Allow negative paramIndex as reversed-order in ParamFlowRule (#549)

* Allow negative paramIndex in ParamFlowRule

Signed-off-by: Carpenter Lee <hooleeucas@163.com>

sentinel-extension/sentinel-parameter-flow-control/src/main/java/com/alibaba/csp/sentinel/slots/block/flow/param/ParamFlowChecker.java

@@ -52,7 +52,7 @@ final class ParamFlowChecker {
         }
 
         // Get parameter value. If value is null, then pass.
-        Object value = args[paramIdx];
+        Object value = valueAt(args, paramIdx);
         if (value == null) {
             return true;
         }
@@ -64,6 +64,18 @@ final class ParamFlowChecker {
         return passLocalCheck(resourceWrapper, rule, count, value);
     }
 
+    private static Object valueAt(Object[] args, int paramIdx) {
+        Object value = null;
+        if (paramIdx < 0) {
+            if (-paramIdx <= args.length) {
+                return args[args.length + paramIdx];
+            }
+        } else {
+            value = args[paramIdx];
+        }
+        return value;
+    }
+
     private static boolean passLocalCheck(ResourceWrapper resourceWrapper, ParamFlowRule rule, int count,
                                           Object value) {
         try {
@@ -112,7 +124,7 @@ final class ParamFlowChecker {
                 int itemThreshold = rule.getParsedHotItems().get(value);
                 return ++threadCount <= itemThreshold;
             }
-            long threshold = (long) rule.getCount();
+            long threshold = (long)rule.getCount();
             return ++threadCount <= threshold;
         }
 

sentinel-extension/sentinel-parameter-flow-control/src/main/java/com/alibaba/csp/sentinel/slots/block/flow/param/ParamFlowRuleUtil.java

@@ -31,7 +31,7 @@ public final class ParamFlowRuleUtil {
 
     public static boolean isValidRule(ParamFlowRule rule) {
         return rule != null && !StringUtil.isBlank(rule.getResource()) && rule.getCount() >= 0
-            && rule.getGrade() >= 0 && rule.getParamIdx() != null && rule.getParamIdx() >= 0 && checkCluster(rule);
+            && rule.getGrade() >= 0 && rule.getParamIdx() != null && checkCluster(rule);
     }
 
     private static boolean checkCluster(/*@PreChecked*/ ParamFlowRule rule) {

sentinel-extension/sentinel-parameter-flow-control/src/test/java/com/alibaba/csp/sentinel/slots/block/flow/param/ParamFlowRuleUtilTest.java

@@ -35,7 +35,7 @@ public class ParamFlowRuleUtilTest {
             .setCount(1)
             .setParamIdx(-1);
         assertFalse(ParamFlowRuleUtil.isValidRule(rule4));
-        assertFalse(ParamFlowRuleUtil.isValidRule(rule5));
+        assertTrue(ParamFlowRuleUtil.isValidRule(rule5));
 
         ParamFlowRule goodRule = new ParamFlowRule("abc")
             .setCount(10)