err;
+		/* Unless it is =, resolve top_of_stack[-1] name to value */
+		if (op != TOK_ASSIGN) {
+			err = arith_lookup_val(math_state, top_of_stack - 1);
+			if (err)
+				return err;
+		}
+	}
+	/* Resolve top_of_stack[0] name to value */
 	err = arith_lookup_val(math_state, top_of_stack);
 	if (err)
 		return err;
@@ -360,20 +373,10 @@ arith_apply(arith_state_t *math_state, operator op, var_or_num_t *numstack, var_
 		/* Binary operators */
 		arith_t right_side_val;
 
-		/* Binary operators need two arguments */
-		if (top_of_stack == numstack)
-			goto syntax_err;
-		/* ...and they pop one */
+		/* Pop numstack */
 		NUMPTR = top_of_stack; /* this decrements NUMPTR */
 		top_of_stack--; /* now points to left side */
 
-		if (op != TOK_ASSIGN) {
-			/* Resolve left side value (unless the op is '=') */
-			err = arith_lookup_val(math_state, top_of_stack);
-			if (err)
-				return err;
-		}
-
 		right_side_val = rez;
 		rez = top_of_stack->val;
 		if (op == TOK_BOR || op == TOK_OR_ASSIGN)